rheology and stability of olive oil cream …repository.um.edu.my/1181/1/thesis- tan hsiao...
TRANSCRIPT
RHEOLOGY AND STABILITY OF OLIVE OIL CREAM EMULSION STABILIZED BY SUCROSE FATTY ACID
ESTERS NONIONIC SURFACTANTS
TAN HSIAO WEI
FACULTY OF SCIENCE UNIVERSITY OF MALAYA
KUALA LUMPUR
JULY 2009
RHEOLOGY AND STABILITY OF OLIVE OIL CREAM EMULSION STABILIZED BY SUCROSE FATTY ACID
ESTERS NONIONIC SURFACTANTS
TAN HSIAO WEI
THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE OF MASTER OF SCIENCE
FACULTY OF SCIENCE UNIVERSITY OF MALAYA
KUALA LUMPUR
JULY 2009
ii
ABSTRACT
The influence of oil and surfactant concentration to the stability and rheological behavior of
the olive oil emulsions stabilized with sucrose fatty acid ester was evaluated through an
accelerated aging test at 45oC. The stability of the emulsion in this study was examined by
the appearance of phase separation in the emulsion, mean droplet size and Zeta potential
over one month. The effect of accelerated ageing on the emulsions rheological properties
was investigated using oscillatory measurements and a viscometry test at the interval of one
day, one week and one month of storage time. The droplet size of the emulsions was found
to decrease with the increase in the oil and surfactant concentrations which give effect on
the viscosity and yield stress of the emulsions. The flow curve of the emulsions always
exhibited shear thinning behavior and obeys the Power Law viscosity. The shear thinning
effect of the emulsions was found to be decreased when the oil and surfactant concentration
increased due to the smaller droplet size and narrower size distribution. The dynamic
properties of the emulsions were also affected by the oil and surfactant content which
indicates the stronger structural integrity and greater interdroplet interactions. The
viscoelasticity of the emulsions was enhanced by the increased in the oil and surfactant
concentrations. The emulsions with higher oil composition show greater elasticity which
implies strong dynamic rigidity of the emulsions. The emulsions with 80% oil were the
most stable emulsions with longest shelf-life.
iii
ABSTRAK
Pengaruhan kepekatan minyak dan surfaktan terhadap kestabilan dan sifat rheologi bagi
emulsi yang distabilkan dengan Sucrose Fatty Acid Ester dikaji di bawah keadan terkawal
pada suhu 45oC. Penilaian terhadap kestabilan emulsi dijalankan melalui pengujian tahap
pisahan, saiz titisan, dan Keupayaan Zeta bagi emulsi tersebut dalam tempoh masa satu
bulan. Kesan menua keatas emulsi disiasat dengan menggunakan teknik rincihan secara
berayun dan suatu ujian viscometrik selepas masa penyimpanan sepanjang satu hari, satu
minggu dan satu bulan. Saiz titisan emulsi didapati menurun apabila peratusan kandungan
minyak dan surfaktan yang digunakan dalam penyediaan emulsi meningkat. Penurunan saiz
titisan ini telah memberikan kesan pada kelikatan dan tegasan alah emulsi tersebut.
Lengkungan aliran bagi emulsi tersebut mempamerkan sifat penipisan secara rincihan dan
mengikuti kelikatan Power-Law. Kesan penipisan secara rincihan bagi emulsi ini didapati
menurun apabila peratusan kandungan minyak dan surfaktan yang digunakan meningkat.
Ini disebabkan oleh titsan emulsi yang lebih kecil dan taburan saiz titisan emulsi yang lebih
sempit. Sifat dinamik bagi emulsi tersebut juga dipengaruhi oleh komposisi minyak dan
surfaktan yang menunjukkan kekuatan integriti struktur serta interaksi yang lebih besar
antara titisan emulsi. Sifat viskoelastik bagi emulsi juga dipertingkatkan dengan
peningkatan penggunaan minyak dan surfaktan. Emulsi yang disediakan dengan peratusan
minyak yang lebih tinggi mempunyai sifat keanjalan yang lebih besar dan ini merupakan
suatu tanda kekuatan ketegaran dinamik. Emulsi yang disediakan dengan 80% minyak
merupakan emulsi yang paling stabil dengan tempoh penggunaan yang terpanjang.
iv
ACKNOWLEDGEMENTS
I would like to take the opportunity to express my appreciation to many people that
have made this dissertation possible. First of all, I would like to express my sincere
gratitude to my supervisor Assoc. Prof. Dr. Misni Misran for his valuable guidance,
brilliant discussion, supervision and patience throughout the course of this research.
Special thanks to the Ministry of Science, Technology and Innovation (MOSTI) and
the University of Malaya that have generously been giving financial support towards my
master studies. My heartfelt gratitude also goes out to all lecturers and staffs in the
Department of Chemistry for their assiduous dedication and also the University of Malaya
management.
I would like to render my appreciation to Ms Loh Mei Ying, Mr. Tan Cok King and
Mr. Tiong Ngik Seng for their experiences, advices and guidance on theories and operation
of the instruments. I would also like to thank the members of Colloid and Surfaces
Laboratory for their encouragement and assistance throughout the research.
Last but not least, I would like to extend my deepest gratitude to my beloved
parents, my sister and my dearest friend Mr. Tay Kheng Soo for encouraging and inspiring
me all these years.
v
Table of Contents
ABSTRACT…………………………………………………………………………. ii
ABSTRAK…………………………………………………………………………... iii
ACKNOWLEDGEMENTS………………………………………………………… iv
TABLE OF CONTENTS…………………………………………………………… v
LIST OF FIGURES………………………………………………………………… vii
LIST OF TABLES………………………………………………………………….. xi
LIST OF ABBREVIATIONS……………………………………………………… xii
Chapter 1 General Introduction Page
1.1 Surfactant……………………………………………………………………... 1
1.2 The importance of glycolipid…………………………………………………
1.2.1 Sucrose fatty acid ester………………………………………………...
4
4
1.3 Aggregation of Surfactant to the Formation of Micelle………………………
1.3.1 The Dynamic Micellization……………………………………………
1.3.2 The Micellar Kinetics………………………………………………….
1.3.3 Micellar Kinetics and Emulsion……………………………………….
7
8
9
11
1.4 Emulsion
1.4.1 Hydrophile-Lipophile Balance (HLB)………………………………...
1.4.2 Determination of Emulsion Type……………………………………...
(a) Conductivity Measurement……………………………………….
(b) Dye Solubility Method…………………………………………...
(c) Dispersion in Water………………………………………………
(d) Microscopic Imaging……………………………………………..
1.4.3 Emulsion Stabilization………………………………………………..
1.4.4 Emulsion Applications………………………………………………..
11
14
15
16
16
17
17
18
22
1.5 Olive Oil……………………………………………………………………… 25
vi
1.6 Property of Emulsion…………………………………………………………. 27
1.7 Rheology……………………………………………………………………...
1.7.1 Historical perspective…………………………………………………
1.7.2 Application of Rheology……………………………………………...
29
29
32
1.8 Objective……………………………………………………………………... 34
Chapter 2 Materials and Methods
2.1 Materials ……………………………………………………………………. 35
2.2 Emulsion preparation…………………………………………………………. 35
2.3 Zeta potential measurement…………………………………………………... 36
2.4 Morphology and droplet size analysis………………………………………... 37
2.5 Rheological measurement…………………………………………………… 38
Chapter 3 Results and Discussion
3.1 Phase Separation……………………………………………………………… 40
3.2 The Effect of Surfactant, Oil Concentration and Storage Time on the Droplet
Size.…………………………………………………………………………...
3.2.1 Effect of Surfactant Concentration to the Droplet Size……………….
3.2.2 Aging effect…………………………………………………………...
3.2.3 Effect of oil concentration…………………………………………….
3.3 Zeta potential………………………………………………………………….
43
46
52
55
62
3.4 Rheological Properties of Emulsion System………………………………….
3.4.1 Steady State Behavior…………………………………………………
3.4.1.1 Effect of emulsion compositions on the flow properties……………...
3.4.1.2 Aging effect to the emulsions flow properties………………………..
66
66
66
81
3.5 Dynamic Characteristic ……………………………………………………… 82
4.0 Conclusion…………………………………………………………………
4.1 Future Research………………………………………………………
108
110
vii
5.0 References……………...………...………………………………………. 112
Appendix……………..………………………………………………….. 125
List of Figure Chapter 1 Page
Figure 1.1 Surfactants consist of two parts: (a) the hydrophilic part (head
group) and (b) the hydrophobic part (lipid tail)…………………….
1
Figure 1.2 The structure of Sucrose palmitate, which is a type of sucrose fatty
acid ester…………….........................................................................
5
Figure 1.3 The arrangement of surfactant in different types of aggregation (a)
micelle and (b) inverse micelle..........................................................
7
Figure 1.4 The mechanism of adsorption and desorption of surfactant
monomers under the dynamic equilibrium condition (above CMC),
where 1 1k k−= ……………………………………………………….
8
Figure 1.5 The schematic to show the second relaxation time, τ2 of the micelle 9
Figure 1.6 Schematic illustration of the effect of bulk concentration to
micellization process. (a) When the surfactant concentration is
below the CMC, the solution is very dilute and the kinetic diffusion
of the monomer is fairly important and the diffusion rate follow the
Fick’s First Law where the flux of diffusion (Jx) is proportional to
the concentration gradient ( dCdx
) and gives xdCJ Ddx
= − . (b) The
micelle concentration is small (just above the CMC), a dissemble
boundary is found, where there were two potential regimes, the
micelle zone and the micelle free zone. (c) When the bulk
concentration is above the CMC, a large diffusion flux drives the
micelle directly to the sublayer, continually supplying the
monomer to the surface……………………………………………..
10
Figure 1.7 Distribution of emulsion droplets (a) monodisperse, and (b)
viii
polydisperse………………………………………………………… 12
Figure 1.8 The schematic of (a) oil-in-water emulsion (o/w), (b) water-in-oil
emulsion (w/o), (c) water-in-oil-in-water emulsion (o/w/o) and (d)
oil-in-water-in-oil emulsion (w/o/w)………………………………..
13
Figure 1.9 The chemical structure of (a) Sudan III and (b) crystal violet........... 17
Figure 1.10 The schematic of emulsion destabilization mechanisms…………… 19
Figure 1.11 The schematic of interdroplet pair potential, w(h), where the wA
and wR are the attractive and repulsive interaction potential
respectively; hmin is the minimum separation of the emulsion
droplets which corresponds to the minimum interdroplet interaction
potential, wmin……………………………………………………………………………………
20
Figure 1.12 The schematic to illustrate the kinetic stability of an emulsion with
the activation energy before the system comes to a thermodynamic
stable state………………………………………………………......
20
Figure 1.13 The typical viscoelastic spectrum which is the master showing the
viscoelastic respond of non-Newtonian material…………………...
31
Chapter 2
Figure 2.1 The emulsion dispersed in (a) water very well, but resists the
dispersion in (b) olive oil indicating that the emulsion is an o/w
emulsion……………………………………………………………
36
Chapter 3
Figure 3.1 The emulsion volume fractions for emulsions at (a) day 1, (b) day
7, and (c) day 30…………………………………………………….
40
Figure 3.2 The mean droplet size for emulsions with (a) 50%, (b) 60%, (c)
70% and (d) 80% of disperse phase ……………………..…………
44
Figure 3.3 Micrographs of emulsion in 80% oil with (a) 2 wt%, (b) 5 wt%
and (c) 10 wt% of SFAE surfactants showing the decrease of
droplet size with increase in surfactant concentration……..………..
48
ix
Figure 3.4 The micrographs of dilute emulsion (50% oil) with (a) 2 wt%, (b) 5
wt%, and (c) 10 wt% of surfactant concentration..............................
51
Figure 3.5 The micrographs of emulsions with 50% disperse phase captured at
(a) day 1, (b) day 7 and (c) day 30, showing the coalescence
destabilization……………………………………………………….
53
Figure 3.6 The effect of oil concentration to the droplet size of emulsions over
30 days of storage under accelerating condition. (a) First day, (b)
7th day, and (c) 30th day.…………………………….........................
56
Figure 3.7 The micrographs for emulsions with (a) 50%, (b) 60%, (c) 70%,
and (d) 80% of oil and stabilized with 2 wt% of surfactant. These
micrographs were obtained after one day of storage under
45oC.………………………………………………………………...
60
Figure 3.8 The Zeta potential of emulsions measured in the presence of 0.01
M NaCl solutions for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of
oil concentration ……….…………………………...........................
63
Figure 3.9 The shear rate dependence viscosity of emulsions with a series of
surfactant concentration for (a) 50%, (b) 60%, (c) 70%, and (d)
80% of oil…………………………………………...........................
67
Figure 3.10 The effect of oil concentration to the zero shear viscosity of
emulsion after (a) 1 day, (b) 7 days, and (c) 30 days of storage
under an accelerated condition of 45oC……………………………..
70
Figure 3.11 The yield stress of the emulsions determined using the Herschel-
Bulkely model from the Bohlin rheometer software, showing the
correlation between the mean droplet size and the yield stress of
the emulsion as a function of surfactant concentration for (a) 1st
day, (b) 7th day, and (c) 30th day………..…………………………...
73
Figure 3.12 Schematic representation of structural change when shear applied.
(a) The shear thickening region, (b) the First Newtonian region, (c)
the shear thinning region, and (d) the Second Newtonian region…..
77
Figure 3.13 The droplet size distribution of the emulsions stabilized with 2 wt%
of SFAE for (a) 1st day, (b) 7th day, and (c) 30th day of storage
time.....................................................................................................
77
x
Figure 3.14 The packing of emulsion droplets in the (a) polydisperse system
and (b) monodisperse system showing the droplet packing
efficiency……………………………………………………………
80
Figure 3.15 The γc of the emulsions for (a) 1st day, (b) 7th day, and (c) 30th
day......................................................................................................
83
Figure 3.16 The morphology of emulsions with 80% oil which stabilized by (a)
7 wt%, (b) 8 wt%, (c) 9 wt% and (d) 10wt% of SFAE……………..
85
Figure 3.17 The morphology for the emulsions with 70% oil stabilized with (a)
7 wt%, (b) 8 wt%, (c) 9 wt%, and (d) 10 wt% of SFAE…………...
87
Figure 3.18 The elastic modulus of emulsions at the first day of age obtained
from the strain sweep to establish the linear viscoelastic range. (a)
50%, (b) 60%, (c) 70%, and (d) 80% oil……………………………
90
Figure 3.19 The magnitude of elastic modulus of the emulsions obtained from
the strain sweep measurement for (a) 1st day, (b) 7th day, and (c)
30th day...............................................................................................
93
Figure 3.20 The cohesive energy of the emulsions obtained at (a) 1, (b) 7, and
(c) 30 days of storage periods……………………………………….
95
Figure 3.21 Frequency sweep profile of the emulsions with (a) 50%, (b) 60%,
(c) 70%, and (d) 80% of oil…………………………………............
97
Figure 3.22 The Tan δ of emulsions with (a) 50%, (b) 60%, (c) 70%, and (d)
80% of oil obtained after one day of storage………………………..
102
Figure 3.23 The effect of oil concentration to the Tan δ of emulsions stabilized
with 2 wt% of SFAE after (a) 1 day, (b) 7 days, and (c) 30
days………………………..……………………….......................
105
xi
List of Table Chapter 1 Page Table 1.1 The examples of different types surfactants …………………………. 2
Table 1.2 Some common hydrophilic surfactants found in commercial available
cosmetic products …………………………………………………….
3
Table 1.3 Microbial biosurfactants and their general structures ……………….. 6
Table 1.4 The classification of surfactants accordingly to the HLB values and
the applications………………………………………………………..
15
Table 1.5 The hydrophilic and lipophilic group number obtained by Davies
which are used in the calculation of HLB value of a surfactant..……..
15
Table 1.6 The type of fatty acid present in olive oil and its composition (in the
form of methyl ester)………………………………………………….
26
Table 1.7 The typical shear rate range for various physical operations with
examples………………………………………………………………
33
Chapter 3
Table 3.1: The Power Law index, n of the emulsions for over 30 days of storage
time. All n were smaller than 1 indicating that the emulsions exhibits
shear thinning behavior………………………………………………..
76
xii
List of Abbreviations SFAE Sucrose Fatty Acid Ester
CMC Critical Micelle Concentration
o/w Oil-in-water
w/o Water-in-oil
o/w/o Oil-in-water-in-oil
w/o/w Water-in-oil-in-water
HLB Hydrophilic-Lipophilic Balance
FDA Food and Drug Administration
EU European Union
GC/MS Gas chromatography/mass spectrometry
CHAPTER 1
INTRODUCTION
Chapter 1 Introduction
________________________________________________________________________ - 1 -
1.0 General Introduction
1.1 Surfactant
The word surfactant (Figure 1.1) comes from an abbreviation for surface active
agent. They are amphiphilic molecules, containing both the hydrophilic and
hydrophobic parts. This duality character enables the surfactant to adsorb at the surface
or interface of two or more immiscible phases thereby reducing the surface or interfacial
tension. The ability of surfactant to adsorb at interface may also be used to prevent
aggregation, flocculation and coalescence of emulsion droplets to enhance the emulsion
stability. They replaced the energy rich bulk phase molecules and reduced the surface
free energy of the system, thus reduce the surface/interface tension [1]. Surfactants are
often classified according to the charge of their head group into anionic, cationic,
nonionic, and zwiterionic (Table 1.1). Surfactants have been widely used in household
products, cosmetic, food and pharmaceutical industries [2] as emulsifiers, wetting
agents, cleansers, foaming agents [3], and drug delivery applications [4, 5]. Surfactants
are also used for creating dispersion system such as emulsions and suspensions for
nanoparticles synthesis [3].
Figure 1.1: Surfactants consist of two parts: (a) the hydrophilic part (head group) and (b)
the hydrophobic part (lipid tail).
Chapter 1 Introduction
________________________________________________________________________ - 2 -
Table 1.1: The examples of different types of surfactants
Surfactant Type Example
(a) Anionic
(b) Cationic
(c) Nonionic
(d) Zwiterionic
In cosmetic industry, surfactants are used as emulsifier, wetting agent, cleanser,
foaming agents, solubilizers, conditioners, thickener and also to produce emollients [2,
3]. Thus, these surfactants used in cosmetic formulations are expected to be human
friendly. Ionic/hydrophilic surfactants (Table 1.2) especially anionic surfactants are
commonly used surfactants in most cosmetic formulations. However, there were studies
proven that the surfactants are harmful to human and environment [6-8]. That is because
the toxicity of these anionic surfactants can harm the cell membrane, enzyme activity,
the binding properties of proteins, and other cell components of aquatics and human [6,
7]. Application of these surfactants in pharmaceutical formulations influences the
biological efficiency of the active ingredient in the formulations [9]. It has been reported
Chapter 1 Introduction
________________________________________________________________________ - 3 -
that these ionic surfactants will either bind directly to the drug [10] or by influencing the
adsorption and absorption processes and the partition of drugs between hydrophobic and
hydrophilic compartments in the organ and organisms [11]. In other words, the
surfactants penetrate through the skin and interfere with the function of the cell
membrane [3, 4, 12].
Table 1.2: Some common hydrophilic surfactants found in commercial available
cosmetic products [13].
Class of Surfactant Head Group Structure
Sulfate Carboxylate
Phosphate
Sulfonate
Betaines
Besides that, disposal of these ionic surfactants mentioned earlier has become
another major concern due to their potential to induce ecotoxicity [3]. Owing to the
extensive use of surfactants in our daily life and industrial world, a considerable amount
of these ionic surfactants has been released into the environment causing serious
pollution to the river and sea [6, 14, 15]. Most of the synthetic surfactant are
biorecalcitrant and the ecological impact of these surfactants is the major concern [16].
Thus, there is a preference to use biosurfactants for formulations such as glycolipids
[17], which are nonionic surfactants, biocompatible and biodegradable in nature.
Chapter 1 Introduction
________________________________________________________________________ - 4 -
1.2 The importance of glycolipid
Glycolipid is one type of the biosurfactant [18] and is a popular class of
biosurfactant used in the industrial applications. In recent years, biosurfactants such as
glycolipid have drawn a large attention among the formulators and researches to replace
the synthetic surfactants in house hold products, personal care products, and
pharmaceutical products due to the destructive cause of the synthetic surfactants to the
environment and human [18]. Glycolipids can be naturally produced by bacteria, fungus,
and yeasts as shown in Table 1.3 [18]. They are known for biocompatibility [19],
therefore, is a promising alternative to replace synthetic surfactants in our daily used
products such as cosmetics, pharmaceuticals and foods. Besides that, the glycolipids are
also environmental friendly due to their biodegradable property in nature [20].
1.2.1 Sucrose fatty acid ester
Sucrose fatty acid ester (SFAE) is a glycolipid surfactant (Figure 1.2) and is
non-ionic. It is widely used as an emulsifier in cosmetic and pharmaceutics industries
for a number of years to replace the petroleum based surfactants. The non-toxic nature
of this compound has also led to the extensive use in the formulation of food products
[21], which due to the naturally occur sugar head group. The interests of developing the
sucrose ester were started from the late 1950s as a natural alternative and is
commercially produced [22]. Their amphiphilic character can be controlled within wide
limits by altering both the degree of esterification and the chain length of the ester group,
so that extensive permutations are possible to obtain a required hydrophile-lyophile
balance [23]. Sucrose esters have a range of applications in the food, cosmetics, oral-
Chapter 1 Introduction
________________________________________________________________________ - 5 -
care, detergent, and pharmaceutical industries [24, 25]. Their properties as
antimicrobials are useful in food storage [26], antitumorals [27] and insecticidals [28].
This carbohydrate-based surfactant [29] can be easily synthesize by
esterification process in the presence of a biocatalyst [30, 31], fermentation with
Corynebacaterium Hydrocarboclastus [32]. There is also no concern about the
environmental pollution of this surfactant, due to the biocompatibility and
biodegradability [18, 33].
HO
HO
O
HO
O
HO OH
OH
OH
OO O
Figure 1.2: The structure of Sucrose palmitate, which is a type of sucrose fatty acid ester.
Chapter 1 Introduction
________________________________________________________________________ - 6 -
Table 1.3: Microbial biosurfactants and their general structures [18].
Microorganism Biosurfactant General Structure
Rhodococcus
erythropolis
Sucrose and
fructose lipids;
Trehalose
lipids;
Trehalose
mycolates
Torulopsis
species Sophorolipids
Psedomonas
species Rhamnolipids
Candida
Petrophilum Peptidolipid
Chapter 1 Introduction
________________________________________________________________________ - 7 -
1.3 Aggregation of Surfactant to the Formation of Micelle
Surfactants tend to form aggregates spontaneously in solution to form a
thermodynamically stable structure, so called micelle [34]. The increase of surfactant
concentration in the bulk phase leads to micelle formation. Concentration when the
micelles starts to form is the Critical Micelle Concentration (CMC) [35]. Generally,
there the surfactant aggregates to form the micelle or inverse micelle (Figure 1.3).
However, the shapes and structures of the aggregation are dependent on the nature of
surfactant and the molecular geometry [36]. In a micelle, the hydrophilic head group of
the surfactant is directed to the aqueous phase (polar) allowing the formation of the
hydrogen bonding between the polar head group and water molecules; while the
hydrophobic tail is protected in the core of the micelle from polar environment to reduce
the free energy of the system (Figure 1.3(a)) [37].
Fig. 1.3(a)
oil
Fig. 1.3(b)
water
Figure 1.3: The arrangement of surfactant in different types of aggregation (a) micelle
and (b) inverse micelle.
Chapter 1 Introduction
________________________________________________________________________ - 8 -
1.3.1 The Dynamic Micellization
Figure 1.4: The mechanism of adsorption and desorption of surfactant monomers under
the dynamic equilibrium condition (above CMC), where 1 1k k−= .
The micelles are the reservoir for the surfactant monomer in solution and surface
activity is reduced due to the shielding of the hydrophobic tail in the core of the micelle,
which is sensitive to non-polar environment [34, 38]. In other words, the micelle cannot
adsorb directly to the available surface and interface and only the monomers are
responsible to the surface, interfacial tension reduction and dynamic phenomena, such
as emulsification, wetting and foaming [34]. As the surfactant concentration exceeds
CMC, the amount of micelle and surfactant monomers are in dynamic equilibrium
indicating that the rates of adsorption and desorption between monomers and micelles
are the same (Figure 1.4) [39]. The rate of association and dissociation are in a very fast
time scale (~ 10-6s), and is called the fast relaxation process of micelle [39, 40].
Chapter 1 Introduction
________________________________________________________________________ - 9 -
1.3.2 The Micellar Kinetics
There is another relaxation process of micelle to explain the life-time of micelles
(Figure 1.5). The relaxation time in this process is very important, because the time
scale can vary from millisecond to minutes [39]. The longer the relaxation time scale,
the more stable the micelle is. However, that is not a favor able condition, because the
more stable the micelle becomes, the longer the time that will be taken by the system to
reach dynamic equilibrium at the interface. Figure 1.6 shows the transportation model
proposes by Song et. al. [37]. Besides, there were also some other adsorption kinetic
theories having almost the same concept, saying that the micelles have to be broken into
surfactant monomers before adsorption to the interface [41-43].
Figure 1.5: The schematic to show the second relaxation time, τ2 of the micelle.
Chapter 1 Introduction
________________________________________________________________________ - 10 -
(a)
(b)
(c)
Figure 1.6: Schematic illustration of the effect of bulk concentration to micellization
process. (a) When the surfactant concentration is below the CMC, the solution is very
dilute and the kinetic diffusion of the monomer is fairly important and the diffusion rate
follows the Fick’s First Law where the flux of diffusion (Jx) is proportional to the
concentration gradient ( dCdx
) and gives xdCJ Ddx
= − . (b) The micelle concentration is
small (just above the CMC), a dissemble boundary is found, where there were two
potential regimes, the micelle zone and the micelle free zone. (c) When the bulk
concentration is above the CMC, a large diffusion flux drives the micelle directly to the
sublayer, continually suppling the monomers to the surface [37, 44].
Chapter 1 Introduction
________________________________________________________________________ - 11 -
1.3.3 Micellar Kinetics and Emulsion
The micelle kinetic adsorption has a correlation with the emulsification and
several other dynamic processes such as wetting and foaming. During emulsion
preparation, energy is applied to break down the oil layer into small oil droplets. The
process increased the oil-water interfacial area. Thus, more surfactant monomers are
required to stabilize these oil droplets against destabilization process. Therefore, the
amount of surfactant used in emulsification has to be well above the CMC, in order to
maintain the equilibrium between micelles and monomers in the interfacial layer and
bulk phase. For this reason, a micelle with shorter life time is favorable.
Certainly, very stable micelles face a difficulty to supply sufficient amount of
monomers onto the large interfacial area. Owing to their long relaxation time, τ2 they are
only able to release a small amount of surfactant molecules to the bulk, creating low
monomer flux. As a result, the oil droplets are not well covered by the surfactant,
increasing the interfacial tension and eventually the oil droplet size increases [45-48].
1.4 Emulsion
An emulsion is a system consisting of two immiscible liquids dispersed in one
another and is thermodynamically unstable [1]. An emulsion can be kinetically stable
(long-term stability) with the presence of surfactant in the system by creating an energy
barrier to flocculation and coalescence [1, 49] and exists in a metastable state [50]. The
droplets size of emulsion is generally greater than 100 nm [1, 51]. The appearance of an
emulsion is turbid, because the droplets are larger than the wavelength of visible light
[51]. In real emulsions, the size distribution of droplets is generally polydisperse as
described in Figure 1.7 [52].
Chapter 1 Introduction
________________________________________________________________________ - 12 -
The mean droplet size (d) and polydispersity (p) are normally used to characterize
emulsions (Eq. 1 and 2) [53]. According to Leal-Calderin, F., et. al. [53], an emulsion
with P ≤ 25% is considered as monodisperse system. However, other researchers have
different opinion about the polydispersity [54]. They believe that monodispersity is a
relatively term. They had proposed that a system with 10% of polydispersity in the
droplets size is acceptable to characterize as a monodisperse system.
4
3
i ii
i ii
N dd
N d=∑
∑ (1)
3
3
1 i i ii
i ii
N d d dp
d N d
−=
∑
∑ (2)
d is the median droplet diameter and iN is the total number of droplets with diameter id .
(a)
(b)
Figure 1.7: Distribution of emulsion droplets (a) monodisperse, and (b) polydisperse.
Chapter 1 Introduction
________________________________________________________________________ - 13 -
The emulsion can be dispersed in several manner and the most common are oil-in-
water (o/w), water-in-oil (w/o), and double emulsion such as oil-in-water-in-oil (o/w/o)
and water-in-oil-in-water (w/o/w) (Figures 1.7 and 1.8) [1]. The dispersion type of
emulsion is highly dependent on the nature of surfactant used in the system and
emulsion preparation [1, 55]. According to Bancroft’s postulate, the phase in which the
surfactant is most soluble is the continuous phase [1, 56]. In order words, hydrophobic
surfactant will form w/o emulsion; hydrophilic surfactant will form o/w emulsion. As
indicated above, the double emulsions can be prepared (Figure 1.8(c) and 1.8(d)) by
dispersing w/o in water continuum or o/w in oil continuum. For example, a w/o/w
emulsion is prepared by emulsifying simple w/o emulsion in an aqueous solution of a
high Hydrophile-Lipophile Balance value surfactant and vice versa [57, 58].
(a) (b)
(c)
(d)
Figure 1.8: The schematic of (a) oil-in-water emulsion (o/w), (b) water-in-oil emulsion
(w/o), (c) water-in-oil-in-water emulsion (o/w/o) and (d) oil-in-water-in-oil emulsion
(w/o/w).
Chapter 1 Introduction
________________________________________________________________________ - 14 -
1.4.1 Hydrophile-Lipophile Balance (HLB)
The HLB value is an indication of the solubility of surfactant. The concept was
introduced by Griffin in 1954 [59] and is extended by Davies in 1957 [1, 60]. This
theory proposed that the hydrophobic surfactants have relatively low HLB value
compared to hydrophilic surfactants (Table 1.4). These hydrophobic surfactants are
predicted to be suitable for the formation of w/o emulsion, whereas the hydrophilic
surfactants are good for the formation of o/w emulsion. Therefore the continuous phase
is in no need to be predominating in quantity of the material. For example, an o/w
emulsion can be formed by using oil to water ratio of 8:2 with hydrophilic surfactant.
There were several methods to calculate the HLB value of a surfactant. Equation
(3) shows the calculation of HLB value from the structure of the surfactant. Davies who
extended the concept in 1957 proposed another calculation of HLB value (Eq. (4))
which is based on the group contribution method (Table 1.5) [1, 60], where Σ
hydrophilic = n × hydrophilic group number; Σ hydrophobic = n × hydrophobic group
number and n is the repeating number of the particular group. However, Davies’s
method is not suitable for some of the surfactant especially nonionic surfactants due to
the lack of group numbers [61, 62].
20 H
H L
MHLBM M
= ×+
(3)
HM is the molecular weight of the hydrophilic part and LM is the molecular weight of
the lipophilic part [1, 63].
7HLB hydrophilic lipophilic= ∑ − ∑ + (4)
Chapter 1 Introduction
________________________________________________________________________ - 15 -
Table 1.4: The classification of surfactants accordingly to the HLB values and the
applications [64].
Solubility of Surfactant in Water HLB Value Applications
Insoluble 4-5 w/o
Partially soluble 6-9 Wetting agent
Translucent to clear 10-12 Detergent
Very soluble 13-18 o/w
Table 1.5: The hydrophilic and lipophilic group number obtained by Davies which are
used in the calculation of HLB value of a surfactant [36].
Hydrophilic group HLB Value Lipohilic Group HLB Value
-SO4Na 35.7 -CH- -0.475
-COOK 21.1 -CH2- -0.475
-N 9.4 =CH- -0.475
Free Ester 2.4 -CH3 -0.475
Free Alcohol 1.9
-CO2H 2.1
-O- 1.3
1.4.2 Determination of Emulsion Type
As mentioned in section 1.2, there are several types of emulsions. In general,
o/w and w/o emulsions is the most common emulsion we came across. Therefore it is
necessary to differentiate these two types of emulsions. There are several methods that
have been used to differentiate types of emulsions such as conductivity measurement
[65], dye, microscopic imaging [1] and dispersion in water.
Chapter 1 Introduction
________________________________________________________________________ - 16 -
(a) Conductivity measurement
The test is based on the principle of the good conductivity of the aqueous phase
[66]. This method is commonly used where high resistance indicated oil as the
continuous phase, and low resistance indicated water as the continuous phase [65].
According to Hanna, S.A. [67], the o/w emulsion passes a current of 10-13 mA, while
for the w/o emulsion it is only 0.1 mA or less. Therefore a crude experiment is
performed by dipping two electrodes attached to an electric circuit with a light indicator
into the test emulsion (in the present of some electrolyte). Owing to the ability of the
o/w to conduct electric, the light goes on if the test emulsion is an o/w emulsion and
vice versa [67]. This method is useful during the study of an emulsification technique so
called the Phase-Inversion Temperature (PIT) emulsification technique to produce a
fine and more stable emulsion [68, 69].
(b) Dye Solubility Method
This method has the same purpose as the previous method that is to identify the
type of continuous phase of an emulsion [67]. Generally, water or oil soluble dyes such
as Sudan III and crystal violet which is oil-soluble and water-soluble dyes respectively
are chosen to perform the test (Figure 1.9). For example, Sudan III which is the oil
soluble dye will turn w/o emulsion into red colour, while an o/w emulsion will turn to
blue colour as the water soluble is used. Although this method is relatively easy to
perform, nevertheless there are risks and safety precautions when using these dyes.
According to Refat, N.A. et. al., the International Agency for Research on Cancer has
classified the sudan dye as category 3 carcinogen [70], therefore this dye has been
Chapter 1 Introduction
________________________________________________________________________ - 17 -
categorized as illegal food additives (according to FDA and EU) [71] and is advised not
to have any contact with eyes and skin.
(a)
(b)
Figure 1.9: The chemical structure of (a) Sudan III and (b) crystal violet.
(c) Dispersion in water
This method is a crude method which operates on the basis of the ability of an
emulsion to be diluted by its continuous phase. For an o/w emulsion, the emulsion is
expected to able being diluted with water and gives a well dispersion. On the other hand,
if an oil phase of the o/w emulsion is used to dilute this emulsion, then the emulsion
tend to resist dispersion [1, 66].
(d) Microscopic imaging
Observation through the microscope is the direct way to determine the type of an
emulsion. Owing to the difference of reflective index for two liquids, it helps to
distinguish the type of emulsion. A liquid with higher reflective index will appear
brighter under a simple optical microscope and vice versa [1, 72]. For example, the
Chapter 1 Introduction
________________________________________________________________________ - 18 -
reflective index for olive oil ranges from 1.46 to 1.47 [73], while the reflective index for
water is 1.33. According to the principal rule that higher reflective index shows brighter
image, therefore it is expected that the oil droplet will look brighter than the water
droplet [74-76]. This microscopic technique is a more detail technique as compared to
others. This is because one can observe the microscopic structure and also determine if
there is any formation of mix emulsion (o/w and w/o emulsion) which other methods
are not providing this information. Beside, this technique also allows a direct
observation of the droplet shape and measurement of the droplet size.
1.4.3 Emulsion Stabilization
As discussed before, emulsions are thermodynamically unstable and there are
various factors affecting the stability, eventually phase separation. These factors do not
only come from the internal such as the interfacial properties, but also from the external
such as the storage time and conditions (temperature and humidity), action of bacteria,
and mechanical agitation [77].
Emulsion stability is refered to the ability of an emulsion to resist change with
time. Since the emulsion is thermodynamically unstable, they are expected to undergo
destabilization after a period of time leading to a total phase separation [51]. For this
reason, an emulsifier is used to increase the stability of the emulsion system as
discussed in section 1.1. The instability of emulsion discussed is referred to physical
instability such as sedimentation/creaming, flocculation, coalescence, and Ostwald
ripening.
Chapter 1 Introduction
________________________________________________________________________ - 19 -
Figure 1.10: The schematic of emulsion destabilization mechanisms.
Chapter 1 Introduction
________________________________________________________________________ - 20 -
Figure 1.11: The schematic of interdroplet pair potential, w(h), where the wA and wR are
the attractive and repulsive interaction potential respectively; hmin is the minimum
separation of the emulsion droplets which corresponds to the minimum interdroplet
interaction potential, wmin [78].
Figure 1.12: The schematic to illustrate the kinetic stability of an emulsion with the
activation energy before the system comes to a thermodynamic stable state.
Chapter 1 Introduction
________________________________________________________________________ - 21 -
(i) Sedimentation/creaming of emulsion droplets happens due to the density
difference between the two phases and are forms of gravitational separation [50]. For
example, in creaming process (Figure 1.10(d)), the oil droplet (o/w emulsion) moves
upward to the surface due to its lower density as compared to that of water. This
gravitational instability rate is affected by the flocculation mechanism [79]. Flocculation
(Figure 1.10(a)) is an aggregation process of two or more droplets to form flocs [80,
81]. Flocculation only happens after collision of droplets. After collision, particles may
either move away from each other or form permanent aggregate [82]. This is highly
dependent to the types of interaction (attractive and repulsive) between the droplets [81].
When the attractive force is dominant, collision of droplets will leads to floc formation
(Figure 1.11). The flocculation is a reversible process, since the droplets will re-
disperse after subjected to a gentle agitation. This flocculation process enhances the
gravitational separation rate and is a significant destabilization process in dilute
emulsion, It decreases the shelf life of the emulsion [83].
(ii) Coalescence is another emulsion destabilization mechanism. Coalescence is a
process whereby two or more droplets merge together to form a single larger droplet
which is the most thermodynamically stable condition (Figure 1.10(b)) [84]. This
process can only happen when the droplets are close together and the interfacial
membrane between the droplets is disrupted [84]. In general, the forces acting between
the droplets and the resistance of droplets against membrane rupture are the major
factors affecting the coalescence process and is important for a concentrated emulsion
[84]. The stiffness of the interfacial layer is the key to the droplet coalescence, which
creates an energy barrier that has to be overcome before the thermodynamic stable state
is reached (Figure 1.12). Therefore, it is necessary to introduce a strong interfacial layer
to an emulsion in order to enhance the emulsion stability. According to Figure 1.11,
Chapter 1 Introduction
________________________________________________________________________ - 22 -
the coalescence often happens when the droplets are in a very close distance. At this
point, the attractive force is greater than the repulsive force and causes failure of the
interfacial layer to protect the droplets. Consequently, the droplets will merge and the
energy of the droplet will fall into a deep minimum wmin, which is an irreversible
process.
(iii) Ostwald ripening (Figure 1.10(c)) is a diffusive transfer of disperse phase from
smaller droplets to larger droplets under the influence of Laplace pressure difference [85,
86]. The Ostwald ripening destabilization occurs especially in a polydisperse emulsion
[87] facilitated by the presence of micelle in the continuous phase [88-90]. The micelles
solubilize the oil molecules and transported them from one droplet to another [88]. In
other words, the micelles enhance the Ostwald ripening by increasing the solubility of
oil in water, allowed the oil molecules diffused from the small droplet to the larger one.
However, the rate of destabilizations can decrease by having small emulsion
droplet size, increasing the viscosity of the disperse medium, lowering the difference in
density between the two phases or/and creating an energy barrier at the oil and water
interface [1].
1.4.4 Emulsion Applications
Emulsions are widely used as commercial products (such as foods, cosmetics,
and paints) [91, 92] and oil recovery processes [93, 94]. Emulsions are used in cosmetic
industry as lotions, creams, moisturizers, and shower gels. The main functions of
emulsion in cosmetic industry are moisturizing and occlusion to prevent the loss of
water from the skin [1, 95]. The emulsion is a good system for both the hydrophobic
Chapter 1 Introduction
________________________________________________________________________ - 23 -
and hydrophilic substance (active ingredients) such as drugs and vitamins in cosmetic
and pharmaceutical formulations for various applications [96].
As in pharmaceutical industry, emulsions act as a carrier for active ingredients
or drugs [96]. Although most of the drugs are water soluble, and can be injected through
aqueous solutions or water-in-oil emulsions, nevertheless reports have shown that the
delivery of drugs through fatty emulsions are more effective [97-99]. The application of
emulsion in pharmacy as parental emulsion is relatively important especially for the
treatment of critical ill patient [100]. A parental emulsion is a special oil-in-water
emulsion that used to feed the patients whose medical condition makes them unable to
eat normally [101]. Hence, careful selection of component for the formulation has to be
stringently considered.
The oil phase of the formulation has to be either paraffinic or vegetable base,
and only nonionic emulsifiers are suitable. Besides that, the emulsion viscosity has to be
as low as 1 mPa s and the droplet size of the emulsion has to be small (of less than 5μm)
in order to avoid blockage of the vessels during drug delivery and reduce the risk of
toxicity [102]. Certainly, the emulsion has to remain stable at least for 1-2 years and
also at high temperature. At high temperature, destabilization process will be more
pronounced especially coalescence of emulsion droplets can lead to an increase of
droplet size which is totally undesirable during application. For this reason, the
emulsions are always kept under refrigerated condition (~ 3-4oC).
O/W emulsions are often used for intramuscular injection which is the injection
of drugs directly into muscle. Besides that, these emulsions are also used in
chemotherapy treatment for cancer [102]. Therefore, the emulsions have to be able to
reach the target and breakup to release the active compounds on site efficiently. The
efficiency of these emulsions is highly depending on the type of emulsifier used. In
Chapter 1 Introduction
________________________________________________________________________ - 24 -
some condition, modifying the applied emulsifiers is necessary in order to enhance the
efficiency of drug loading and delivering.
Cosmetic products had a very large global market and valued at $125.7 billion in
the year of 1998 [103]. Today, the Asia-Pacific region has become the most valuable
cosmetic market in the world and is leading in global skin care products market.
According to the Euromonitor International’s market report, the Asia-Pacific is the
world’s second largest cosmetics and toiletries market with sales of $62.1 billion in the
year of 2005 [104]. Emulsion is generally used as a vehicle carrying the cosmetic active
ingredients in cosmetic products. Unlike pharmaceutical emulsions, most cosmetic
emulsions are care products which achieved caring and preventing effect on the site of
application. The skin and hair are the most common sites of application.
Playing almost the same role as the pharmaceutical emulsions, cosmetic
emulsions also delivered cosmetically active compounds to target side of application.
Since the cosmetic emulsions are applicable through the skin, hair and nails, emulsions
are more likely to play roles of caring and preventing to the outer layer of our human
body. Besides carrying specific component for special effects such as anti-aging and
cleansing, the cosmetic emulsions have more important function which is to create a
protective layer against external potential damaging factors such as UV radiation. By
applying a suitable and appropriate cosmetic emulsion containing specific active
ingredients helps to improve the appearance of the outermost organs. For examples, skin
dehydration can be prevented through the application of moisturizer which contain
hydrating agent that can retard moisture loss from the skin [105].
An effective cosmetic emulsion has to fulfill several criteria in order to have
maximum performances. First and foremost, the cosmetic emulsions should have long
shelf life and stable at a wide range of temperature. Since the action of cosmetic
emulsions is limited to the outer organ, therefore a pleasant feeling upon application and
Chapter 1 Introduction
________________________________________________________________________ - 25 -
the consistency to achive this pleasant feeling are relatively important such as
moisturizers. The purpose of using moisturizer is to increase the water-holding capacity
and protect the deposit of oily material from the environment that we are exposed to
[106]. Therefore, sticky and greasy product definitely influences consumer preference.
It is also an important point that the emulsions should not cause any skin irritation or
allergenic effect upon application. For this reason, the raw ingredients for the
formulation have to be biocompatible.
Emulsions are also applicable in other industries such as food, agriculture, paint,
paper coating, lubrications, petroleum extraction, bitumen emulsion and etc [102].
Emulsions can easily entrap both hydrophobic and hydrophilic active ingredients by
reducing the usage of organic solvent where some of them are very toxic and is a
potential carcinogenic compound. Moreover, the homogeneity of emulsions makes it to
be more easily spread thereby enhances the spreadability of the active ingredients at the
application site. This helps in reducing toxicity of the concentrate active ingredients.
1.5 Olive Oil
Olive, which is famous for its valued fruit and oil, has long been cultivated in
the Mediterranean Basin and is one of the important parts in the Mediterranean-style
diet. The Mediterranean-style diet is famous in reducing the risk of heart and other
chronic diseases by lowering the low-density lipoprotein cholesterol level due to the
high content of monounsaturated and polyunsaturated fatty acids in nuts [107, 108].
The olive oil is widely used as cooking oil, salad dressing, fuel for traditional oil lamp,
and more importantly some applications in formulating cosmetics and pharmaceuticals
products such as skin and hair moisturizer. This is due to the ability of olive oil to
Chapter 1 Introduction
________________________________________________________________________ - 26 -
promote smooth and radiant complexion, maintaining the elasticity of the skin, and also
conditioning and adding shine to hair.
Table 1.6: The type of fatty acid present in olive oil and its composition (in the form of
methyl ester).
Composition Percentage (mol/mol) IUPAC Name
Oleic acid 56.0 – 83.0 Octadec-9-enoic acid
Palmitic acid 7.5 – 20.0 Hexadecanoic acid
Linoleic acid 3.5 – 20.0 9,12-octadecadienoic acid
Stearic acid 0.5 – 3.5 Octadecanoic acid
Palmitoleic acid 0.3 – 3.5 Hexadec-9-enoic acid
Linolenic acid 0.0 – 1.5 9,12,15-octadecatrienoic
acid
Myristic acid 0.0 – 0.5 Tetradecanoic acid
Others Minor -
Olive oil is the richest source of the monounsaturated fatty acid, the oleic acid.
According to the Recommended International Standard for Olive Oil, Virgin and Refine
(Table 1.6), olive oil has a high composition of unsaturated fatty acid especially oleic
acid (56% - 83%) [109]. These monounsaturated fatty acids are proven to be beneficial
to human which help to reduce canser risk. According to epidemiological studies, they
had report and show evidences that the monounsaturated fatty acids reduce the cancer
risk especially in breast canser [110-114]. There are also several studies discussing and
proposing method and formula using olive oil to skin canser [115, 116]. Their results
show that the number of tumors in the mice pretreated with olive oil has significantly
Chapter 1 Introduction
________________________________________________________________________ - 27 -
reduced as compared to the untreated mice, which indicated that the olive oil can
effectively protect the skin and reduce the risk of having skin cancer [117, 118].
Beside internal use, olive oil which is a type of essential oil is also suitable for
external application such as carrier oil in aromatherapy treatment for body massage. The
function of carrier oil is to dilute the concentrate essential before applying to the skin.
This is because the potential of the concentrate essential oil such as Lavender and
Jasmine to cause skin irritation if applied to the skin without further dilution.
The olive oil is also a rich source of antioxidants such as flavenoid polyphenols
and vitamins A, D, E, and K. These natural antioxidants are able to provide protection
to the skin against the sun. The flavenoid polyphenols are also effective in healing of
sunburn, lowering the cholesterol level, blood pressure, and the risk of coronary disease
[119, 120]. Owing to the extensive antioxidant and benefits of olive oil to human
especially the skin, olive oil is selected to be the oil phase of the emulsions in this study.
1.6 Properties of Emulsion
The basic condition for the formation of emulsion is by mixing two immiscible
liquid such as oil and water. As mentioned in section 1.4, that system is not stable.
Therefore another component, the surfactant is employed to ‘hold’ the droplets, ensure
that the droplets are well dispersed in the continuous phase and stable against
destabilization process. Upon addition of surfactant, the complexity of emulsion system
increases which involves various types of interaction forces affecting the microstructure
of the emulsion such as droplet size and droplet size distribution. Since there is a strong
relation between microstructure and macroscopic properties of an emulsion, the changes
that happen in the emulsion microstructure will affect the macroscopic properties of the
emulsion such as their rheology. The rheology is related to the study of the flowability
Chapter 1 Introduction
________________________________________________________________________ - 28 -
and elasticity of the emulsion. For examples, there are differences in the viscosity of the
coarse and fine emulsions, by which the fine emulsion will have a higher viscosity and
elasticity as compared to the coarse emulsion.
At the commercialization view point, high viscosity, low oil content and fine
droplet size emulsion is required. This is because the high viscosity will enhance the
emulsion stability; lower oil content will decrease greasy feeling on the skin; while fine
droplet size enhance the delivery of active ingredients into the skin. For these reasons,
additives such as thickeners, co-surfactants and other additives for specific application
are added into the emulsion during formulation. These additives’ are commonly used in
formulating cosmetic creams and food emulsions. The additives such as thickeners are
often used to act as viscosity modifiers in order to modify the viscosity of the
continuous phase to obtain more stable emulsion. Besides, it also enhances the
properties of the emulsion and promotes the pleasant feeling upon applications. For
example, in cosmetic, the viscosity of the creams is related to the spreading of the
creams on the skin; in food, the emulsion should have good mouthfeel and chewability.
The spreading and chewing properties can be monitored by studying the
rheology of the emulsions such as the viscosity. Beside the viscosity, there is another
important rheological property of emulsion which is the viscoelasticity that tells the
elasticity of the emulsion whether it is more solid-like or liquid-like. The study of
viscoelastic properties of an emulsion is also providing useful information about long-
term stability of the emulsions and the stability of the droplets against destabilization
processes such as coalescence and flocculation [121]. Consequently, the study of
rheology of emulsion is essential especially in the industries of cosmetic and
pharmaceutics to evaluate the properties and stability of the products.
Chapter 1 Introduction
________________________________________________________________________ - 29 -
1.7 Rheology
1.7.1 Historical perspective
The word rheology comes from the Greek words which carry the meaning of
flow for “rheo” and “-ology” for study of [122]. The term rheology was first introduced
by Professor Bingham from Lafayette College. Rheology is an independent scientific
discipline, studying the deformability and flow properties of a matter under an applied
stress or strain [123, 124]. The definition of rheology mentioned above was accepted by
the American Society of Rheology in 1929 [124].
For the past 300 years, behaviors of perfect liquids and solids were represented
mathematically by Newton’s law and Hooke’s law respectively, assuming that these
were the universal laws [125, 126]. However, there are a lot of materials in the world
that have an intermediate mechanical behavior in between the perfect solids and liquids
which cannot be described by the classical theories. Scientists began to have doubt
when Wilhelm Weber and James Clerk Maxwell found the non-ideal behavior of silk
threads and fluids respectively. Wilhelm Weber’s experimental results show that the silk
threads are not fully elastic material; Maxwell’s fluids were found to have some elastic
properties which were mathematically proved [125].
According to the response of silk threads and elastic fluid, they were classified
as the typical viscoelastic and non-Newtonian materials. A viscoelastic material will
recover after being subjected to small deformation showing the elasticity of the material,
while a non-Newtonian material has a time and stress dependent viscosity providing
information on the continuous flowability [127]. In order to have better understanding
of the terms viscoelastic and Newtonian or non-Newtonian, one may consider the
behavior of cheese and honey. According to the data collected from rheological
measurements, cheese was showing a viscoelastic response [128, 129], while honey is
Chapter 1 Introduction
________________________________________________________________________ - 30 -
the typical Newtonian fluid which the viscosity is independent of time and applied
stress [130-132].
The typical viscoelastic response of non-Newtonian liquid as a function of
frequency is presented in Figure 1.13. This viscoelastic spectrum (Figure 1.13) is
obtained from the frequency sweep at a wide range of frequency with a fix strain. The
spectrum can be divided into five regions, which are the terminal, transitions, plateau
and glassy regions. Owing to the capability of the instrument used in this study, only
regions II and III were detected for all the emulsions in this study. Region I, which is
the terminal region, is obtained at a very low frequency range and the material will
behave as a liquid showing predomination of G’. As the frequency increases, the
magnitude of G’ increases and become dominant against the G”, which is the plateau
zone. Before this plateau zone, there is a transition zone where the two G’ and G”
curves crossover at t*. This crossover point represented the relaxation time of the tested
sample. This point is important for designing products in industries because it provides
curing information especially in polymer industry producing epoxy resins [133].
Chapter 1 Introduction
________________________________________________________________________ - 31 -
Figure 1.13: The typical viscoelastic spectrum which is the master showing the
viscoelastic respond of non-Newtonian material [133].
These rheological properties discussed above, can be studied by using a
rheometer. In the earlier days, viscometer was the only developed as an instrument to
measure the flow behavior of liquid. Due to the advancement of technology in recent
years, development of advanced rheometer is no longer a dream. The very first
rotational rheometer was produced in the year 1990 [134]. This type of rheometer not
only allows temperature control, but also able to vary the rotational speeds which enable
the scientist and rheologist to investigate the material properties as a function of shear
rate. Soon later, the oscillation rheometer is available with greater function. This
modern rheometer is able to control the stress in the range of ≤ 1 m N/m to more than 1
N/m with a wide range of temperature control from -150oC to 300oC. With the help of
these advanced instruments, more and more information about the rheological behavior
of materials can be obtained.
Chapter 1 Introduction
________________________________________________________________________ - 32 -
1.7.2 Application of Rheology
Owing to the fact that rheology can give a better picture of the behavior of a
material, therefore it is widely used as a tool to test the texture and flow behavior of
industrial products especially in the processing industries such as foods [135-138],
cosmetic [139-143], pharmaceutical [144-146], polymer [147-149], coating [150, 151],
and oil processing [152, 153]. The rheological results can help the scientists and the
manufacturers to have better understanding of the products. The rheological results also
enable scientists to estimate the products’ quality such as elasticity, viscosity,
deformability, storage, shelf life and intermolecular interactions, due to the ultra-
sensitivity toward microstructure of a material.
Also, in commercial emulsions, factors such as appearance and flow properties
of the final products are very important. Consumers are often very sensitive to the
texture, the mouth feel, and flavor perception of the products. In order to influence the
consumers’ buying preference, the rheological properties such as the stickiness, the
viscosity, and the elasticity of the products have to be controlled with care. Sometimes,
it is necessary to modify the properties of their products in order to improve product
quality. In this case, rheology is a good tool to perfectly accomplish the task. For
example, the residue emptying behavior of products from its container and this is very
important for the products that packed in pump disperser bottle. This type of products
such as body shampoo, hair shampoo and body lotion has to be able to flow to the
bottom of the container to guarantee the optimal of emptying. When the residues
emptying are low, that means the sagging viscosity of the product is high and has to be
modified. By referring to the shear rate range shown in Table 1.7, the viscosity of
specific situation such as sagging, levelling and rubbing can be identified and enable the
Chapter 1 Introduction
________________________________________________________________________ - 33 -
researchers or formulators to modify the flow behavior in order to maximize the
products performance.
Table 1.7: The typical shear rate range for various physical operations with examples
[154, 155].
Situation Shear Rate Range (s-1) Examples
Sedimentation of fine
powders in liquids 10-6-10-3
Medicines, paints, salad
dressing
Leveling 10-2-10-1 Paints, printing ink
Draining of under gravity 10-1-101 Toilet bleaches, paints,
coatings
Extruders 100-102 Foods
Dip coating 101-102 Paints, confectionery
Mixing and stirring 101-103 Liquid manufacturing
Pipe flow 100-103 Pumping liquids, blood
flow
Brushing 103-104 Painting
Rubbing 104-105 Skin creams, lotions
High-speed coating 104-106 Paper manufacture
Spraying 105-106 Atomisation, spray drying
Lubrication 103-107 Bearings, engines
Besides food and cosmetic creams, rheology is also applicable to study of
natural phenomena such as deformation of natural rocks [156-159], wastewater
treatment industry [160, 161], and biological problems [162, 163]. Overall, rheology is
a useful and informative discipline in science which had provided a lot of information
for the food, cosmetic and pharmaceutical industries as well as the natural phenomena
studies.
Chapter 1 Introduction
________________________________________________________________________ - 34 -
1.8 Objective
1. To prepare olive oil cream emulsions stabilized by sucrose fatty acid esters.
2. To study the stability of the emulsions through physically phases separation and
rheological properties.
3. To determine the effect of oil–water ratio, surfactant concentration and emulsion
age on the stability and rheological properties of the emulsion.
CHAPTER 2
MATERIALS AND METHODS
Chapter 2 Materials and Methods
________________________________________________________________________ - 35 -
2.0 Materials and Methods
2.1 Materials
Deionized water with resistivity of 18.2 Ωcm-1, supplied from a Barnstead
Diamond Nanopure Water Purification unit couple with a Barnstead DiamondTM RO
unit from Barnstead International, Iowa USA was used in all emulsion preparation. A
commercial grade extra virgin olive oil from brand Laleli with maximum acidity 0.8%,
density 0.9091 g/ml, and viscosity 9.562 m Pas at 30oC was used as received. Nonionic
surfactant sucrose fatty acid ester (SFAE) was obtained from TCI, Tokyo Kasei Co.
LTD, Japan. GC/MS analysis of SFAE was found to be containing sucrose myristate
0.29%, sucrose palmitate 83.99%, and sucrose stearate 15.72%. Polyethylene glycol
dodecyl ether (Brij35P) was purchased from Fluka.
2.2 Emulsion preparation
Preparation of aqueous phase was performed by dissolving 2 – 10 wt% of SFAE
and 0.5 wt% Brij35P (acting as a solubilizer due to low solubility of SFAE in water)
into different volumes of deionized water (20%, 30%, 40% and 50% from total volume)
followed by sonication for 5 minutes and then heated with stirring at 60oC until clear gel
like appearance was observed. Preparation of oil phase was performed by homogenizing
2 – 10 wt% of SFAE into different volumes of olive oil (50%, 60%, 70% and 80% from
total volume) with an IKA Labortechnik homogenizer model T25 from IKA, Germany
and followed by treatment at 60oC for 5 minutes. The emulsions were prepared by
pouring the oil phase into the aqueous phase and homogenized with the homogenizer at
fix 13,000 rpm for 5 minutes. As the speed of homogenizer and the homogenation time
Chapter 2 Materials and Methods
________________________________________________________________________ - 36 -
are factors affecting the droplet size of the emulsions, therefore these parameters were
fixed in order to obtain a more comparable and reliable results. The emulsions were
then kept under accelerating condition at 45oC for further analysis. Emulsion stability
test was performed by observing emulsion phase separation over 30 days.
Dispersion test was used to identify the type of emulsion. The dispersion test
was carried out by placing a drop of emulsion on water and oil. The emulsion tends to
disperse in water (Figure 2.1 (a)), but no dispersion was observed in oil (Figure 2.1
(b)). Thus, the prepared emulsion is an o/w emulsion. Beside, the micrographs of these
emulsions were also a good evidence to prove that these are o/w type of emulsions.
(a) (b)
Figure 2.1: The emulsion dispersed in (a) water very well, but resists the dispersion in
(b) olive oil indicating that the emulsion is an o/w emulsion.
2.3 Zeta potential measurement
Zeta potential of the emulsion droplets was measured by using Malvern
ZetaSizer Nano Series (Malvern Instruments, UK). The samples were prepared by
diluting approximately 0.02 g of emulsion with 5 ml of 0.01 M of sodium chloride
solution. Finally, the solutions were conditioned for 20 minutes at 30oC before carrying
Chapter 2 Materials and Methods
________________________________________________________________________ - 37 -
out the measurements. The Zeta potential of the emulsions was measured on the 1st day,
7th days and 30th days after the emulsions were prepared. The results were obtained by
applying the Henry equation,
2 ( )3Ef kaU εζη
= (5)
where EU is the electrophoretic mobility, ε is the dielectric constant, ζ is the zeta
potential, η is the viscosity of the continuous phase, and ( )f ka is the Henry’s function
[164]. In this study, the calculation of Zeta potential is done by using the Smoluchowski
approximation where the ( )f ka is 1.5. That is due to the properties of the emulsion
droplets which were dispersed in electrolytes containing more that 10-3 molar of salt and
droplet size greater than 0.2 μm. According to equation (5), the Zeta potential of the
emulsions was calculated from the electrophoretic mobility of the droplets. Under an
electrical field, the droplets tend to move towards the electrode with opposite charge.
Their velocity is then measured by a technique call Laser Doppler Velocimetry and is
expressed in unit of field strength as their mobility.
2.4 Morphology and droplet size analysis
The integrity, aggregation and the droplet size distribution of the emulsion were
observed and determined by using a light polarizing microscope (Leica model PM RXP ,
New York) equipped with a JVC Color Video camera (model KY F550) and Leica
QWin image analysis software. The measurements of droplets size was performed
without any further dilution of the emulsions. Approximately 3000 droplets were
Chapter 2 Materials and Methods
________________________________________________________________________ - 38 -
randomly chosen from 10 micrographs captured from 10 different drops of the
emulsions. The droplet size was reported as the volume-weighted mean droplet size (d4,3)
as shown in the following equation, where in is the number of droplets with diameter
id [165].
4
4,33
i i
i i
n dd
n d=∑
∑ (6)
2.5 Rheological measurement
A stress/rate controlled Bohlin CVO-R Rheometer (Malvern Instrument UK)
with a temperature controller, was employed to measure the rheological properties of
the emulsion. The measurements were performed under the temperature of 30.0 ± 0.1oC
(-40 to 180oC, Peltier Plate system from Bohlin Instrument Ltd.) with 4o/40 mm cone
and plate geometry and gap of 0.150 mm. In order to obtain the proper parameter and
reliable data from the frequency sweep, amplitude sweep was first performed at a
controlled strain mode with applied strain in the range of 0.0001 to 1 unit and at fixed
frequency of 1 Hz; followed by the frequency sweep, which was performed at a
controlled strain mode (a very low deformation strain was chosen from the linear
viscoelastic profile from amplitude sweep) with frequency varying from 0.001 to 10 Hz.
All the measurements were performed on day 1, 7, and 30.
n
Kσ γ= & (7)
Chapter 2 Materials and Methods
________________________________________________________________________ - 39 -
The steady rheological behavior of the emulsions was measured at a controlled
rate mode varying from 0.0001 to 50 s-1. The samples were allowed relaxing for 10
minutes after loaded to the plate before the measurement started. All the shear data
obtained from the increasing shear rate measurement were fitted to the Power- Law (Eq.
7) and Hershel-Bulkley model (Eq. 8) using the Bohlin rheometer software (Gemini
150). The σ and γ& are the shear stress and shear rate respectively; K is the consistency
index and n is the flow behavior index, so called the Power-Law index describing the
non-Newtonian behavior of liquids . For a shear thinning fluid, 0 < n < 1; for a shear
thickening fluid, n > 1, and a Newtonian fluid give n = 0.
no Kσ σ γ= + & (8)
γ& is the shear rate, the oσ is the yield stress; K is the dimensionless constant, n is the
power index measuring the degree of thinning (n < 1 for shear thinning behavior).
CHAPTER 3
RESULTS AND DISCUSSION
Chapter 3 Results and Discussion
________________________________________________________________________ - 40 -
3.0 Results and Discussion
3.1 Phase Separation
The stability of emulsion was first determined by observing the physical phase
separation which greater phase separation indicates lower emulsion stability with
shorter shelf life. According to Figure 3.1, the emulsions phase separation was
dependent on the oil-water ratio, surfactant concentration and storage time. As the
surfactant concentration and oil-water ratio increased, the emulsions phase separation
decreased which indicated the increase of emulsion stability. This happens to all
emulsions except the emulsion with 80% oil. These emulsions with 80% oil were the
most stable emulsions that showing no phase separation even after 30 days of storage
under 45oC. This result shows that the emulsions with 80% oil have a long shelf life.
Fig. 3.1(a)
1 2 3 4 5 6 7 8 9 10 110.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Surfactant concentration (wt%)
Em
ulsi
on V
olum
e Fr
actio
n (%
)
50% Oil 60% Oil 70% Oil 80% Oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 41 -
Fig. 3.1(b)
1 2 3 4 5 6 7 8 9 10 110.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Surfactant concentration (wt%)
Em
ulsi
on V
olum
e Fr
actio
n (%
)
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.1(c)
1 2 3 4 5 6 7 8 9 10 110.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Em
ulsi
on V
olum
e Fr
actio
n (%
)
Surfactant concentration (wt%)
50% Oil 60% Oil 70% Oil 80% Oil
Figure 3.1: The emulsion volume fractions for emulsions at (a) day 1, (b) day 7, and (c)
day 30.
Chapter 3 Results and Discussion
________________________________________________________________________ - 42 -
The emulsions which were prepared with 50%, 60%, and 70% oil showed a
significant phase separation. Among these emulsions, emulsions with 50% oil were the
most unstable. According to the experimental result, emulsions with 50% oil which
were stabilized by 2- 4 wt% of surfactant show immediate phase separation (Figure
3.1(a)). After 30 days of storage under 45oC, all emulsions with 50% oil show phase
separation even the one that was stabilized with 10wt% of SFAE (Figure 3.1(c)). This
means that these emulsions were having a short shelf life.
The major factor causing the emulsion phase separation is the creaming process
which is a type of gravitational destabilization process (Figure 1.10(d)). In the case of
oil-in-water emulsion, the oil tends to move upward under gravity due to the difference
in density between the continuous phase and the disperse phase [166]. The droplets are
accumulated at the top of the emulsion due to the creaming effect, resulting in oiling off
(formation of an oil layer on top of the emulsion) after some time [167]. The creaming
rate of emulsion is highly dependent on the density of the disperse and continuous phase,
viscosity of the continuous phase, droplet size and the droplet concentration [168].
These will be discussed in the following sections.
Chapter 3 Results and Discussion
________________________________________________________________________ - 43 -
3.2 The Effect of Surfactant, Oil Concentration and Storage Time on
the Droplet Size.
The droplet size of an emulsion is an important parameter which influences the
colloidal stability and rheological properties such as the flow and deformations of the
emulsion. Emulsion with smaller droplet size enhances the skin penetration of drugs and
active ingredients through skin [169, 170]. The droplet size of the emulsions decreased
when the oil and surfactant concentration increased (Figures 3.2 and 3.3). In this study,
results (Figures 3.1 and 3.2) show that the smaller the droplet size, the greater the
emulsion stability and this agreed with many other research works on emulsion with
different composition and type of oil [168, 171]. The droplet size of emulsions with
50% and 60% oil decreased dramatically (about 60%) with increase from 2 to 10 wt%
of surfactant concentration. On the other hand, there was only a slight decrease in
droplet size of emulsions with 70% and 80% oil, showing a trend that no significant
decrease in the droplet size as the surfactant concentration further increased from 7 to
10 wt%. Besides, the aging of emulsion also affected the droplet size very much
especially in more dilute emulsions (50% and 60% oil). The experimental result shows
that there was a significant increase in the droplet size after 30 days of storage (Figures
3.2(a) and (b)). However, the aging effect was not considerable observed in the
concentrated emulsions of 70% and 80% oil content. The decreased of droplet size of
concentrated emulsions was insignificant as compared to the dilute emulsions (Figure
3.2).
Chapter 3 Results and Discussion
________________________________________________________________________ - 44 -
Fig. 3.2(a)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Surfactant Concentration (wt%)
Mea
n D
ropl
et S
ize,
d (m
m)
Day 1 Day 7 Day 30
Fig. 3.2(b)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Mea
n D
ropl
et S
ize,
d (m
m)
Surfactant Concentration (wt%)
Day 1 Day 7 Day 30
Chapter 3 Results and Discussion
________________________________________________________________________ - 45 -
Fig. 3.2(c)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Surfactant Concentration (wt%)
Mea
n D
ropl
et S
ize,
d (m
m)
Day 1 Day 7 Day 30
Fig. 3.2(d)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Mea
n D
ropl
et S
ize,
d (m
m)
Surfactant Concentration (wt%)
Day 1 Day 7 Day 30
Figure 3.2: The mean droplet size for emulsions with (a) 50%, (b) 60%, (c) 70% and (d)
80% of disperse phase.
Chapter 3 Results and Discussion
________________________________________________________________________ - 46 -
3.2.1 Effect of Surfactant Concentration to the Droplet Size
There were several methods used to control the droplet size of the emulsion. One
is introducing the surfactant into the system to reduce the interfacial tension, so that the
homogenizer can easily disrupt the droplets under low interfacial tension leading to the
formation of smaller droplet [172]. Another is to vary the speed of homogenizer and the
duration of emulsification during sample preparation through secondary
homogenization [173]. However, the second technique is dependent on the
homogenizer’s capability and is not an economic method. Besides that, the age of
emulsion is also affecting the emulsion droplet size, due to the thermodynamic
instability of the emulsions as presented in Figure 3.2.
Before proceeding further to the discussion on the effect of the surfactant
concentration on the emulsions properties, it is necessary to discuss about the reason of
choosing 2 wt% of SFAE surfactant to be the minimum amount of surfactant to stabilize
the emulsions. This is to make sure that there was enough supply of surfactant
molecules to the oil and water interface. That is also a basic condition to attain in the
formation of emulsion, because the formulation will fail if insufficient amount of
surfactant is used. According to the experimental results (Figure 3.1), a full emulsion
was obtained with this minimum surfactant concentration and was stable for 30 days
under 45oC of storage condition. This indicates that the minimum amount of surfactant
concentration used in this study is sufficient and able to produce a stable emulsion.
The decrease of emulsion stability with respect to the decrease of droplet size
when increased of surfactant concentration is expected and can be explained with
creaming destabilization process. According to the ideal Stokes’ Law for the creaming
rate of emulsion, the gravitational instability rate of emulsion is directly proportional to
the square root of the droplet radius (Eq. 9) [172]. For example, when the droplet size
Chapter 3 Results and Discussion
________________________________________________________________________ - 47 -
increased from 25 μm (8 wt% of surfactant in emulsion with 60% oil) to 50 μm (3 wt%
of surfactant in emulsion with 60% oil), the creaming rate for the emulsion with droplet
size of 50 μm is four times greater than the smaller one. Thus, the emulsion with droplet
size of 50 μm (3 wt% of surfactant, 60% oil) is much more unstable compared to 25 μm
droplet (Figure 3.1). Equations 10 and 11 predict the minimum droplet size and the
surfactant concentration used in order to produce an emulsion with longer shelf life
[172].
2
2 1
1
2 ( )9Stokes
grv ρ ρη
−= − (9)
Stokesv is the Stokes’ creaming velocity, g is the acceleration due to gravity, r is the
droplet radius, 1ρ and 2ρ are the density of the continuous phase and disperse phase
respectively, 1η is the viscosity of the continuous phase. The sign for the v indicating
the type of destabilization processes, creaming (+) or sedimentation (-).
min3 sat
s
rc
φΓ= (10)
minr (m) is the minimum size of stable droplet that can be produced, satΓ (kg m-2) is the
excess surface concentration of the surfactant at saturation at the oil-water interface, φ
is the disperse phase volume fraction (Eq. 3), sc (kg m-3) is the concentration of
surfactant in the emulsion.
Chapter 3 Results and Discussion
________________________________________________________________________ - 48 -
D
E
VV
φ = (11)
DV is the volume of emulsion droplets and EV is the total volume of the emulsion.
Fig. 3.3(a)
Chapter 3 Results and Discussion
________________________________________________________________________ - 49 -
Fig. 3.3(b)
Fig. 3.3(c)
Figure 3.3: Micrographs of emulsion in 80% oil with (a) 2 wt%, (b) 5 wt% and (c) 10
wt% of SFAE surfactants showing the decrease of droplet size with increase in
surfactant concentration.
Chapter 3 Results and Discussion
________________________________________________________________________ - 50 -
When the surfactant concentration increased, the amount of surfactant adsorb at
the droplet surface increased. The droplets were well protected due to close arrangement
of surfactant layer at the droplet surface. When droplets approach at proximately close
enough to each other, repulsive force dominated and the droplets repelled each other
keeping them apart. At low surfactant concentration, the energy barrier created by the
surfactant layer at droplet surface is relatively low due to the dynamic property of the
interfacial layer. The hydrophobic interaction between the droplets came into
consideration due to the increased exposure of the nonpolar surface (oil) with the polar
region (water). The interaction between the uncovered droplet surface areas with the
aqueous continuous phase is thermodynamically unfavorable. Therefore, the system will
try to minimize these unfavorable energy by minimizing the contact area [174, 175].
Chapter 3 Results and Discussion
________________________________________________________________________ - 51 -
Fig. 3.4(a)
Fig. 3.4(b)
Chapter 3 Results and Discussion
________________________________________________________________________ - 52 -
Fig. 3.4(c)
Figure 3.4: The micrographs of dilute emulsion (50% oil) with (a) 2 wt%, (b) 5 wt%,
and (c) 10 wt% of surfactant concentration.
3.2.2 Aging effect
The changes of the droplet size as a function of time is obviously a kinetic effect,
which can be affected by the dynamics, colloidal interactions and interfacial properties
of the emulsion [176, 177]. Kinetic effect is used to elaborate the long term stability of
the emulsions. Generally, the emulsions with smaller droplet size have longer shelf life,
but are the more thermodynamically unstable due to the large interfacial area (Eq. 12).
formation configG A T SγΔ = Δ − Δ (12)
Chapter 3 Results and Discussion
________________________________________________________________________ - 53 -
formationGΔ is the Gibbs free energy of the system, γ is the interfacial tension, AΔ is the
difference in the total interfacial area before and after emulsification, T is the absolute
temperature and configSΔ is the configurational entropy of the droplets in the system.
As previously mentioned in Chapter 1, the kinetic stability of emulsion depends
on its activation energy, which is attributed to the adsorbed surfactant layer (Figure
1.12). According to McClements, D.J. [176], the emulsions have to overcome this
activation energy before they reach the thermodynamic stable state. For a long term
stability emulsion, the activation energy is generally 20 times greater than the thermal
energy of the system (> 20kT) [178]. However, due to the dynamic property of the
surfactant layer, the stability of droplets is changing from time to time.
Fig. 3.5(a)
Chapter 3 Results and Discussion
________________________________________________________________________ - 54 -
Fig. 3.5(b)
Fig. 3.5(c)
Figure 3.5: The micrographs of emulsions with 50% disperse phase captured at (a) day
1, (b) day 7 and (c) day 30, showing the coalescence destabilization.
Chapter 3 Results and Discussion
________________________________________________________________________ - 55 -
For dilute emulsions (< 70% oil), the droplets were in continual motion, because
of the low viscosity of the emulsions and droplets were not closely packed (Figure 3.5).
The droplets collided with each other when they move around under Brownian motion
and gravity [84]. The motion may lead to coalescence destabilization after collisions
when there was insufficient surfactant to perfectly cover the droplets’ surface at low
surfactant concentration (< 4 wt%) [179, 180]. As droplets moving toward each other,
the surface-to-surface distance between the droplets was decreased. At closer distance,
there is an energy barrier created by the adsorbed surfactant, which is the repulsive
force is dominating. As previously discussed, the droplets has to overcome the energy
barrier (the activation energy) before they coalescence and fall into the deep minimum
(w(hmin)) (Figures 1.11 and 1.12) [176]. Due to desorption of surfactant resulting from
the dynamic behavior of the interfacial layer, the activation energy decreased [179] and
creating a greater contact area between the droplets. As a result, the droplets can easily
overcome the energy barrier and merge to form a larger droplet [81].
3.2.3 Effect of oil concentration
In this study, the mean droplet size is found to be decreasing when the oil
concentration increased from 50% to 80% (Figures 3.1 and 3.4) [181]. The droplets
were also becoming close packed when the oil concentration increased from 50% to
80% (Figures 3.6 and 3.7) due to the increase of droplet concentration. However, no
flocs were observed even though they were all closely packed. The most concentrated
emulsion (80% oil) was the most stable emulsion showing high droplet concentration
compared to others (Figure 3.7). The high droplet concentration enhances the stability
of the emulsion, because their movements were blocked by each other [167]. Thus, this
slows down the creaming rate, followed by the destabilization of emulsion.
Chapter 3 Results and Discussion
________________________________________________________________________ - 56 -
From the droplet size analysis results, the less concentrated emulsion (lower oil
concentration) has greater droplet size (Figures 3.4 and 3.5). Since the minimum
amount of surfactant used in this study was sufficient to produce a stable emulsion with
80% oil, in general, the droplet size of the emulsions with 50% oil at fix surfactant
concentration should be smaller than those with 80%. But experimental results show the
other way round. This may due to the effect of the emulsion viscosity.
Fig. 3.6(a)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Surfactant Concentration (wt%)
Mea
n D
ropl
et S
ize,
d (μ
m)
50% oil 60% oil 70% oil 80% oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 57 -
Fig. 3.6(b)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Surfactant Concentration (wt%)
Mea
n D
ropl
et S
ize,
d (μ
m)
50% oil 60% oil 70% oil 80% oil
Fig. 3.6(c)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
50
60
70
80
90
Surfactant Concentration (wt%)
Mea
n D
ropl
et S
ize,
d (μ
m)
50% oil 60% oil 70% oil 80% oil
Figure 3.6: The effect of oil concentration to the droplet size of emulsions over 30 days
of storage under accelerating condition. (a) First day, (b) 7th day, and (c) 30th day.
Chapter 3 Results and Discussion
________________________________________________________________________ - 58 -
In this study, the emulsions were prepared by varying the water to oil ratio
(WOR). When the emulsion was said to be low oil concentration, it is to understand that
the water or aqueous part of that emulsion increased. Certainly, when the water portion
increased, the viscosity of the emulsions will decrease as the emulsions were diluted. In
other words, the viscosity of the continuous phase ( 1η ) is decreased. In addition, the
droplet concentration was relatively low in the emulsion with low oil concentration;
they were not in a close pack condition. That indicates the mobility of the droplets
increases which enhances the collision frequency (FG) [81]. As the droplets were able to
move freely in the emulsion induced by the gravitational force, the impact of collision
between the droplets is high and may result in the disruption of the interfacial layer,
consequently the droplets merge to form a larger droplet [84]. The formation of larger
droplet enhanced the creaming rate of the droplets, thus increased the phase separation
and decreasing the emulsion stability. The above explanations were supported by
equation 13 which shows the relationship between the FG and 1η . The equation 13
shows very obvious that the FG increased with the decrease of 1η . Beside 1η , a
polydisperse system also contributes to the FG which due to the fact that larger droplet
will move to the top more quickly than the smaller one under gravity.
2 2 21 2 2 1 1 2
3 31 1 2
( )( )[ ]8G
g r r r rFr r
ρφ φπη
Δ − += (13)
g is the gravitational force, Δρ is the difference in density between the droplets and the
surrounding liquid, iφ is the disperse volume fraction with droplet radius ir .
On the other hand, as for higher oil concentration emulsions (≥ 70% oil), the
droplets were closely packed (Figure 3.7). The collision efficiency is also lower than
Chapter 3 Results and Discussion
________________________________________________________________________ - 59 -
that in the dilute emulsions due to limitation of motion [81, 84]. In such close distance,
repulsive forces between the droplets are dominant and the droplets keep repelling each
other. This helps the droplets keeping a distance with each other. Although there was
some increase of the mean droplet size for these concentrated emulsions (Figure 3.2),
but the increase was negligible. Therefore, a conclusion can be made that the
concentrated emulsions were stable against the growing of droplet size in the studied
period.
As discussed above, the increase of oil concentration enhances the emulsion
stability. Consequently, the emulsion stability can be controlled by altering the droplet
size and concentration. However, this method is not favorable in formulation of
industrial products, because the increase of the oil concentration may decrease flavor
perceptions, texture of the products, and consumer acceptability [182-185].
Chapter 3 Results and Discussion
________________________________________________________________________ - 60 -
Fig. 3.7(a)
Fig. 3.7(b)
Chapter 3 Results and Discussion
________________________________________________________________________ - 61 -
Fig. 3.7(c)
Fig. 3.7(d)
Figure 3.7: The micrographs for emulsions with (a) 50%, (b) 60%, (c) 70%, and (d)
80% of oil and stabilized with 2 wt% of surfactant. These micrographs were obtained
after one day of storage under 45oC.
Chapter 3 Results and Discussion
________________________________________________________________________ - 62 -
3.3 Zeta potential
The Zeta potential is a common technique being used to study the emulsions
stability. The magnitude of the Zeta potential carries information on emulsion stability
and tendency for droplet flocculation. According to the literatures’ [164, 186], rule of
thumb that is usually being practiced the droplet flocculation is when the Zeta potential
is less than the absolute value of ± 25 mV and the greater the magnitude implies the
higher the stability of the emulsion system. In our case, experimental results of the Zeta
potential for the emulsions over 30 days of storage in 45oC oven condition were
presented in Figure 3.8.
In the presence of 0.01 M of NaCl electrolyte, the Zeta potential for the
emulsions with 50% and 60% of oil were found to be in the range of -12 mV to -24 mV
and -12 mV to -18 mV for emulsions with 70% and 80% oil. The results indicated that
the droplet stability in the emulsions with 70% and 80% oil was lower than the
emulsions with 50% and 60% oil which contradicts with the other stability test results
that have shown greater emulsion stability with increase in the oil concentration (Figure
3.1). However, the Zeta potential of all emulsions increased in magnitude as the
surfactant concentration was increased indicating increase of emulsion stability. The
Zeta potential of all emulsions was also found to decrease with the emulsion storage
time. These results agreed with other stability tests performed on the emulsions.
Chapter 3 Results and Discussion
________________________________________________________________________ - 63 -
Fig. 3.8(a)
1 2 3 4 5 6 7 8 9 10 11
-24
-22
-20
-18
-16
-14
-12
-10
Surfactant Concentration (wt%)
Zeta
Pot
entia
l, ζ
(mV
)
1st day 7th days 30th days
Fig. 3.8(b)
1 2 3 4 5 6 7 8 9 10 11
-24
-22
-20
-18
-16
-14
-12
-10
Surfactant Concentration (wt%)
Zet
a Po
tent
ial,
ζ (m
V)
1st day 7th days 30th days
Chapter 3 Results and Discussion
________________________________________________________________________ - 64 -
Fig. 3.8(c)
1 2 3 4 5 6 7 8 9 10 11
-24
-22
-20
-18
-16
-14
-12
-10
Zet
a Po
tent
ial,
ζ (m
V)
Surfactant Concentration (wt%)
1st day 7th days 30th days
Fig. 3.8(d)
1 2 3 4 5 6 7 8 9 10 11
-24
-22
-20
-18
-16
-14
-12
-10
Surfactant Concentration (wt%)
Zet
a P
oten
tial,
ζ (m
V)
1st day 7th days 30th days
Figure 3.8: The Zeta potential of emulsions measured in the presence of 0.01 M NaCl
solutions for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil concentration.
The surface charge of all emulsions in this study was negative although the
emulsions were stabilized with nonionic surfactant, exhibiting similarity with the naked
oil droplet as reported by a number of researchers [187-190]. Several suggestions have
been given to explain this phenomenon; first is the spontaneous adsorption of hydroxyl
Chapter 3 Results and Discussion
________________________________________________________________________ - 65 -
ion from the aqueous phase to the surfactant head group through hydrogen bonding or
desorption of the hydrogen ions from the droplet surface to the aqueous phase to form
hydronium ion [188, 191, 192]. Nevertheless, the concentration of these ions (hydroxyl
and hydronium ions) is relatively low (~ 10-7 M), therefore one may consider the role of
the water dipoles [193]. This has the influence on the negative surface charge on the
droplet. Chibowski and Waksmundzki [194] had also proposed that the water dipoles
may have some contribution to the electrical double layer structure [191, 194]. However,
there was strong experimental evidence to prove the hypothesis of hydroxyl ion
adsorption as the origin of the droplet potential [188-191]. Thus, this hypothesis
plausibly is the best explanation on the negative Zeta potential obtained in this study.
The decrease of Zeta potential when oil concentration increased may be due to
the decrease of the droplet mobility. As has previously been discussed, the increase of
oil concentration is accompanied with an increase of droplet concentration. Thus, once
the droplet concentration increases, the available space will become crowded. The
separation distance between droplets will also decrease, thus limit the mobility of the
droplets. Besides that, in such crowded environment, there was also an increase of the
possibility for the electrical double layers between the adjacent droplets to overlap. As
the overlapping occur, the osmotic pressure which arises from the difference between
the ionic concentration in the overlapping region and the bulk phase will act and force
the droplets apart from each other [195]. That may cause a delay in the mobility of the
droplets which leads to a decrease in Zeta potential.
Chapter 3 Results and Discussion
________________________________________________________________________ - 66 -
3.4 Rheological Properties of Emulsion System
3.4.1 Steady State Behavior
Steady state behavior on emulsions is relatively important in order to monitor
the texture of the formulation, and also the consumer acceptability of the final products.
Besides that, the flow properties of emulsions also provide information on product
processing, handling, storage and mechanical behavior [196]. However, the viscosity is
always the favored parameter for the chemists especially those with cosmetic, foods,
and pharmaceutical industries to evaluate the emulsion stability [197]. Since the
viscosity of the emulsion arises from the friction between droplets, the interdroplet
forces are now an important factor influencing the viscosity of the emulsions. Therefore,
it is reasonable to say that the factors that contributed to the interdroplet interactions
such as the mean droplet size, polydispersity of droplets, and the presence of additives
such as thickener [198] and polysaccharides [199, 200], the surfactant concentration ,
oil concentration and the age of the emulsions can influence the viscosity of emulsion.
3.4.1.1 Effect of emulsion compositions on the flow properties
The shear viscosity versus shear rate profile (flow curve) for the emulsions is
presented in Figure 3.9. The flow curves show four clear different regions, at very low
shear rate (< 0.005 s-1), the viscosity increased with the shear rate showing shear
thickening behavior. The second region starts to show independence of viscosity to the
shear rate, which is the characteristic Newtonian plateau. However, the plateau is short
and the range is getting shorter when the emulsion became concentrated (Figure 3.9)
[201]. The noticed short plateau is the typical flow behavior of emulsions without
Chapter 3 Results and Discussion
________________________________________________________________________ - 67 -
addition of any viscosity modifier [201]. A zero shear viscosity (ηo) of the emulsions
was determined by extrapolating the Newtonian plateau [202]. The third region, which
is the shear thinning region, starts after exceeding the yield value. In this region, the
viscosity decreased smoothly (up ≈ 0.1 s-1). Finally, the viscosity decreased dramatically
and continuously with high slope. The dependence of the viscosity to the shear rate as
discussed above indicated that the emulsions were typical non-Newtonian in behavior.
Fig. 3.9(a)
1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01
0.1
1
10
100
1000
10000
2 wt% 3 wt% 4 wt% 5 wt% 6 wt% 7 wt% 8 wt% 9 wt% 10 wt%
Vis
cosi
ty, η
(Pas
)
Shear Rate (s-1)
Chapter 3 Results and Discussion
________________________________________________________________________ - 68 -
Fig. 3.9(b)
1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01
0.1
1
10
100
1000
10000
Shear Rate (s-1)
Vis
cosi
ty, η
(Pas
)
2 wt% 3 wt% 4 wt% 5 wt% 6 wt% 7 wt% 8 wt% 9 wt% 10 wt%
Fig. 3.9(c)
1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01
0.1
1
10
100
1000
10000
Shear Rate (s-1)
Vis
cosi
ty, η
(Pas
)
2 wt% 3 wt% 4 wt% 5 wt% 6 wt% 7 wt% 8 wt% 9 wt% 10 wt%
Chapter 3 Results and Discussion
________________________________________________________________________ - 69 -
Fig. 3.9(d)
1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01
0.1
1
10
100
1000
10000
Shear Rate (s-1)
Vis
cosi
ty, η
(Pas
)
2 wt% 3 wt% 4 wt% 5 wt% 6 wt% 7 wt% 8 wt% 9 wt% 10 wt%
Figure 3.9: The shear rate dependence viscosity of emulsions with a series of surfactant
concentration for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil.
Chapter 3 Results and Discussion
________________________________________________________________________ - 70 -
Fig. 3.10(a)
1 2 3 4 5 6 7 8 9 10 110
2000
4000
6000
8000
10000
12000
14000
Zero
She
ar V
isco
sity
, η(P
as)
Surfactant concentration (wt%)
50% oil 60% oil 70% oil 80% oil
Fig. 3.10(b)
1 2 3 4 5 6 7 8 9 10 110
2000
4000
6000
8000
10000
12000
14000
Zero
She
ar V
isco
sity
, η (P
as)
Surfactant Concentration (wt%)
50% oil 60% oil 70% oil 80% oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 71 -
Fig. 3.10(c)
1 2 3 4 5 6 7 8 9 10 110
2000
4000
6000
8000
10000
12000
14000
Surfactant Concentration (wt%)
Zer
o Sh
ear V
isco
sity
, η (P
as)
50% oil 60% oil 70% oil 80% oil
Figure 3.10: The effect of oil concentration to the zero shear viscosity of emulsion after
(a) 1 day, (b) 7 days, and (c) 30 days of storage under an accelerated condition of 45oC.
The zero shear viscosity and yield stress of the emulsions increased with
surfactant and oil concentration as shown in Figures 3.10 and 3.11, indicating
structural integrity arising from the strong colloidal interaction between the droplets.
The yield stress is the stress that has to be overcome before the emulsion starts to flow
[203]. As discussed before, the system with higher surfactant concentration tends to
form a denser interfacial layer which is incompressible [204, 205]. Hence, the droplets
in such sterically stabilized system is usually characterized as “hard sphere” [123]. The
strength of interaction forces (mainly the attractive and repulsive interactions) between
the droplets for the hard sphere system (high surfactant concentration emulsion) was
relatively greater than the one with lower surfactant concentration. In the absence of the
strong sterically repulsive effect, the droplets in the emulsions with lower surfactant
concentration were able to pack more efficiently even at low shear. Therefore, the
droplets were easily aligning themselves with the shear field to initiate flow.
Chapter 3 Results and Discussion
________________________________________________________________________ - 72 -
Oil concentration in an emulsion is another factor affecting the flow behavior of
the emulsion [168, 206]. When the oil concentration increases, the droplet size is getting
smaller and resulting in higher droplet concentration, lower polydispersity and smaller
mean separation distance between the droplets when compared to the emulsions with
lower oil concentration (Figures 3.6 and 3.7) [168]. Since the flow property of fluid
arises from friction between the liquid layers as they slip pass each other [207], when
the droplets are at close distance, the hydrodynamic repulsive force becomes dominant
leading to an increase in the friction between the droplets as external shear is applied
[74, 208].
In addition, the attractive force which is also one of the colloidal interactions
plays an important role to the increase in viscosity and yield stress. The magnitude of
viscosity and yield stress depend on the strength of the attractive force between the
droplets [209]. The decrease of droplet size resulting from the increase in the disperse
phase volume fraction leads to the increase in the total droplet surface area. When the
total surface area of the droplet increase, the strength of the attractive force will also
increase. Thus, greater stress is required to initiate flow when high attractive force is
holding the droplets resulting in high viscosity and yield stress (Figure 3.12) [209].
Chapter 3 Results and Discussion
________________________________________________________________________ - 73 -
Fig. 3.11(a)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
Yiel
d St
ress
, σY (P
a)
Surfactant Concentration (wt%)
0
15
30
45
60
75
90
Mea
n D
ropl
et S
ize,
d (n
m)
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.11(b)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
Surfactant Concentration (wt%)
Yie
ld S
tress
, σY
(Pa)
50% Oil 60% Oil 70% Oil 80% Oil
0
15
30
45
60
75
90
Mea
n D
ropl
et S
ize,
d (μ
m)
Chapter 3 Results and Discussion
________________________________________________________________________ - 74 -
Fig. 3.11(c)
1 2 3 4 5 6 7 8 9 10 110
10
20
30
40
Yie
ld S
tress
, σY (P
a)
Surfactant Concentration (wt%)
0
15
30
45
60
75
90
Mea
n D
ropl
et S
ize,
d (μ
m) 50% Oil
60% Oil 70% Oil 80% Oil
Figure 3.11: The yield stress of the emulsions determined using the Herschel-Bulkely
model from the Bohlin rheometer software, showing the correlation between the mean
droplet size and the yield stress of the emulsion as a function of surfactant concentration
for (a) 1st day, (b) 7th day, and (c) 30th day.
The theoretical model that relates the viscosity (η ) and the yield stress ( Yσ ) is
given by the following equation [81],
1 ( )o
Y
η ηη η σσ
∞∞
−= +
+ (14)
where σ is a particular shear stress, oη is the zero viscosity (first Newtonian plateau),
and η∞ is the infinity viscosity (second Newtonian plateau). The relationship between
the Yσ and droplet radius, r is shown in the following equation,
Chapter 3 Results and Discussion
________________________________________________________________________ - 75 -
3YkTr
σβ
= (15)
k is the Boltzmann constant, T is the temperature and β is a dimensionless constant
with a value of 0.431 [81, 210]. The equation shows that the Yσ is inversely
proportional to the droplet size which implies that the increase in the droplet size will
decrease the Yσ of the emulsions. This theoretical prediction shows a strong agreement
with the experimental results shown in Figure 3.9.
The flow behavior of the emulsions was also investigated using the Power-Law
Index, n (Table 3.1). The Power-Law Index is decreasing towards zero when the oil
concentration is increased. The decrease in the Power-Law Index as a function of
surfactant concentration is profound for the emulsions with lower oil concentration (<
70% oil) after 1 day of storage under 45oC. The experimental result shows that the
Power-Law Index for the emulsions with lower oil concentration decreased after 30
days of storage. This indicates the increase in the degree of thinning for the emulsions
after the storage period. However, the effects of storage time and surfactant
concentration were insignificant in the emulsions with higher oil concentration (≥ 70%
oil).
Chapter 3 Results and Discussion
________________________________________________________________________ - 76 -
Table 3.1: The Power Law index, n of the emulsions for over 30 days of storage time.
All n were smaller than 1 indicating that the emulsions exhibits shear thinning behavior.
Age Surfactant
ConcentrationPower Law index, n
(wt%) 50% Oil 60% Oil 70% Oil 80% Oil
1st day 2 0.75 0.67 0.22 0.21
3 0.86 0.66 0.12 0.23
4 0.73 0.55 0.04 0.17
5 0.83 0.60 0.04 0.19
6 0.64 0.54 0.11 0.21
7 0.24 0.17 0.18 0.17
8 0.23 0.19 0.20 0.21
9 0.22 0.22 0.19 0.24
10 0.23 0.21 0.23 0.23
7th days 2 0.65 0.19 0.15 0.18
3 0.69 0.30 0.12 0.20
4 0.70 0.16 0.14 0.23
5 0.62 0.16 0.17 0.23
6 0.64 0.21 0.19 0.21
7 0.59 0.17 0.16 0.21
8 0.51 0.15 0.23 0.23
9 0.44 0.17 0.22 0.22
10 0.35 0.15 0.19 0.21
30th days 2 0.21 0.08 0.19 0.21
3 0.11 0.14 0.18 0.26
4 0.17 0.11 0.20 0.25
5 0.15 0.12 0.21 0.26
6 0.12 0.22 0.17 0.22
7 0.27 0.19 0.19 0.20
8 0.29 0.15 0.23 0.18
9 0.35 0.18 0.22 0.22
10 0.30 0.17 0.22 0.22
Chapter 3 Results and Discussion
________________________________________________________________________ - 77 -
Figure 3.12: Schematic representation of structural change when shear applied. (a) The
shear thickening region, (b) the First Newtonian region, (c) the shear thinning region,
and (d) the Second Newtonian region.
Fig. 3.13(a)
-10 0 10 20 30 40 50
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
Droplet Size, d (μm)
Pop
ulat
ion
50% Oil 60% Oil 70% Oil 80% Oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 78 -
Fig. 3.13(b)
-10 0 10 20 30 40 50
0
200
400
600
800
1000
1200
Droplet Size, d (μm)
Pop
ulat
ion
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.13(c)
-10 0 10 20 30 40 50-200
0
200
400
600
800
1000
1200
Droplet Size, d (μm)
Pop
ulat
ion
50% Oil 60% Oil 70% Oil 80% Oil
Figure 3.13: The droplet size distribution of the emulsions stabilized with 2 wt% of
SFAE for (a) 1st day, (b) 7th day, and (c) 30th day of storage time.
Chapter 3 Results and Discussion
________________________________________________________________________ - 79 -
The Power-Law equation (Eq. 7) is used to illustrate the shear thinning effect of
an emulsion, while the Power-Law Index, n is an indication of the degree of thinning.
The shear thinning region begins right after the yield stress of the emulsions is exceeded
where the viscosity of the emulsion decreased with shear rate is observed. Shear
thinning behavior is initiated from the perturbation of the original structure of the
emulsion and followed by rearrangement into an ordered layer when shear is applied
(Figure 3.12) [211]. Thus, the droplet structural orientation and the polydispersity are
important factors because they affect the packing efficiency of the droplets especially
for a densely packed system.
The width of the droplet size distribution curves for those emulsions which are
shown in Figure 3.13 increased with storage time and decreased with surfactant and oil
concentration. The distribution of droplets for the emulsions clearly shows that the
frequency of larger droplet size increased after 30 days of storage (Figure 3.13(c)). This
implies that the droplets in the aged emulsions can pack more efficiently than the
freshly prepared emulsions. According to the literature [212, 213], the maximum
disperse phase volume fraction, mφ (oil concentration) for the droplets to be randomly
close packed in concentrated emulsion is 61% for monodisperse and 71% for
polydisperse system, further addition of the φ will cause droplets to deform leading to
hexagonal close packing. However, the magnitude of the mφ is polydispersity
dependence, higher polydisperse droplets tend to pack more efficiently at higher volume
fraction when shear is applied and vice versa (Figure 3.14). In this case, the
polydisperse droplets can easily slip pass each other and can easily align themselves
along the flow line induced by the movement of the geometry at lower shear stress ,
thus reducing the viscosity [214].
Chapter 3 Results and Discussion
________________________________________________________________________ - 80 -
(a)
(b)
Figure 3.14: The packing of emulsion droplets in the (a) polydisperse system and (b)
monodisperse system showing the droplet packing efficiency.
The shear thinning behavior of the emulsions can also be predicted using
Equations 14 and 15. This shear thinning effect is an evidence of the competition
between the thermal and hydrodynamic forces [215]. At the first Newtonian region
( σ ‹ Yσ ), no flow is being observed and the droplets experience thermal force
(Brownian). When the emulsions started to flow (σ › Yσ ), the hydrodynamic force
came into consideration and dominated over the thermal force [215]. At high shear
stress (σ » Yσ ), the droplets become organized into layers along the line of shear field
and closer to each other, hence, the hydrodynamic shear force dominates (second
Newtonian plateau) [81]. For large droplets, the Yσ is often so low (Figure 3.11) that the
shear thinning effects is hardly observed as compared to the emulsions with smaller
droplets [81]. These were supported by the experimental results where the Power-Law
index, n for the emulsions with larger droplet size was greater and closer to 1 (Table
3.1). Therefore, a conclusion can be drawn where the degree of thinning of the
emulsion decreased with the increase of mean droplet size [216].
Chapter 3 Results and Discussion
________________________________________________________________________ - 81 -
3.4.1.2 Aging effect to the emulsions flow properties
Aging effect has led to the decrease of the emulsions stability resulting from an
increase of droplet size. The increase of droplet size had weaked the interdroplet
interactions, thus lowering the viscosity, yield stress and degree of thinning of the
emulsions [135, 217]. Among the emulsions examined in this study, the emulsions with
50% oil were the most unstable, which show phase separation for all range of surfactant
concentration after 30 days of storage. The mean droplet size was increased and the
flow properties show significant changes after 30 days of storage time. On the other
hand, the aging effect was negligible in the very stable emulsions which are the one
with 80% oil. According to the stability test, there was no phase separation observed
form these emulsions throughout the 30 days of storage time. There were also no
significant increase in the droplet size of these emulsions over time (Figure 3.2) which
implied that the emulsions with 80% oil were relatively stable against destabilization
processes.
Upon aging, the viscosity of emulsions especially the dilute emulsions (< 70%)
decreased. The reduction in viscosity of these emulsions was due to the increase of the
droplet size [74]. The droplet concentration in dilute emulsions was low as compared to
the concentrated emulsions (Figures 3.4 and 3.7). As a result, the droplets were more
mobile and free to move, thus enhancing droplets collision leading to the coarsening of
droplets. The appearance of large droplets coexisting with the smaller ones
(polydisperse system) influences the close packing of the droplets by locating
themselves (the smaller droplets) in between the larger droplets. Therefore, the droplets
can pack more efficiently when they were forced to be close to each other as stress is
applied (Figure 3.22 (b)). When the applied stress exceeded the yield stress of the
emulsion system, the droplets started to flow by aligning themselves into the shear line.
Chapter 3 Results and Discussion
________________________________________________________________________ - 82 -
Many polydiserse systems started to flow at lower shear field due to low interaction
between the fine and coarse droplets thus lowering the yield stress as well as the shear
thinning effect of the emulsions. In addition, the larger droplet can easily slip pass over
the smaller ones when they were forced to move forward. As a result, the flow
properties of the coalescenced emulsions decreased.
3.5 Dynamic Characteristic
The viscoelastic properties of the emulsions were investigated using the strain
and frequency sweep measurements. In the strain sweep measurement, the oscillation
frequency is fixed at 1 Hz in order to obtain the linear viscoelastic region; any higher
frequency will rupture the internal structure of the emulsion. The purpose to perform
this measurement is to obtain the limits of linearity of the emulsions [218]. In this linear
region, all rheological parameters were remaining constant and independent of the strain
amplitude. The parameters start to change with the applied strain when it exceeded the
critical strain (γc) of the emulsions [219]. This γc is identified as the minimum strain
where emulsion shows departure from linearity.
Chapter 3 Results and Discussion
________________________________________________________________________ - 83 -
Fig. 3.15(a)
1 2 3 4 5 6 7 8 9 10 110.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
0.055
Crit
ical
Stra
in, γ
c
Surfactant Concentration (wt%)
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.15(b)
1 2 3 4 5 6 7 8 9 10 110.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
0.055
Surfactant Concentration (wt%)
Crit
ical
Stra
in, γ
c
50% Oil 60% Oil 70% Oil 80% Oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 84 -
Fig. 3.15(c)
1 2 3 4 5 6 7 8 9 10 110.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
0.055
Surfactant Concentration (wt%)
Crit
ical
Stra
in, γ
c
50% Oil 60% Oil 70% Oil 80% Oil
Figure 3.15: The γc of the emulsions for (a) 1st day, (b) 7th day, and (c) 30th day.
The critical strain, γc of emulsions in different oil concentration are shown in
Figure 3.15. The γc of the emulsions increased with the oil volume fraction and
surfactant concentration, but decreased with the emulsions’ age. It can be observed from
the findings that, surfactant and oil concentration affected γc the most compared to the
emulsion age. The ageing effect is only significantly affected the γc in more dilute
emulsions which is the emulsions with 50% and 60% oil. The γc is increased more than
100% as the surfactant concentration increased from 2 wt% to 10 wt%; and almost 80%
when the oil concentration increased from 50% to 80%. These implied that the
emulsions with high surfactant and oil concentration were able to recover even after
being subjected to large deformation strain.
Chapter 3 Results and Discussion
________________________________________________________________________ - 85 -
Fig. 3.16(a)
Fig. 3.16(b)
Chapter 3 Results and Discussion
________________________________________________________________________ - 86 -
Fig. 3.16(c)
Fig. 3.16(d)
Figure 3.16: The morphology of emulsions with 80% oil which stabilized by (a) 7 wt%,
(b) 8 wt%, (c) 9 wt% and (d) 10wt% of SFAE.
Chapter 3 Results and Discussion
________________________________________________________________________ - 87 -
The rapid increase of critical strain of emulsions with 80% oil when the SFAE
concentration increased above 7 wt% (Figure 3.15) implied that the highly packed
droplets (Figure 3.16) have develop a strong structural due to the great interdroplet
interaction between the droplets which correspond to the droplet size and droplet
concentration of the emulsions system. According to the morphology of the emulsions
shown in Figure 3.16, the droplet size was fine and highly packed as compared to those
emulsions prepared with lower oil and surfactant concentration (Figure 3.17). Since the
strength of the interdroplet interactions corresponds to the mean separation distance
between the droplets, the highly packed emulsion system will therefore has greater
interdroplet interaction forces. This high interdroplet interaction strength was able to
hold the droplets and withstand the large deformation forces applied during the strain
sweep test.
Fig. 3.17(a)
Chapter 3 Results and Discussion
________________________________________________________________________ - 88 -
Fig. 3.17(b)
Fig. 3.17(c)
Chapter 3 Results and Discussion
________________________________________________________________________ - 89 -
Fig. 3.17(d)
Figure 3.17: The morphology for the emulsions with 70% oil stabilized with (a) 7 wt%,
(b) 8 wt%, (c) 9 wt%, and (d) 10 wt% of SFAE.
Chapter 3 Results and Discussion
________________________________________________________________________ - 90 -
Fig. 3.18(a)
1E-4 1E-3 0.01 0.1 1
10
100
1000
Ela
stic
Mod
ulus
, G' (
Pa)
Strain,γ (%)
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Fig. 3.18(b)
1E-4 1E-3 0.01 0.1 11
10
100
1000
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Elas
tic M
odul
us, G
' (Pa
)
Strain,γ (%)
Chapter 3 Results and Discussion
________________________________________________________________________ - 91 -
Fig. 3.18(c)
1E-4 1E-3 0.01 0.1 11
10
100
1000
Ela
stic
Mod
ulus
, G' (
Pa)
Strain, γ (%)
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Fig. 3.18(d)
1E-4 1E-3 0.01 0.1 11
10
100
1000
Strain, γ(%)
Ela
stic
Mod
ulus
, G' (
Pa)
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Figure 3.18: The elastic modulus of emulsions at the first day of age obtained from the
strain sweep to establish the linear viscoelastic range. (a) 50%, (b) 60%, (c) 70%, and (d)
80% oil.
Chapter 3 Results and Discussion
________________________________________________________________________ - 92 -
The strain sweep profiles not only give information on the critical strain but also
provide useful information about the elastic component of the emulsions. Figures 3.18
and 3.19 show increasing trend in the elastic modulus of the emulsions with surfactant
and oil concentration which means that the interactions between droplets are relatively
strong. Comparing the results shown in Figures 3.15 and 3.19, a trend of increasing
elastic modulus accompanying with the increase of γc is observed. This further
supported the fact that increasing of the interdroplet interaction strength as the
surfactant and oil concentration increased.
21 '2c cE G γ= (16)
These interdroplet interactions can be represented as the cohesive force, Ec
which can be to estimated using the elastic modulus (G’) and γc of the emulsions (Eq.
16). The results show that the cohesive energy was increasing when the surfactant and
oil concentration increased (Figure 3.20). According to Tadros, T. [121], the cohesive
energy is related to the structure of the emulsion system which correlated to the droplet
size and number of contact area between the droplets. In other word, the droplet
concentration and the packing of the droplets directly influence the strength of the
cohesive force.
By observing and comparing the morphology of the emulsions with 70% and
80% oil (Figures 3.16 and 3.17), the number of droplets was increasing as the oil
concentration increased from 70% to 80% at fix surfactant concentration. As a result,
the number of contacts within the droplets increases. That causes the increase in the
cohesive energy of the emulsion system (Figure 3.20).
Chapter 3 Results and Discussion
________________________________________________________________________ - 93 -
Beside the oil concentration, the surfactant concentration is also an important
factor affecting the energy of an emulsion system. According to the results shown in
Figure 3.20, the cohesive energy increased significantly with surfactant concentration
especially for the emulsions with 80% oil stabilized with 7 wt% of SFAE and above.
The dramatic increase of the cohesive force was due to the highly packed systems
related to the interdroplet interactions that had previously been mentioned.
Fig. 3.19(a)
1 2 3 4 5 6 7 8 9 10 110
200
400
600
800
1000
1200
1400
1600
1800
2000
2200 50% Oil 60% Oil 70% Oil 80% Oil
Elas
tic M
odul
us, G
' (P
a)
Surfactant Concentration (wt%)
Chapter 3 Results and Discussion
________________________________________________________________________ - 94 -
Fig. 3.19(b)
1 2 3 4 5 6 7 8 9 10 110
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Surfactant Concentration (wt%)
Ela
stic
Mod
ulus
, G' (
Pa)
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.19(c)
1 2 3 4 5 6 7 8 9 10 110
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Surfactant Concentration (wt%)
Ela
stic
Mod
ulus
, G' (
Pa)
50% Oil 60% Oil 70% Oil 80% Oil
Figure 3.19: The magnitude of elastic modulus of the emulsions obtained from the strain
sweep measurement for (a) 1st day, (b) 7th day, and (c) 30th day.
Chapter 3 Results and Discussion
________________________________________________________________________ - 95 -
Fig. 3.20(a)
1 2 3 4 5 6 7 8 9 10 110.0
0.5
1.0
1.5
2.0
2.5
3.0
E c (Jm
-3)
S u rfactan C oncentra tion (w t% )
50% oil 60% oil 70% oil 80% oil
Fig. 3.20(b)
1 2 3 4 5 6 7 8 9 10 110.0
0.5
1.0
1.5
2.0
2.5
3.0
Ec (J
m-3)
Surfactan Concentration (w t% )
50% oil 60% oil 70% oil 80% oil
Chapter 3 Results and Discussion
________________________________________________________________________ - 96 -
Fig. 3.20(c)
1 2 3 4 5 6 7 8 9 10 110.0
0.5
1.0
1.5
2.0
2.5
3.0
E c (Jm
-3)
Surfactant Concentration (w t% )
50% oil 60% oil 70% oil 80% oil
Figure 3.20: The cohesive energy of the emulsions obtained at (a) 1, (b) 7, and (c) 30
days of storage periods.
All frequency sweep tests for the emulsions were performed in the linear
viscoelastic region based on the amplitude sweep profile to reduce the possibility of
introducing great damage to the microstructures of emulsion. After considering the
amplitude sweep profile of all emulsions, fix strain amplitude of 0.4% was chosen as the
applied strain in order to obtain a comparable data. The frequency sweep profile is used
to illustrate the response of the emulsions’ modulus with frequency. As shown in
Figure 3.21, the elastic modulus (G’) and the viscous modulus (G”) of the emulsions
showing the domination of G’ over G”. Also, as the frequency increased, the
magnitudes of G’ and G” increased, the distance between the two curves also getting
larger. The magnitude of G’ and G” increased with surfactant concentration as well.
These emphasize the elastic character of the emulsions.
Chapter 3 Results and Discussion
________________________________________________________________________ - 97 -
Fig. 3.21(a)
0.01 0.1 1 1010
100
1000
10000 G' G"
Frequency (Hz)
Ela
stic
Mod
ulus
, G' (
Pa)
10
100
1000
10000 2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt% V
iscous Modulus, G
" (Pa)
Fig. 3.21(b)
0.01 0.1 1 1010
100
1000
10000
Ela
stic
Mod
ulus
, G' (
Pa)
Frequency (Hz)
G' G"
10
100
1000
10000
Vis
cous
Mod
ulus
, G" (
Pa)
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Chapter 3 Results and Discussion
________________________________________________________________________ - 98 -
Fig. 3.21(c)
0.01 0.1 1 1010
100
1000
10000
Ela
stic
Mod
ulus
, G' (
Pa)
Frequency (Hz)
G' G"
10
100
1000
10000
Vis
cous
Mod
ulus
, G" (
Pa)
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Fig. 3.21(d)
1E-3 0.01 0.1 1 1010
100
1000
10000
G' G"
10
100
1000
10000
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Elas
tic M
odul
us, G
' (P
a)
Frequency, f (Hz)
Viscous Modulus, G
" (Pa)
Figure 3.21: Frequency sweep profile of the emulsions with (a) 50%, (b) 60%, (c) 70%,
and (d) 80% of oil.
Chapter 3 Results and Discussion
________________________________________________________________________ - 99 -
However, there was a slight difference in the frequency sweep profiles between
the dilute emulsions (< 70% oil) and the concentrated one (≥ 70% oil). As previously
discussed, the frequency sweep can be divided into five regions (Figure 1.12).
According to the experimental results shown in Figures 3.21(a) and (b), the frequency
sweep profiles for dilute emulsions show the transition and plateau zone of the
mechanical spectrum (Figure 1.12). But, the frequency profiles for concentrated
emulsions show only the plateau zone (Figures 3.21(c) and (d)).
In the case of dilute emulsions, the G” is found to dominate over the G’ in the
low frequency region. The Tan δ of the emulsions helps to clarify this point by giving
magnitude greater than 1 (Figure 3.22), which implies that the emulsions flow at low
frequency showing liquid like behavior. The magnitude of G’ and G” continuously
increased when the frequency increased and the rate of increase for G’ is greater than
G”. Finally, the two curves crossover at a frequency point at which G’=G”. This
crossover frequency is the characteristic frequency ( *ω ) which the inverse of this *ω
representing the relaxation time ( *t ) of the system (Eq. 17) [121]. The *ω is also a
transition point where the emulsion changes its behavior from liquid like to solid like.
After the *ω , the magnitude of G’ and G” continuously increased and this time the G’
dominated over the G” which indicated that the emulsions started to behave like solid.
On the other hand, there was no crossover point observed at the same studied
frequency range for concentrated emulsions. The G’ for these concentrated emulsions
was always dominating over the G” in the studied frequency range. In other word, the
crossover frequency had shifted to lower frequency region. This is due to high
relaxation time ( *t ) of the emulsions. The *t of an emulsion system is the time required
for the deformed system to reform under condition of low stress or rest [133].
According to the experimental results, the *t is increasing with the oil concentration. In
other word, the concentrated emulsions need longer time for it to recover once the
Chapter 3 Results and Discussion
________________________________________________________________________ - 100 -
emulsion structure breakdowns as compared to more dilute emulsion system. The Tan δ
of these concentrated emulsions is lower than 1 which is well-proven that these
emulsions exhibit solid like behavior.
**
1tω
= (17)
In a dilute emulsion system, owing to the weak interdroplet interactions, the
systems were able to behave like a liquid after being subjected to low stress for long
time (low frequency). In the case of concentrated emulsions, no domination of the G”
over the G’ was observed when the test was carried out under same condition as applied
to the dilute emulsion. That means the applied stress is able to be stored in the elastic
component although at low stress condition. This indicated the presence of strong
interactions among the emulsion droplets. According to Tadros, T. [219], G’ is a
measure of the energy stored elastically in the system, while G” is the measure of the
energy dissipated as heat during viscous flow. The input energy which comes from the
shear stress is stored elastically and represented as G’. Therefore, a decrease in the
magnitude of the imaginary viscous component (G”) is observed. When the frequency
further decreases, most of the input energy is no longer able to be stored, but will
dissipated through viscous flow [220].
The frequency sweep profile also shows decrease of G’ slope and the flattening
of G” happened when the oil concentration increased [173]. That is illustrated by the
results shown in Figure 3.21, which shows G’ >> G” and the modulus is almost
independence of frequency. This development of plateau region maybe attributed to an
increase of interactions among the droplets, as a consequence of the decreased of
droplet size and polydispersity [221]. Besides, this is also an indication of the ability of
Chapter 3 Results and Discussion
________________________________________________________________________ - 101 -
the emulsions to resist any structural changes under stress (as the frequency increased)
[222].
In this study, however, the effect of surfactant concentration on the viscoelastic
property of emulsion is negligible compared to the effect of oil concentration (Figures
3.22 and 3.23). Those were clearly reflected in the Tan δ of the emulsions. The Tan δ of
the emulsion with 50% oil is 2 times greater than that of the 80% emulsion system. The
magnitude of Tan δ decreased significantly with increase of the oil concentration
indicating the great elasticity of the emulsions with 80% oil [222]. Tan δ (Eq. 18) is a
dimensionless loss factor measuring the amount of energy loss during the test cycle
[223]. The emulsion with Tan δ greater than 1 behaves like a liquid because the G” is
larger than G’; while a solid like emulsion gives Tan δ lower than 1 (G’>G”) [224].
"'
GTanG
δ = (18)
The lesser influence of the surfactant concentration to the elastic property of
emulsion is illustrated in Figure 3.22. That figure indicated that there was no
complicated network formed among the adsorbed layers between droplets. This also
implied the low contribution of surfactant to the dynamic rigidity of the emulsions.
Chapter 3 Results and Discussion
________________________________________________________________________ - 102 -
Fig. 3.22(a)
1E-3 0.01 0.1 1 100.1
1
Frequency, f (Hz)
Tan
δ
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Fig. 3.22(b)
1E-3 0.01 0.1 1 100.1
1
Frequency, f (Hz)
Tan
δ
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Chapter 3 Results and Discussion
________________________________________________________________________ - 103 -
Fig. 3.22(c)
1E-3 0.01 0.1 1 100.1
1
Frequency, f (Hz)
Tan
δ
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Fig. 3.22(d)
1E-3 0.01 0.1 1 100.1
1
Frequency, f (Hz)
Tan
δ
2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%
Figure 3.22: The Tan δ of emulsions with (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil
obtained after one day of storage.
Chapter 3 Results and Discussion
________________________________________________________________________ - 104 -
As mentioned before, G’ is a measure of the energy stored elastically in the
system, while G” is the measure of the energy dissipated as heat during viscous flow. In
general, more and more energy will be stored elastically in the system when the
frequency increases and the G” tends to decrease to zero. Based on the experimental
results (Figure 3.22), the elasticity of emulsion is said to be almost independent of the
surfactant concentration. But, the magnitude of G’ was increasing in both strain sweep
and frequency sweep with the increase of surfactant concentration. This is due to the
decrease of droplet size and polydispersity of the droplets. As the droplet size and
polydispersity decreased, the interfacial area increased; thus, increased the surface-to-
surface interactions. As a result, the magnitude of G’ increased. However, the increase
of G’ is accompanied with the increasing of G” that specifies the energy dissipation
from the system. In other word, the amount of energy stored in the system when the
surfactant concentration increases will then be dissipated from the system. That
explained why the increase of surfactant concentration does not increase the interdroplet
interaction between the droplets.
Chapter 3 Results and Discussion
________________________________________________________________________ - 105 -
Fig. 3.23(a)
1E-3 0.01 0.1 1 100.1
1
Tan
δ
Frequency, f (Hz)
50% Oil 60% Oil 70% Oil 80% Oil
Fig. 3.23(b)
1E-3 0.01 0.1 1 100.1
1
50% Oil 60% Oil 70% Oil 80% Oil
Tan
δ
Frequency, f (Hz)
Chapter 3 Results and Discussion
________________________________________________________________________ - 106 -
Fig. 3.23(c)
1E-3 0.01 0.1 1 100.1
1
50% Oil 60% Oil 70% Oil 80% Oil
Tan
δ
Frequency, f (Hz)
Figure 3.23: The effect of oil concentration to the Tan δ of emulsions stabilized with 2
wt% of SFAE after (a) 1 day, (b) 7 days, and (c) 30 days of storage duration.
The increase of elasticity when the oil concentration increased is explained with
the effect of droplet size and droplet concentration. Unlike electrostatically stabilized
system, the sterically stabilized system involved a short range interaction. According to
Tadros, T.F. [225], the elastic modulus attributed from the hydrodynamic and surface
force is very dependent to the surface-to-surface separation distance. These interactions
between droplets become stronger when the droplets getting closer to each other. At
lower oil concentration, the droplets separation distances were large and is comparable
to droplet radius. In this case, the droplets were considered loosely packed and were still
able to diffuse with slower rate. Due to the large separation distances between the
droplets, the interaction among the droplets is relatively weak.
Chapter 3 Results and Discussion
________________________________________________________________________ - 107 -
The increased of oil concentration resulting in the increased of droplet
concentration and decreasing the interdroplet separation distance. As the interdroplet
separation distance decreased some overlap of the interfacial layer occur. That causes
rapid increase of the repulsive forces and eventually the droplets repel each other. When
the droplets were in close distance, the hydrodynamic interaction is important to act as a
repulsive force to prevent the droplets coming into close contact. Meanwhile, the
attractive forces increase by pulling the droplets towards each other. These interdroplet
forces therefore act like a “spring” holding the droplets and the droplets end up
vibrating with a small amplitude which pronounced the elastic property [226]. As stated
by Tadros, T.F. [225], in such concentrated condition, the droplets interact with many
neighbors and the repulsive force produces a specific order among the droplets to the
extent that a highly develop structure is reached. However, the droplets will merge or
rupture at high shear stress region when the droplets are too close to each other whereby
the attractive forces are greater than the repulsive forces. When that happens, loss of
elasticity can be observed by showing a decreasing in magnitude of G’. Fortunately, the
viscoelastic profiles of the emulsions do not show this phenomenon, which indicates the
structural rigidity and high droplet stability although subjected to high stress (high
frequency).
CHAPTER 4
CONCLUSION
Chapter 4 Conclusion
________________________________________________________________________ - 108 -
4.0 Conclusion
This study has demonstrated the preparation of olive oil-in-water emulsion
stabilized with nonionic surfactant. The emulsions were prepared with different
composition of surfactant and oil and were kept for 30 days under 45oC for stability and
rheological test. The microstructure of the emulsions was studied under polarizing
microscope, while the Bohlin CVO-R rheometer was employed for characterization of
the macroscopic properties of the emulsions.
The stability of the formulated emulsions was found to be increased with the
concentration of oil and surfactant. The phase separation of the emulsions decreased
with an increase of surfactant and oil concentration indicating the greater stability of
droplets against the gravitational creaming destabilization process. However, the 80%
oil emulsions were the only emulsions that remain stable without phase separation for
30 days of storage period under accelerated condition. The other emulsions show
obvious phase separation after or within the examined period. The emulsions instability
can be explained from creaming destabilization, which causes the migration of oil
droplets to the top of the emulsions. According to the experimental results, the effect of
oil is more significant to affect the shelf life of the emulsion compared to the effect of
surfactant.
The influences of oil and surfactant concentration to the microstructure of the
emulsions were clearly shown in the micrographs. The droplet size decreased with the
increase of surfactant and oil concentration. The droplet concentration was increased
with the increase of the oil and surfactant concentration. The droplet size and droplet
concentration are the key to these destabilization processes that influence the
interdroplet interaction. This is due to the strong relationship between the droplets size
and droplet concentration with the strength of the interaction forces.
Chapter 4 Conclusion
________________________________________________________________________ - 109 -
The droplet size and droplet concentration also affected the rheological behavior
of the emulsions. A very significant change of the rheological behavior of the emulsions
was the viscosity of the emulsions. The viscosity of emulsions was increased with the
increase of surfactant and oil concentration. This was due to the increase of droplet
concentration which affected the mobility of the droplets. That also increased the
interdroplet interaction which increased the yield stress of the emulsions. Besides, the
viscoelastic property of the emulsions was also affected by the changes of the
microstructure. The increase in the oil concentration had increased the droplet
concentration which enhanced the elasticity of the emulsions.
The stability and the rheological properties of the emulsions decreased with
storage time. After being kept under accelerated condition for 30 days, the droplet size
of these emulsions increased, which indicated the microstructural rearrangement. The
viscosity and the viscoelastic behavior decreased significantly after 30 days of storage
time. However, the emulsions with 80% oil were the only emulsions that are stable
against phase separation. The changes in the droplet size and rheological behavior of
these emulsions were negligible. In order word, these 80% oil emulsions have the
longest shelf life as compared to the other emulsions that had been prepared in this
study.
Chapter 4 Conclusion
________________________________________________________________________ - 110 -
4.1 Future Research
It can be observed that the increase of the oil and surfactant concentration
enhanced the emulsions stability and rheological properties. But, due to the economical
concern, the increase in the surfactant concentration is not a desirable method to use as a
way to improve the emulsion stability, while the increase of oil content in the emulsion
formulations will decrease the perception of the emulsions. Therefore, the emulsions
must have lower oil content in order to create pleasant feeling when applied on the skin.
However, the emulsions formulated from low oil concentration have pronounced phase
separation within short period after preparation. Thus, the rheological properties of this
emulsion have to be modified to increase stability and the shelf life of the emulsion. In
this case, there are several factors that have to be considered.
One of the factors is to modify the viscosity of the continuous phase of the
emulsions by adding thickener into the emulsion in order to create a continuous phase
with high viscosity to control the movement of the droplets. The increase of the
continuous phase viscosity will decrease the mobility, the collision frequency as well as
the collision efficiency of the droplets. That is also able to prevent sedimentation or
creaming destabilization processes.
Thickener such as polymers were used in formulation in order to improve the
flow ability (viscosity modifier), shelf life and intergrity of an emulsion. In this study,
polysaccharides such as xantham gum is chosen as a thickener due to the factor of being
“natural”. Xantham gum is one of the examples of a high molecular weight
extracellular polysaccharide biopolymer produced by the bacterium Xanthomonas
campestris [227]. It has a broad application range including food, cosmetic, and
pharmaceutical industries [228] due to the human friendly nature. Xantham gum exists
in aqueous media with an ordered rigid chain conformation [228], thus is suitable as
Chapter 4 Conclusion
________________________________________________________________________ - 111 -
stabilizer and thickener in water-based (oil-in-water) emulsion system. However, there
are also some interactions between the surfactant added into the emulsions system and
the polysaccharides. These interactions may affect the function of the polysaccharides
and the surfactants that will indirectly influence the stability of the emulsion.
In addition, the chemical composition such as pH and electrolyte concentration
as well as the viscosity of the emulsion are also important factors affecting the emulsion
stability [229]. Therefore, the investigation of the influence of thickener, pH, and
concentration of electrolytes to the emulsions system stabilized with glycoipids is
required in the future.
CHAPTER 5
REFERENCES
Chapter 5 References
________________________________________________________________________ - 112 -
5.0 References
1. Somasundaran, P., Wines, T.H., Metha, S.C., Garti, N., and Farinato, R., Emulsions and Their Behavior, in Surfactants in Personal Care Products and Decorative Cosmetics, L.D. Rhein, Schlossman, M., O'Lenick, A. and Somasundaran, P, Editor. 2007, CRC Press: New York. p. 504.
2. Balzer, D., Varwig, S., and Weihrauch, M., Viscoelasticity of personal care products. Colloids and Surfacs A: Physicochemical and Engineering Aspects, 1995. 99: p. 233-246.
3. Somasundaran, P., Chakrabotry, S., Deo, P., Deo, N., and Somasundaran, T., Contribution of Surfactant to Personal Care Products, in Surfactants in Personal Care Products and Decorative Cosmetics, L.D. Rhein, Schlossman, M., O'Lenick, A. and Somasundaran, P, Editor. 2007, CRC Press: New York. p. 121.
4. Kostarelos, K., Rational design and engineering of delivery systems for therapeutics: biomedical exercises in colloid and surface science. Advances in Colloid and Interface Sciences, 2003. 106: p. 147-168.
5. Xia, Q., Hao, X.Z., Lu, Y.Y., Xu, W., and Wei,H., Production of drug-loaded lipid nanoparticles based on phase behaviors of special hot microemulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2008. 313-314: p. 27-30.
6. Cserha´ti, T., Forga´cs, E., and Oros, G., Biological activity and environmental impact of anionic surfactants. Environment Internatinal, 2002. 28: p. 337-348.
7. Rocha, A.J.S., Gomes, V., Ngan, P.V., Passos, M.J.A.C.R., and Furia, R.R., Effects of anionic surfactant and salinity on the bioenergetics of juveniles of Centropomus parallelus (Poey). Ecotoxicology and Environmental Safety, 2007. 68: p. 387-404.
8. Aidar, E., Sigaud-Kutner, T.C.S., Nishihara, L., Schinke, K.P., Braga, M.C.C., Farah, R.E., and Kutner, M.B.B., Marine phytoplankton assays: effects of detergents. Marine Environmental Research, 1997. 43 (1/2): p. 55-68.
9. Gould, L.A., Lansley, A.B., Alison, B., Brown, M.B., Forbes, B., and Martin, G.P., Mitigation of surfactant erythrocyte toxicity by egg phosphatidylcholine. Journal of Pharmacy and Pharmacology 2000. 52: p. 1203-1209.
10. Seedher, N., In vitro study of the mechanism of interaction of trifuoperazine dihydrochloride with bovine serum albumin. Indian Journal of Pharmaceutical Sciences, 2000. 62: p. 16-20.
11. Yushmanov, V.E., Perussi, J.R., Imasato, H., and Tabac, M., Interaction of papaverine with micelles of surfactants with different charge studied by 1H-NMR. Biochimica et Biophysica Acta 1994. 1189: p. 74-80.
12. Bucks, D.A.W., Hostynek, J.J., Hinz, R.S., and Guy, R.H., Uptake of Two Zwitterionic Surfactants into Human Skin in Vivo. Toxicology and Applied Pharmacology, 1993. 120: p. 224-227.
13. Eastoe, J., ed. Surfactant Aggregation and Adsorption at Interfaces. Colloid Science: Principles, Methods and Applications, ed. T. Cosgrove. 2005, Blackwell Publishing: Oxford.
14. Odokuma, L.O., and Okpokwasili, G.C., Seasonal influences of the organic pollution monitoring of the New Calaber river, Nigeria. Environmental Monitoring and Assessment, 1997. 45: p. 43-57.
15. Baglimieri, C., Cenciarini, J., Fernex, F., Pucci, R., and Vassiere, R., Problems of stirage of various substances found in the interstitial waters on the surface
Chapter 5 References
________________________________________________________________________ - 113 -
sediments of the French continental shelf. Program of Water Technology, 1980. 12: p. 79-88.
16. Holmberg, K., Jönsson, B., Kronberg, B., and Lindman, B., ed. The Ecological Impact of Surfactants is of Growing Importance. Sufactants and Ploymers in Aqueous Solution. 2003, John Wiley & Sons, Ltd.: West Sussex.
17. Brown, M.J., Biosurfactants for cosmetic applications. International Journal of Cosmetic Science, 1991. 13: p. 61-64.
18. Healy, M.G., Devine, C.M., and Murphy, R., Microbial Production of Biosurfactants. Resources, Conservation and Recycling, 1996. 18: p. 41-57.
19. Söderlind, E., Wollbratt, M., and von Corswant, C., The Usefulness of Sugar Surfactants as Solubilizing Agents in Parenteral Formulations. International Journal of Pharmaceutics, 2003. 252: p. 61-71.
20. Youan, B.C., Hussain, A., and Nguyen, N.T., Evaluation of Sucrose Esters as Alternative Surfactants in Microencapsulation of Proteins by the Solvent Evaporation Method. AAPS PharmSci, 2003. 5: p. Article 22.
21. Yamada, T., Kawase, N., and Ogimoto, K. Y., 1980. 29: p. 543. 22. Farone, W.A., and Serfass, R., Sugar-ester Manufacturing Process, L. Kimball
Chase Technologies, Editor. 1996: United State. 23. Herrington, T.M., Harvey, B.A., Midmore, B.R., and Sahi, S.S. , Properties of
sucrose esters, in Surfactants in Lipid Chemistry : Recent Synthetic, Physical, and Biodegradative Studies T.J.H. P., Editor. 1992, The Royal Society of Chemistry: UK.
24. Lerk, P.C., Sucker, H.H., and Eicke, H.F. , Micellization and Solubilization Behavior of Sucrose Laurate, a New Pharmaceuical Excipient. Pharmaceutical Development and Technology, 1996. 1: p. 27-36.
25. Watanabe, T., Sucrose fatty acid esters-past, present and future. Foods Food Ingr J Jpn, 1999. 180: p. 18-25.
26. Marshallm, D.L., and Bullerman, L.B., Antimicrobial properties of sucrose fatty acid esters, in Carbohydrate polyesters as fat substitutes, C.C.a.S. Akoh, B.G. , Editor. 1994, Marcel Dekker: New York. p. 149-167.
27. Okabe, S., Saganuma, M., Tada, Y., Ochiai, Y., Sueoka, E., Kohya, H., Shibata, A., Takahashi, M., Mizutani, M., Matsuzaki, T., and Fujiki, H., Disaccharide esters screened for in hibition of tumor necrosis factor-α release are new anticancer agents. Japanese Journal of Cancer Research, 1999. 90: p. 669-676.
28. Puterka, G.J., Farone, W., Palmer, T., and Barrington, A. , Structurefunction relationships affecting the insecticidal and miticidal activity of sugar esters. Ecotoxicology, 2003. 96: p. 636-644.
29. Allen, D.K., and Tao, B.Y., Carbohydrate-Alkyl Ester Derivatives as Biosurfactants. Journal of Surfactants and Detergents, 1999. 2: p. 383-390.
30. Habulin, M., S abeder, S., and Knez, Z. , Enzymatic synthesis of sugar fatty acid esters in organic solvent and in supercritical carbon dioxide and their antimicrobial activity Journal of Supercritical Fluids 2008. 45: p. 338-345.
31. Reyes-Duarte, D., López-Cortés, N., Ferrer, M., Plou, F.J., and Ballesteros, A. , Parameters Affecting Productivity in the Lipase-catalysed Synthesis of Sucrose Palmitate Biocatalysis and Biotransformation, 2005. 23: p. 19-27.
32. Zhao, Y., and Wang, N., Bio-method of Preparing Sucrose Ester - Biosurfactant Journal of Beijing University of Chemical Technology 1996. 23: p. 10.
33. Tokiwa, Y., Raku, T., Kitagawa, M., and Kurane, R., Preparation of polymeric biosurfactant containing sugar and fatty acids esters. Clean Produsts and Processes, 2000. 2: p. 108-111.
Chapter 5 References
________________________________________________________________________ - 114 -
34. Holmberg, K., Jönsson, B., Kronberg, B., and Lindman, B., Surfactants Aggregate in Solution, in Sufactants and Ploymers in Aqueous Solution. 2003, John Wiley & Sons, Ltd.: West Sussex.
35. Rosen, M.J., ed. Micelle Formation by Surfactants. 3rd Edition ed. Surfactants and Interfacial Phenomena. 2004, John Wiley & Sons, Inc.: New Jersey.
36. McClements, D.J., Molecular Characteristics, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 102-103.
37. Song, Q., Couzis, A., Somasundaran, P., and Maldarelli, C., A Transport Model for the Adsorption of Surfactant from Micelle Solutions onto a Clean Air/Water Interface in the Limit of Rapid Aggregate Dissambly Relative to diffusion and Supporting Dynamic Tension Experiments. Colloids and Surfaces A: Physicochemical Engineering Aspects, 2006. 282-283: p. 162-182.
38. Colegate, D.M., and Bain, C.D., Adsorption Kinetics in Micellar Solutions of Nonionic Surfactants. Physical Review Letters, 2005. 95: p. 1-4.
39. Partist, A., Kanicky, J.R., Shukla, P.K., and Shah, D.O., Importance of Micellar Kinetics in Relation to Technological Processes. Journal of Colloid and Interface Science, 2002. 245: p. 1-15.
40. Oh, S.G., Klein, S.P., and Shah, D.O., Effect of Micellar Life-Time on the buble dynamics in Sodium Dodecyl Sulfate Solutions. AIChE Journal, 1992. 38: p. 149-152.
41. Danov, D.K., Vlahovska, P.M., Horozov, T., and Dushkin, C.D., Adsorption from Micellar Surfactant Solutions: Nonlinear Theory and Experiment. Journal of Colloid and Interface Science, 1996. 183: p. 223-235.
42. Fainerman, V.B., and Makievski, A.V. , Micelle Dissociation Kinetics Study by Dynamic Surface Tension of Micellar Solutions. Colloids and Surfaces, 1993. 1993: p. 249-263.
43. Fainerman, V.B., Adsorption Kinetics from Concentrated Micellar Solutions of Ionic Surfactants at the Water-Air Interface. Colloids and Surfaces, 1992. 62: p. 333-347.
44. Chang, C.H., and Franses, E.I., Adsorption Dynamics of Surfactants at Air/Water Interface: A Critical Review of Mathematical Models, Data, and Mechanisms. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995. 100: p. 1-45.
45. Patist, A., Kanicky, J.R., Shukla, P.K., and Shah, D.O., Importance of Micellar Kinetics in Relation to Technological Processes. Journal of Colloid and Interface Science, 2002. 245: p. 1-15.
46. Patist, A., Oh, S.G., Leung, R., and Shah, D.O., Kinetics of Micellization: Its Significance to Technological Processes. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001. 176: p. 3-16.
47. Patist, A., Jha, B.K., Oh, S.G., and Shah, D.O., Importance of Micellar Relaxation Time on Detergent Properties. Journal of Surfactants and Detergents, 1999. 2: p. 317-324.
48. Oh, S.G., and Shah, D.O., Micellar Lifetime: Its Relevence to Various Technological Processes. Journal of Dispersion Science and Technology, 1994. 15: p. 297-316.
49. Marshall, D.L., and Bullerman, L.B., Antimicrobial properties of sucrose fatty acid esters, in Carbohydrate polyesters as fat substitutes, C.C.a.S. Akoh, B.G. , Editor. 1994, Marcel Dekker: New York.
50. McClements, D.J., General Characteristics of Food Emulsions, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 2.
Chapter 5 References
________________________________________________________________________ - 115 -
51. Becher, P., and Schick, M.J., Macroemulsions, in Nonionic Surfactants Physical Chemistry M.J. Schick, Editor. 1987, Marcel Dekker, INC.: New York. p. 1138.
52. Ho, K.Y., Kalman, K., and Wasan, D.T., Dynamic Film and Interfacial Tensions in Emulsion and Foam Syetems. Journal of Colloid and Interface Science, 1997. 187: p. 29-44.
53. Leal-Calderin, F., Schmitt, V., and Bibette, J., Emulsion Preparation and Characterization, in Emulsion Science: Basic Principles. 2007, Springer: New York. p. 20-21.
54. Princen, H.M., The Structure, Mechanics, and Rheology of Concentrated Emulsions and Fluid Foams, in Encyclopedic Handbook of Emulsion Technology, J. Sjoblom, Editor. 2001, Marcel Dekker, Inc.: New York. p. 243-278.
55. Kabalnov, A., Tarara, T., Arlauskas, R., and Weers, J. , Phospholipids as Emulsion Stabilizers- Phase Behavior versus Emulsion Stability. Journal of Colloid and Surface Science, 1996. 184: p. 227-235.
56. Bancroft, W.D., The theory of emulsificationV. Journal of Physical Chemistry, 1913. 17: p. 501.
57. Pal., R., Viscosity models for multiple emulsions. Food Hydrocolloids, 2008. 22: p. 428-438.
58. Goubault, C., Pays, K., Olea, D., Bibette, J., Schmitt, V., and Leal-Calderon, F., Shear Rupturing of Complex Fluids: Application to the Preparation of Quasi-Monodisperse W/O/W Double Emulsions Langmuir, 2001. 17: p. 5184.
59. Griffin, W.C., Classification of Surface-active agents by "HLB". Journal of the Society of Cosmetic Chemists, 1949. 1: p. 311-326.
60. Davies, J.T. Quantitative kinetic theory of emulsion type. I Physical chemistry of the emulsifying agent. in Proceeding of the International Congress on Surface Activity. 1957. London.
61. Guo, X.W., Rong, Z.M., and Ying, X.G., Calculation of hydrophile -lipophile balance for polyethoxylated surfactants by group contribution method. Journal of Colloid and Interface Science, 2006. 298: p. 441-450.
62. Schott, H., Comments on hydrophile-lipophile balance systems. Journal of Colloid and Interface Science, 1989. 133(2): p. 527-529.
63. Stig, E., and Friberg, P.B., Microemulsion: Structure and Dynamics 2000, Boca Raton: CRC.
64. Holmberg, K., Jönsson, B., Kronberg, B., and Lindman, B., Emulsions and Emulsifiers, in Surfactants and Plymers in Aqueous Solution. 2003, John Wiley & Sons, Ltd.: West Sussex. p. 451-471.
65. Erik, T., and Sjöblom, J., Emulsion stabilization by means of combined surfactant multilayer (D-phase) and asphaltene particles. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003. 228: p. 131-142.
66. Ali, J., Baboota, S., and Ahuja, A. Emulsion. 17 September 2008 [cited 2008 08 October 2008]; Available from: http://www.pharmpedia.com/Emulsion.
67. Hanna, S.A., Quality Assurance, in Pharmaceutical Dosage Forms: Disperse Systems, H.A. Lieberman, Rieger, M.M., and Banker, G.S., Editor. 1998, Informa HealthCare: New York. p. 423 - 479.
68. Allouche, J., Tyrode, E., Sadtler, V., Choplin, L., and Salager, J-L. Emulsion Morphology Follow-up by Simultaneous In Situ Conductivity and Viscosity Measurements During A Dynamic Temperature-Induced Transitional Inversion. in 3rd International Symposium on Food Rheology and Structure. 2003. Lappersdorf, Germany : Kerschensteiner Verlag.
Chapter 5 References
________________________________________________________________________ - 116 -
69. Allouche, J., Tyrode, E., Sadtler, V., Choplin, L., and Salager, J-L., Simultaneous Conductivity and Viscosity Measurements as a Tecnique to Track Emulsion Inversion by the Phase-Inversion-Temperature Method. Langmuir, 2004. 20: p. 2134-2140.
70. Refat, N.A., Ibrahim, Z.S., Moustafa, G.G., Sakamoto, K.Q., Ishizuka, M., and Fujita, S., The Induction of Cytochrome P450 1A1 by sudan Dyes. Journal of Biochemical and Molecular Toxicology, 2008. 22: p. 77-84.
71. The Food (Hot Chilli and Hot Chilli Products) (Emergency Control) (England) (Amendment) Regulations 2004, in ISBN 13: 9780110484877 20 June 2003, The Stationery Office: United Kingdom.
72. Rosen, M.J., Surfactants and Interfacial Phenomena, 3rd. 2004, New York: John Wiley & Sons, Inc.
73. Codex Standard for Olive Oil, Virgin and Refined, and For Refined Olive-Pomace Oil, in Codex Stan 33, F.W.F. Standard, Editor. 2001.
74. Pal, R., Effect of Droplet Size on the Rheology of Emulsions. AIChE Journal, 1996. 42: p. 3181 - 3190.
75. Pal, R., Shear Viscosity Behavior of Emulsions of Two Immiscible Liquids. Journal of Colloid and Interface Science, 2000. 225: p. 359-366.
76. Wengst, J., and Daniels, R. Influence of A Hydrophilic Polymer on A Polymer Stabilized W/O Emulsion. in Proceeding International Meeting on Pharmaceutics, Biophamaceutics and Pharmaceutical Technology. 2004. Nuremberg.
77. Myers, D., Some Other Factors Affecting Emulsion Stability, in Surfaces, Interfaces, and Colloids: Principles and Applications. 1999, John Wiley & Sons, Inc.: New York. p. 286-288.
78. McClements, D.J., Colloidal Interactions, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 366.
79. Myers, D., Surfaces, Interfaces, and Colloids: Principles and Application 2nd. 1999, New York: John Wiley & Sons, Inc. p. 493.
80. Dukhin, S.S., Sjoblom, J., and Sæther, Ø., Kinetics of Brownian and gravitational coagulation in dilute emulsions, in Emulsions and Emulsion Stability, J. Sjoblom, Editor. 1996, Marcel Dekker: New York. p. 662.
81. McClements, D.J., Flocculation, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 195.
82. Dukhin, S.S., Sjoblom, J., and Sæther, Ø., Hydrodynamics of Flocculation: Main Notions in Emulsions and Emulsion Stability, J. Sjoblom, Editor. 2006, Taylor & Francis Group, LLC: New York. p. 662.
83. Luyten, H., Jonkman, M., Kloek, W., and van Vliet, T., Creaming behaviour of dispersed particles in dilute xanthan solutions, in Food Colloids and Polymers: Stability and Mechanical Properties, E. Dickinson, and Walstra, P., Editor. 1993, Royal Society of Chemistry, Cambridge. p. 438.
84. McClements, D.J., Coalescence, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 210.
85. Leal-Calderin, F., Schmitt, V., and Bibette, J., Ostwald Ripening, in Emulsion Science: Basic Principles. 2007, Springer Science+Business Media: New York.
86. Dickinson, E., Ritzoulis, C., Yamamoto, Y., and Logan, H., Ostwald ripening of protein-stabilized emulsions: effect of transglutaminase crosslinking. Colloids and Surfaces B: Biointerfaces, 1999. 12: p. 139-146.
87. Somasundaran, P., Wines, T.H., Metha, S.C., Garti, N., and Farinato, R., Ostwald Ripening, in Surfactants in Personal Care Products and Decorative
Chapter 5 References
________________________________________________________________________ - 117 -
Cosmetics, L.D. Rhein, Schlossman, M., O'Lenick, A. and Somasundaran, P, Editor. 2007, CRC Press: New York. p. 504.
88. Kabalnov, A.S., Can Micelles Mediate a Mass Transfer between Oil Droplets? Langmuir, 1994. 10: p. 680-684.
89. Weiss, J., Hermann, N., and McClements, D.J., Ostwald Ripening of Hydrocarbon Emulsion Droplets in Surfactant Solutions. Langmuir, 1999. 15: p. 6652-6657.
90. Taylor, P., Ostwald Ripening in Emulsions. Colloids and Surfaces 1995. 99: p. 175-185.
91. Larsson, K., Emulsions in the Food Industry, in Emulsions - A Fundamental and Practical Approach, J. Sjöblom, Editor. 1992, Kluwer Academic Publishers: Netherlands. p. 41-49.
92. Junginger, H.E., Pharmaceutical Emulsions and Creams, in Emulsions - A Fundamental and Practical Approach, J. Sjoblom, Editor. 1992, Kluwer Academic Publisher: Netherlands. p. 189-205.
93. Tadros, T.F., Surfactants. 1984, London: Academic Press Inc. 94. Schick, M.J., Non-ionic Surfactants. 1966, New York: Marcel Dekker. 95. Sakai, Y., Suzuki, M., Ohara, Y., and Okabe, S. , Resolving the Conflict of a
Simultaneously Highly Moisturizing and Occlusive Emulsion Film, in IFSCC Magazine 2006. p. 23-28.
96. Muchtar, S., and Benita, S., Emulsions as Drug Carriers for Ophthalmic Use. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994. 91: p. 181-190.
97. Collins-Gold, L., Feichtinger, N., and Wärnhem,T., Are Lipid Emulsions the Drug Delivery Solution? Modern Drug Discovery, 2000. 3(3): p. 44-46.
98. Nishikawa, M., Takakura, Y., and Hashida, M., Biofate of Fat Emulsions, in Submicron Emulsions in Drug Targeting and Delivery, S. Benita, Editor. 1998, Harwood Academic Publishers: New York. p. 99-118.
99. Lyons, R.T., and Carter, E.G., Lipid Emulsions for Intravenous Nutrition and Drug Delivery, in Lipid Technologies and Applications, F.D. Gunstone, and Padley, F.B., Editor. 1997, Marcel Dekker, Inc.: New York. p. 535-556.
100. Cury-Boaventura, M.F., Gorjão, R., De Lima, T.M., Fiamoncini, J., Torres, R.P., Mancini-Filho, J., Soriano, F.G., and Curi, R., Effect of Olive Oil-Based Emulsion on Human Lymphocyte and Neutrophil Death. Journal of Parenteral and Enteral Nutrition, 2008. 32: p. 81-87.
101. Cuéllar, I., Bullón, J., Forgarini, A.M., Cárdenas, A., and Briceño, M.I. , More Efficient Preparation of Parenteral Emulsions or How to Improve a Pharmaceutical Recipe by Formulation Engineering. Chemical Engineering Science, 2005. 60: p. 2127-2134.
102. Chappat, M., Some Applications of Emulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994. 91: p. 57-77.
103. Milstein, S.R., Bailey, J.E., and Halper, A.R. , Definition of Cosmetics, in Handbook of Cosmetic Science and Technology, A.O. Barel, Paye, M., and Maibach, H.I., Editor. 2001, Marcel Dekker, Inc.: New York. p. 5-18.
104. Asia-Pacific: Stepping Up the Pace, in Global Cosmetic Industry. 2007, Allured Publishing.
105. Oddo, L.P., and Shannon, K., Sensory Testing, in Handbook of Cosmetic Science and Technology, A.O. Barel, Paye, M., and Maibach, H.I., Editor. 2001, Marcel Dekker, Inc.: New York. p. 845-857.
Chapter 5 References
________________________________________________________________________ - 118 -
106. Johnson, A.W., The Skin Moisturizers Marketplace, in Skin Moisturization, J.J. Leyden, and Rawlings, A.V., Editor. 2002, Marcel Dekker, Inc.: New York. p. 9-16.
107. Willett, W.C., and Hu, F.B., Optimal Diets for Prevention of Coronary Heart Disease. The Journal of the American Medical Association, 2002. 288: p. 2569-2578.
108. Kris-Etherton, P.M., Zhao, G., Binkoski, A.E., Coval, S.M., and Etherthon, T.D., The Effects of Nuts on Coronary Heart Disease Risk Nutrition Reviews, 2001. 59: p. 103-111.
109. Lipworth, L., Martínez, M.E., Angell, J., Hsieh, C.C., and Trichopoulos, D., Olive Oil and Human Cancer: An Assessment of the Evidence. Preventive Medicine, 1997. 26: p. 181-190.
110. van Gils, C.H., Peeters, P.H.M., Bueno-de-Mesquita, B., et. al., Consumption of Vegetables and Fruits and Risk of Breast Cancer. The Journal of the American Medical Association, 2005. 293: p. 183-193.
111. Smith-Warner, S.A., Spiegelman, D., Yaun, S.S, Adami, H.O, Beeson, W.L., van den Brandt, P.A., Folsom, A.R., Fraser, G.E., Freudenheim, J.L., Goldbohm, R.A., Graham,S., Miller, A.B., Potter, J.D., Rohan, T.E., Speizer, F.E., Toniolo, P., Willett, W.C., Wolk, A., Zeleniuch-Jacquotte, A., and Hunter, D.J., Intake of Fruits and Vegetables and Risk of Breast Cancer: A Pooled Analysis of Cohort Studies. The Journal of the American Medical Association, 2001. 285: p. 769-776.
112. Willett, W.C., Diet and Cancer. The Oncologist, 2000. 5: p. 393-404. 113. Menendez, J.A., Vellon, L., Colomer, R., and Lupu, R., Oleic Acid, The Main
Monounsaturated Fatty Acid of Olive Oil, Suppresses Her-2/neu (erb B-2) Expression and Synergistically Enhances The Growth Inhibitory Effects of Trastuzumab (Herceptin) in Breast Cancer Cells with Her-2/neu Oncogene Amplification. Annals of Oncology, 2005. 16: p. 359-371.
114. Simonsen, N.R., Navajas, J.F.C, Martin-Moreno, J.M., Strain, J.J, and et. al. , Tissue Stores of Individual Monounsaturated Fatty Acids and Breast Cancer: the EURAMIC Study. American Journal of Clinical Nutrition, 1998. 68: p. 134-141.
115. Gallardo, V., Munoz, M., and Ruiz, M.A., Formulations of Hydrogels and Lipogels with Vitamin E. Journal of Cosmetic Dermatology, 2005. 4: p. 187-192.
116. Afaq, F., Adhami, V.M., Ahmad, N., and Mukhtar, H., Botanical Antioxidants for Chemoprevention of Photocarcinodenesis. Frontiers in Bioscience, 2002. 7: p. 784-792.
117. Ichihashi, M., Ueda, M., Budiyanto, A., Bito, T., Oka, M., Fukunaga, M., Tsuru, K., and Horikawa, T., UV-Induced Skin Damage. Toxicology 2003. 189: p. 21-39.
118. D'Angelo, S., Ingrosso, D., Migliardi, V., Sorrentino, A., Donnarumma, G., Baroni, A., Masella, L., Tufano, M.A., Zappia, M., and Galletti, P., Hydroxytyrosol, A Natural Antioxidant from Olive Oil, Prevents Protein Damage Induced by Long-Wave Ultraviolet Radiation in Melanoma Cells. Free Radical Biology & Medicine, 2005. 38: p. 908-919.
119. Visioli, F., and Galli, C., Olive Oil Polyphenols and Their Potential effects on Human Health. Journal of Agricultural and Food Chemistry, 1998. 46: p. 4292- 4296.
120. Visioli, F., and Galli, C, Biological Properties of Olive Oil Phytochemicals. Critical Reviews in Food Science and Nutrition, 2002. 42: p. 209 - 221.
Chapter 5 References
________________________________________________________________________ - 119 -
121. Tadros, T., Application of Rheology for Assessment and Prediction of Long-Term Physical Stability of Emulsions. Advances in Colloid and Interface Science, 2004. 108-109: p. 227-258.
122. Morrison, F.A., What is Rheology Anyway?, in The Industrial Physicist 2004, American Institude of Physics: New York. p. 29-31.
123. McClements, D.J., Emulsion Rheology, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton.
124. Barnes, H.A., Hutton, J.F., and Walters, K., What is Rheology?, in An Introduction to Rheology. 1993, Elservier Science Publishers: Netherlands.
125. Barnes, H.A., Hutton, J.F., and Walters, K., Historical Perspective, in An Introduction to Rheology. 1993, Elsevier Science New York.
126. Malkin, A.Y., Introduction. Rheology: subject and language, in Rheology Fundamentals. 1994, ChemTec Publishing.
127. Ramiswamy. Rheology. 1996 [cited 2008 27 October ]; Available from: http://www.rpi.edu/dept/chem-eng/Biotech-Environ/RHEOS/rheos.htm.
128. Muliawan, E.B., and Hatzikiriakos, S.G., Rheology of Mozzarella Cheese. International Dairy Journal, 2007. 17: p. 1063-1072.
129. Sanchez, C., Beauregard, J.L., Chassagne, M.H., Bimbenet, J.J., and Hardy, J., Effects of Processing on Rheology and Structure of Double Cream Cheese. Food Research International, 1995. 28: p. 547-552.
130. Sopade, P.A., Halley, P., Bhandari, B., D'Arcy, B., Doebler, C., and Caffin, N., Application of the Williams-Landel-Ferry Model to the Viscosity-Temperature Relationship of Australian Honeys. Journal of Food Engineering, 2002. 56: p. 67-75.
131. Lazaridou, A., Biliaderis, C.G., Bacandritsos, N., and Sabatini, A.G., Composition, Thermal and Rheological Behaviour of Selected Greek Honeys. Journal of Food Engineering, 2004. 64: p. 9-21.
132. da Costa, C.C., and Pereira, R.G., The Influence of Propolis on the Rheological Behaviour of Pure Honey. Food Chemistry, 2002. 76: p. 417-421.
133. Barnes, H.A., Linear Viscoelastic and Time Effects, in A Handbook of Elementary Rheology. 2000, University of Wales: Aberystwyth. p. 81-105.
134. Brummer, R., A trip Back in Time, in Rheology Essentials of Cosmetic and Food Emulsions. 2006, Springer-Verlag Berlin Heidelberg: Germany. p. 5-13.
135. Lorenzo, G., Zaritzky, N., and Califano, A., Modelling rheological properties of low-in-fat o/w emulsions stabilized with xanthan/guar mixture. Food Research International, 2008. 41: p. 487-494.
136. Liu, H., Xu, X.M., and Guo, Sh.D. , Rheological, texture and sensory properties of low-fat mayonnaise with different fat mimetics. LWT-Food Science and Technology, 2007. 40: p. 946-954.
137. Martinez, I., Riscardo, M., and Franco, J.M., Effect of salt content on the rheological properties of salad dressing-type emulsions stabilized by emulsifier blends. Journal of Food Engineering, 2007. 80: p. 1272-1281.
138. Ahmed, J., Prabhu, S.T., Raghavan, G.S.V., and Ngadi, M. , Physico-chemical, rheological, calorimetric and dielectric behavior of selected Indian honey. Journal of Food Engineering, 2007. 79: p. 1207-1213.
139. Bummer, R., and Godersky, S., Rheological studies to objectify sensations occurring when cosmetic emulsions are applied to the skin. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1999. 152: p. 89-94.
140. Abu-Jdayil, B., Mohameed, H.A., Sa'id, M. and Snobar, T., Rheological characterization of hair shampoo in the presence of dead sea salt. International Journal of Cosmetic Science, 2004. 26: p. 19-29.
Chapter 5 References
________________________________________________________________________ - 120 -
141. Gaspar, L.R., and Maia Campos, P.M.B.G., Rheological behavior and the SPF of sunscreens. International Journal of Pharmaceutics, 2003. 250: p. 35-44.
142. White, D.A., Fisk, I.D., Mitchell, J.R., Wolf, B., Hill, S.E. and Gray, D.A., Sunflower-seed oil body emulsions: Rheology and stability assessment of a natural emulsion. Food Hydrocolloids, 2008. 22: p. 1224-1234.
143. Guaratini, T., Gianeti, M.D., and Campos, P.M.B.G.M., Stability of cosmetic formulations containing esters of Vitamins E and A: Chemical and physical aspects. International Journal of Pharmaceutics, 2006. 327: p. 12-16.
144. Zumalacárregui, L., Vázquez, M., Estévez, T., Aguilera, A., and Hardy, E., Rheological studies of interferon creams. Applied Rheology, 2004. 14: p. 251-255.
145. Di Mambro, V.M., Maia Campos, P.M.B.G., and Fonseca, M.J.V., Physical and chemical stability of different formulations with superoxide dismutase. Pharmazie, 2004. 59: p. 786-790.
146. Jibry, N., Heenan, R.K., and Murdan, S., Amphiphilogels for Drug Delivery: Formulation and Characterization. Pharmaceutical Research, 2004. 21: p. 1852-1861.
147. Karg, R.F., Boozer, C.E., and Benefield, R.E., Injection Molding of Elastomers. Rubber World, 1985. 192: p. 14-19.
148. Macaúbas, P.H.P., and Demarquette, N.R., Interfacial Tension, Morphology and Linear Viscoelasticity Behavior of PP/PS Blends. Polímeros 1999. 9: p. 71-77.
149. Altstaedt, V., Werner, P., and Sandler, J., Rheological, Mechanical and Tribological Properties of Carbon-nanofibre Reinforced Poly (ether ether ketone) Composites. Polímeros, 2003. 13: p. 218-222.
150. Kikic, I., Lapasin, R., Torriano, G., and Papo, A., Processing Intermediates for High-Build Paints. Journal of Coatings Technology 1979. 51: p. 29-33.
151. Nsib, F., Ayed, N., and Chevalier, Y., Matting Agent Concentration and its Effect on the Colour and the Rheology of Matted Coatings. Journal of Applied Sciences, 2008. 8: p. 1527-1533.
152. Martín-Alfonso, M.J., Martínez-Boza, F., Partal, P., and Gallegos, C., Influence of Pressure and Temperature on the Flow Behavior of Heavy Fuel Oils. Rheologica Acta, 2006. 45: p. 357-365.
153. Mothé, C.G., Correia, D.Z., de França, F.P., and Riga, A.T., Thermal and Rheological Study of Polysaccharides for Enhanced Oil Recovery. Journal of Thermal Analysis and Calorimetry, 2006. 85: p. 31-36.
154. Brummer, R., Basic Physical and Mathematical Principles, in Rheology Essentials of Cosmetic and Food Emulsions. 2006, Springer-Verlag Berlin Heidelberg: Leipzig. p. 25-50.
155. Barnes, H.A., What is Flow and Deformation?, in A Handbook of Elementary Rheology. 2000, University of Wales: Aberystwyth. p. 5-10.
156. Murrell, S.A.F., Rheology of the lithosphere - Experimental indications. Tectonophysics, 1976. 24: p. 5-24.
157. Treagus, S.H., and Treagus J.E., Studies of strain and rheology of conglomerates Journal of Structural Geology, 2002. 24: p. 1541-1567.
158. Michibayashi, K., Structural Geology of Peridotie and Rheology of teh Uppermost Mantle. Nihon Reoroji Gakkaishi, 2006. 34: p. 291-300.
159. Schrank, C.E., Boutelier, D.A., and Cruden, A.R., The Analogue Shear Zone: from Rheology to Associated Geometry Journal of Structural Geology, 2008. 30: p. 177-193.
Chapter 5 References
________________________________________________________________________ - 121 -
160. Caldino, V.I.A., Bonola, A. I., and Salgado, M. G., Laboratory determination of the rheological parameters of water-sediment mixtures for the calculation of sludge and debris flows. Ingenieria Hidraulica en Mexico, 2002. 17: p. 27-35.
161. Chu, C.R., Lee, D.J., and Tay, J.H., Gravitational sedimentation of flocculated waste activated sludge. Water Research, 2003. 37: p. 155-163.
162. Dintenfass, L., Influence of plasma proteins on the in vivo and in vitro rheological properties of blood. Clinical Hemorheology, 1985. 5: p. 191-206.
163. Janmey, P.A., Georges, P.C., and Hvidt, S., Basic Rheology for Biologists. Methods in Cell Biology 2007. 83: p. 1,3-27.
164. Mirhosseini, H., Tan, C.P., Hamid, N.S.A., and Yusof, S., Effect of Arabic Gum, Xanthum Gum and Orange Oil Contents on ζ-Potential, Conductivity, Stability, Size Index and pH of Orange Beverage Emulsion. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2008. 315: p. 47-56.
165. Worrasinchai, S., Suphantharika, M., Pinjai, S., and Jamnong, P., β-Glucan Prepared from Spent Brewer's Yeast as A Fat Replacer in Mayonnaise Food Hydrocolloids, 2006. 20: p. 68-78.
166. Robins, M.M., Emulsions - Creaming Phenomena. Current Opinion in Colloid and Interface Science, 2000. 5: p. 265-272.
167. McClements, D.J., Gravitational Separation, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press: Boca Raton.
168. Chanamai, R., and McClements, D.J., Dependence of Creaming and Rheology of Monodisperse Oil-in-Water Emulsions on Droplet Size and Concentration. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2000. 172: p. 79-86.
169. Izquierdo, P., Wiechers, J.W., Escribano, E., García-Celma, M.J., Tadros, T.F., Esquena, J., Dederen, J.C., and Solans, C., A Study on the Influence of Emulsion Droplet Size on the Skin Penetration of Tetracaine. Skin Pharmacology and Physiology, 2007. 20: p. 263-270.
170. Sonneville-Aubrun, O., Simonnet, J.T., and Alloret, F.L'. Nanoemulsions: A New Vehicle for Skincare Products Advances in Colloid and Interface Science, 2004. 108-109: p. 145-149.
171. Partal, P., Guerrero, A., Berjano, M., and Gallegos, C., Influence of Concentration and Teperature on the Flow Behavior of Oil-in-Water Emulsions Stabilized by Sucrose Palmitate. Journal of the American Oil Chemists' Society, 1997. 74: p. 1203-1212.
172. McClements, D.J., Emulsifier Type and Concentration, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 366.
173. Romero, A., Cordobés, F., and Puppo, M.C., Rheology and Droplet Size Distribution of Emulsions Stabilized by Crayfish Flour. Food Hydrocolloids, 2008. 22: p. 1033-1043.
174. Alaimo, M.H., and Kumosinski, T.F., Investigation of Hydrophobic Interactions in Colloidal and Biological Systems by Molecular Dynamics Simulations and NMR Spectroscopy. Langmuir, 1997. 13: p. 2007-2018.
175. McClements, D.J., Hydrophobic Interactions, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton.
176. McClements, D.J., Kinetics, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton. p. 366.
177. Friberg, S.E., Emulsion Stability, in Emulsions- A fundamental and Practical Approach, J. Sjöblom, Editor. 1992, Kluwer Academic Publishers: Dordrecht. p. 1-24.
Chapter 5 References
________________________________________________________________________ - 122 -
178. Friberg, S., Emulsion Stability, in Food Emulsions, S. Friberg, and Larsson, K., Editor. 1997, Marcel Dekker: New York.
179. Sonneville-Aubrun, O., Bergeron, V., Gulik-Krzywicki, T., Jönsson, B., Wenner-ström, H., Linder, P., and Cabane, B., Surfacant Film in Biliquid Foams. Langmuir, 2000. 16: p. 1566.
180. Hazlett, R.D., Stability of Macroemulsions. Colloids and Surfaces, 1988. 29: p. 53-59.
181. Silletti, E., Vingerhoeds, M.H., van Aken, G.A., and Norde, W., Rheological Behavior of Food Emulsions Mixed with Saliva: Effect of Oil Content, Salivary Protein Content, and Saliva Type. Food Biophysics, 2008: p. Article in Press.
182. Brauss, M.S., Linfoth, R.S.T., Cayeux, I., Harvey, B., and Taylor, A.J., Altering the Fat Content Affects Flavor Release in a Model Yogurt System. Journal of Agricultural and Food Chemistry, 1999. 47: p. 2055-2059.
183. Malone, M.E., and Appelqvist, I.A.M., Gelled Emulsion Particles for the Controlled Release of Lipophilic Volatiles During Eating. Journal of Controlled Release, 2003. 90: p. 227-241.
184. Malone, M.E., Appelqvist, I.A.M., and Norton, I.T., Oral Behaviour of Food Hydrocolloids and Emulsions. Part 2. Taste and Aroma Release. Food Hydrocolloids, 2003. 17: p. 775-784.
185. Bayárri, S., Smith,T., and Hollowood, T., The Role of Rheological Behaviour in Flavour Perception in Model Oil/Water Emulsions. European Food Research and Technology, 2007. 226: p. 161-168.
186. Leiberman, H.A., Reiger, M.M., and Banker, G.S., Pharmaceutical Dosage Forms: Disperse Systems. Vol. 2. 1989, New York: Merchel Dekker.
187. Ahmad, K., Ho, C.C., Fong, W.K., and Toji, D., Properties of Palm oil-in-Water Emulsions Stabilized by Nonionic Emulsifiers. Journal of Colloid and Interface Science, 1996. 181: p. 595-604.
188. Ho, C.C., and Ahmad, K., Electrokinetic Behavior of Palm Oil Emulsions in Dilute Electrolyte Solutions. Journal of Colloid and Interface Science, 1999. 216: p. 25-33.
189. Klinkesorn, U., Sophanodora, P., Chinachoti, P., and McClements, D.J., Stability and Rheology of Corn Oil-in-Water Emulsions Containing Maltodextrin Food Research International, 2004. 37: p. 851-859.
190. Hsu, J.P., and Nacu, A., Behavior of Soybean Oil-in-Water Emulsion Stabilized by Nonionic Surfactant Journal of Colloid and Interface Science, 2003. 259: p. 374-381.
191. Marinova, K.G., Alargova, R.G., Denkov, N.D., Velev, O.D., Petsev D.N., Ivanov, I.B., and Borwankar, R.P. , Charging of Oil-Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions. Langmuir, 1996. 12: p. 2045-2051.
192. Ho, O.B., ed. Surfactant-Stabilized Emulsions from an Electrokinetic Perspective. Interfacial Electrokinetics and Electrophoresis, ed. Á.V. Delgado. Vol. 106. 2002, Marcel Dekker, Inc.: New York. 991.
193. Chibowski, E., and Wiącek, A., ed. Electrokinetics of n-Alkane Oil-in-Water Emulsions. Interfacial Electrokinetics and Electrophoresis, ed. Á.V. Delgado. Vol. 106. 2002, Marcel Dekker, Inc.: New York. 907-931.
194. Chibowski, E., and Waksmundzki, A., A Relationship Between the Zeta Potential and Surface Free Energy Changes of Sulfur/n-Heptane-Water System. Journal of Colloid and Interface Science, 1978. 66: p. 213-219.
195. Eastman, J., Colloid Staility. Colloid Science: Principles, Method and Applications, ed. T. Cosgrove. 2005, Oxford: Blackwell Publishing Ltd. 36-49.
Chapter 5 References
________________________________________________________________________ - 123 -
196. deMan, J.M., Texture, in Principles of Food Chemistry 1999, Aspen Publishers, Inc.: Maryland. p. 311-312.
197. Romaowski, P.a.S., R., Stability Testing of Cosmetic Products, in Handbook of Cosmetic Science and Technology, A.O. Barel, Paye, M., and Maibach, H.I.,, Editor. 2001, Marcel Dekker, Inc.: New York. p. 771-773.
198. Ekong, E.A., Melbouci, M., Lusvardi, K., and Erazo-Majewicz, P.E., Rheological Additives and Stabilizer, in Handbook of Cosmetic Science and Technology, A.O. Barel, Paye, M., and Maibach, H.I., , Editor. 2001, Marcel Dekker: New York. p. 377-378.
199. Simeone, M., Alfani, A., and Guido, S., Phase diagram, rheology and interfacial tension of aqueous mixtures of Na-caseinate and Na-alginate. Food Hydrocolloids, 2004. 18: p. 463-470.
200. Thaiudom, S., and Goff, H.D., Effect of k-Carrageenan on Milk Protein Polysaccharide mixtures. International Dairy Journal, 2003. 13: p. 763-771.
201. Pal, R., Viscoelastic Properties of Polymer-Thinkened Oil-in-Water Emulsions. Chemical Engineering Science, 1996. 51: p. 3299-3305.
202. Vélez, G., Fernández, M.A., and Muñoz, J., Role of Hydrocolloids in the Creaming of Oil in Water Emulsions. Journal of Agricultural and Food Chemistry, 2003. 51: p. 265-269.
203. Barnes, H.A., The Yield Stress - A Review or Pialphanutaualpha Rhoepsiloniota - Everything Flows? Journal of Non-Newtonian Fluid Mechanics, 1999. 81: p. 133- 178.
204. Hamill, R.D., and Petersen, R.V., Effects of Aging and Surfactant Concentration on the Rheology and Droplet Size Distribution of a Nonaqueous Emulsion. Journal of Pharmaceutical Sciences, 1966. 55: p. 1268-1277.
205. Barnes, H.A., Rheology of Emulsions - A Review. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994. 91: p. 89-95.
206. Akhtar, M., Stenzel, J., Murray, B.S., and Dickinson, E., Factors Affecting the Perception of Creaminess of Oil-in-Water Emulsions. Food Hydrocolloids, 2005. 19: p. 521-526.
207. Macosko, C.W., Rheology: Principles, Measurements and Applications. 1994, New York: VCH Publishers.
208. McClements, D.J., Hydrodynamic Interactions and Nonequilibrium Effects, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Boca Raton.
209. Pal, R., Rheology of Emulsions Containing Polymeic Liquids, in Encyclopedia of Emulsion Technology, P. Becher, Editor. 1996, Marcel Dekker: New York.
210. Krieger, I.M., Rheology of Monodisperse Latices. Advances in Colloid and Interface Science, 1972. 3: p. 111-136.
211. Goodwin, J.W., and Hughes, R.W., Rheology for chemists: an itroduction. 2000, UK: The Royal Society of Chemistry.
212. Derkatch, S.R., Levochov, S.M., Kuhkushkina, A.N., Novosyolova, N.V., Kharlav, A.E., and Matveenko, V.N., Rheological Properties of Concentrated Emulsions Stabilized by Globular Protein in the Presence of Nonionic Surfactant. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007. 298: p. 225-234.
213. Saiki, Y., Prestidge, C.A., and Horn, R.G., Effects of Droplet Deformability on Emulsion Rheology. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007. 299: p. 65-72.
Chapter 5 References
________________________________________________________________________ - 124 -
214. McClements, D.J., Major Factors that Determine Emulsion Rheology, in Food Emulsions: Principles, Practice, and Techniques. 1999, CRC Press LLC: Florida.
215. Howe, A.M., and Clarke, A., Viscosity of Emulsions of Polydisperse Droplets with a Thick Adsorbed Layer. Langmuir, 1997. 13: p. 2617-2626.
216. Pal, R., Shear Viscosity Behaviour of Emulsions of Two Immiscible Liquids. Journal of Colloid and Interface Science, 2000. 225: p. 359-366.
217. Berli, C.L.A., Rheology and Phase Behavior of Aggregating Emulsions Related to Droplet-droplet Interactions. Brazilian Journal of Chemical Engineering, 2007. 24: p. 203-210.
218. Bais, D., Trevisan, A., Lapasin, R., Partal, P., and Gallegos, C., Rheological Characterization of Polysaccharide-Surfactant Matrices for Cosmetic O/W Emulsions. Journal of Colloid and Interface Science, 2005. 290: p. 546-556.
219. Tadros, T.F., Rheological Properties of Emulsion Systems, in Emulsions- A Fundamental and Practical Approach, J. Sjoblom, Editor. 1992, Kluwer Academic Publishers: Netherlands. p. 173-188.
220. Brummer, R., Oscillatory Measurements, in Rheology Essentials of Cosmetic and Food Emulsions. 2006, Springer-Verlag Berlin Heidelberg: Heidelberg. p. 101-117.
221. Sánchez, M.C., Berjano, M., Guerrero, A., and Gallegos, C., Emulsification Rheokinetics of Nonionic Surfactant-Stabilized Oil-in-Water Emulsions. Langmuir, 2001. 17: p. 5410-5416.
222. Thorgeirsdóttir, T.Ó., Thormar, H., and Kristmundsdóttir, T., Viscoelastic Properties of a Virucidal Cream Containing the Monoglyceride Monocaprin: Effects of Formulation Variables: A Technical Note. AAPS PharmSciTech, 2006. 7: p. Article 44.
223. Ma, L., and Barbosa-Cánovas, G.V., Rheological Characterization of Mayonnaise. Part II: Flow and Viscoelastic Properties at Different Oil and Xanthum Gum Concentrations. Journal of Food Engineering, 1995. 25: p. 409-425.
224. Liu, H., Xu, X.M., and Guo, Sh.D., Rheological, Texture and Sensory Properties of Low-Fat Mayonmaise with Different Fat Mimetics. LWT-Food Science and Technology, 2007. 40: p. 946-954.
225. Tadros, T.F., Correlation of Viscoelastic Properties of Stable and Flocculated Suspensions with Their Interparticle Interactions. Advances in Colloid and Interface Science, 1996. 68: p. 97-200.
226. Mewis, J., and Macosko, C.W., Suspension Rheology, in Rheology: Principles, Measurements, and Applications, C.W. Macosko, Editor. 1994, VCH Publishers, Inc.: New York. p. 425-474.
227. Song, K.W., Kuk, H.Y. and Chang, G.S., Rheology of concentrated xanthan gum solutions: Oscillatory shear flow behavior. Korea-Australia Rheology Journal, 2006. 18(2): p. 67-81.
228. Pelletier, E., Viebke, C., Meadows J. and Williams, P.A. , A rheological study of the order-disorder conformational transition of xanthan gum. Biopolymers, 2001. 59: p. 339-346.
229. Tadros, T.F., Applications of Surfactants in Emulsion Formation and Stabilisation, in Applied Surfactants: Principles and Applications. 2005, Wiley-VCH Verlag GmbH & Co. KGaA: Germany. p. 115-185.
APPENDIX
Appendix
________________________________________________________________________ - 125 -
Publication
1. Tan, H.W., Misran, M., Stability of Concentrated Olive Oil-in-Water Emulsion.
Chinese Journal of Chemistry, 2008, 26: p. 1963- 1968
Conference
1. Tan, H.W., Misran, M., Ageing Effect on Rheological Properties of Concentrated
Olive O/W Emulsion. The 2nd Penang International Conference for Young Chemist,
18-20 June 2008, Penang, Malaysia.