interfacial composition, structural and thermodynamic parameters of water/(surfactant+ n butanol)/ n...

ORIGINAL ARTICLE

Interfacial Composition, Structural and ThermodynamicParameters of Water/(Surfactant+n-Butanol)/n-HeptaneWater-in-Oil Microemulsion Formation in Relationto the Surfactant Chain Length

Mrinmoy De • Subhash C. Bhattacharya •

Satya P. Moulik • Amiya K. Panda

Received: 9 July 2009 / Accepted: 1 February 2010 / Published online: 10 March 2010

� AOCS 2010

Abstract Interfacial behavior, structural and thermody-

namic parameters of a water/(surfactant?n-butanol)/

n-heptane water-in-oil (w/o) microemulsion have been

investigated using the dilution technique at different tem-

peratures, and [water]/[surfactant] mole ratios. The cationic

surfactants used were alkyltrimethyl ammonium bromides

(CnTAB, n = 10, 14 and 16) while the nonionic surfac-

tants were polyoxyethylene (20) sorbitan monoalkanoates

(polysorbate), viz., palmitate (PS 40), stearate (PS 60) and

oleate (PS 80). The distribution of cosurfactant between the

oil–water interface and the bulk oil at the threshold level of

stability, and the thermodynamics of transfer of the

cosurfactant from the bulk oil to the interface were eval-

uated. Structural parameters such as the dimensions, pop-

ulation density and effective water pool radius of the

dispersed water droplets in the oil phase and the interfacial

population of the surfactant and cosurfactant have been

evaluated in terms of the surfactant chain length.

Keywords Water-in-oil microemulsion � Surfactant �Cosurfactant � Structural parameter �Thermodynamic parameters � Dilution technique

Introduction

Microemulsions are isotropic, thermodynamically stable

dispersions of oil in water (o/w) or, vice versa (w/o), stabilized

by a surfactant monolayer at the water/oil interface [1–9].

Also there are some bicontinuous microemulsions which are

able to exhibit exceptional properties like the highest solubi-

lization and lowest tension. The present study is dedicated to

the water-in-oil microemulsion. Usually, single-chained

surfactants require a cosurfactant for the stabilization of such

dispersions [4]. Short chain alkanols and amines are quite

often used as cosurfactants. A cosurfactant reduces the

bonding stress during the formation of a dispersed phase.

Studies on microemulsions have been the focus of attention

over a considerable period of time due to their multifaceted

application potentials [10–12]. With the advent of newer

techniques, the scope for varied physicochemical studies are

on the increase. The potential techniques include Small

Angle X-ray Scattering (SAXS), Small Angle Neutron

Scattering (SANS), Quasi Elastic Light Scattering (QELS),

also know as Dynamic Light Scattering (DLS), Nuclear

Magnetic Resonance (NMR), Time Resolved Fluorescence

Quenching (TRFQ), diffusion, electrical conductance,

solvation dynamics, etc. [12–17]. In addition to all the

cited techniques, a relatively simple and unsophisticated

method that hardly requires any instrumentation can be

used to gather very fundamental information on the forma-

tion of w/o microemulsions. This is the method of dilution

[13, 14, 17–19], which is advantageous because of its sim-

plicity. In a w/o microemulsion containing a cosurfactant,

the water phase is linked with the oil by a surfactant mono-

layer at the interface [20]. On the other hand, the cosurfactant

essentially partitions between the oil phase and the interfa-

cial region; it may even moderately or weakly partition

between the water phase and the interface as well. A stable

M. De � A. K. Panda (&)

Department of Chemistry, University of North Bengal,

Darjeeling 734 013, West Bengal, India

e-mail: akpanda1@yahoo.com

M. De � A. K. Panda

Department of Chemistry, Behala College,

Kolkata 700 060, West Bengal, India

M. De � S. C. Bhattacharya � S. P. Moulik

Centre for Surface Science, Department of Chemistry,

Jadavpur University, Kolkata 700032, West Bengal, India

123

J Surfact Deterg (2010) 13:475–484

DOI 10.1007/s11743-010-1186-7

clear four component (water, surfactant, cosurfactant and

oil) microemulsion, can be destabilized by extra addition of

oil to make it inhomogeneous and turbid. To satisfy partition

requirement there is a depletion of cosurfactant at the oil

water interface. Adequate addition of the cosurfactant helps

to restore the system to a renewed state of thermodynamic

equilibrium. This procedure of stabilization and destabili-

zation can be performed a number of times while noting the

amounts of oil and cosurfactants required to achieve the

above described process. By this simple procedure, the dis-

tribution of the cosurfactant between the oil and interfacial

regions can be ascertained to evaluate the thermodynamics

of the microemulsion formation process [1, 21, 22]. Such an

evaluation by other chemical and physical methods is not so

easy. For details of the process and its efficacy we refer to the

works published in the literature [20, 23–25]. The thermo-

dynamics of transfer of the cosurfactant from the oil to the

interface can be quantitatively understood with the help of

mathematical relations. In addition, the structural informa-

tion on the nanodispersions of water in the oil continuum can

also be gathered [1, 21, 22].

In most of the reported studies, different kinds of oils,

surfactants, and cosurfactants have been used. Systematic

investigations with cosurfactant variation have seldom

been done. Recently, Digout et al. [26] have attempted to

understand the effect of variations of oil and cosurfactant

chain lengths on w/o microemulsion formation. In our

previous work, the effect of surfactant head-group variation

has been studied considering cationic, anionic and nonionic

surfactants having an identical alkyl chain length [27]. To

the best of our knowledge, detailed study on microemul-

sion formation by the method of dilution using surfactants

of the same homologous series is unexplored.

In the present study, the dilution method has been

employed to derive different physicochemical parameters

for the formation of water/(surfactant?n-butanol)/n-hep-

tane w/o microemulsions wherein both cationic and non-

ionic surfactants of two series have been used. The

cationics were decyl (C10), tetradecyl (C14) and hexadecyl

(C16) trimethyl ammonium bromides, and the nonionics

polyoxyethylene (20) sorbitan monopalmitate, monostea-

rate and monooleate. The dilution results have been ana-

lyzed and processed to understand the alkyl chain length

effects on the phenomenon of microemulsion formation in

the studied micro heterogeneous systems.

Experimental

Materials

The surfactants decyltrimethyl ammonium bromide (C10TAB),

tetradecyltrimethyl ammonium bromide (C14TAB),

hexadecyltrimethyl ammonium bromide (C16TAB), and three

polysorbates (PS) or sorbitan monoalkanoates esters with 20

ethylene oxide groups per molecule, i.e., monopalmitate,

monostearate and monooleate (respectively, abbreviated

PS40, PS60 and PS80 in what follows), were products from

Sigma Chemicals, USA. They were fairly pure (cationics were

[99% pure) and were further purified using standard pro-

cedures [28–30]. The surfactants did not produce a minimum

in their surface tension (c) versus concentration plots [31–33].

HPLC grade n-heptane and n-butanol were purchased from

Merck, India. Doubly distilled water with a specific resistivity

of 18 MX cm was used during the experiments.

Methods

At a given temperature, water and surfactant at a fixed

[water]/[surfactant] ratio x, were placed in a dry test tube

with a known quantity of n-heptane and then placed in a

thermostated water bath (of accuracy ±0.1 K). The viscous

and turbid solution was then titrated with n-butanol under

stirring to make it just clear. The system was given suffi-

cient time to attain equilibrium; once turbidity had disap-

peared, it was not restored. The volume of n-butanol

required to make the stable microemulsion was noted. Then

a known amount of n-heptane was added to destabilize the

microemulsion. The turbid solution thus formed was then

again titrated with n-butanol until it appeared just clear as

stated above. The volume of n-butanol added was noted

again. This stabilization–destabilization procedure was

repeated several times noting the volumes of n-butanol and

n-heptane at each occasion [20, 34, 35]. The entire pro-

cedure was carried out at different x (5, 10, 15, 20, 25) and

at different temperatures (303, 308, 313, 318 and 323 K).

Each set of experiments was repeated twice and the aver-

age values were used for data processing and analysis.

Theoretical Backgrounds

Evaluation of Thermodynamic Parameters

For a stable microemulsion, the total number of moles of

alkanol (nat ) is [20, 26];

nta ¼ nw

a þ nia þ no

a ð1Þ

where naw, na

i and nao represent the number of moles of

alkanol in water, at the interface and in oil, respectively.

Other conditions remaining the same, the ratios (1)

between the number of moles of cosurfactant (alkanol) in

oil (nao) and the number of moles of oil (no), and (2)

between the mole fraction of alkanol at the interface

(Xai ) and that in the oil (Xa

o) are fixed. For a stable

equilibrium,

476 J Surfact Deterg (2010) 13:475–484

123

k ¼ noa

no

ð2Þ

and Kd ¼Xi

a

Xoa

ð3Þ

With the help of Eq. 2, Eq. 1 can be rewritten as

nta ¼ nw

a þ nia þ kno

or;nt

a

ns

¼ nwa þ ni

a

ns

þ kno

ns

ð4Þ

where ns is the number of moles of surfactant present in the

system.

From Eq. 4, ifnt

a

nsis plotted against no

nsthen k and

nwa þni

a

nscan

be obtained from the intercept (I) and the slope (S) of the

straight line, respectively. naw is generally obtained from the

solubility of the cosurfactant (alkanol) in water [34]. The

aqueous solubility of higher alkanols than n-butanol is very

small and hence for those alkanols, naw = 0 [34, 36].

Equation 3 can also be written in terms of I and S as:

Kd ¼Xi

a

Xoa

¼ nia=ðni

a þ nsÞno

a=ðnoa þ noÞ

¼ni

að1þno

a

noÞ

noa

noðni

a þ nsÞð5Þ

or; Kd ¼að1þ SÞ

S½1þ ðI � nwa

nsÞ�¼ að1þ SÞ

Sð1þ aÞ ð6Þ

where, a ¼ ðI � nwa

ns

Þ ¼ nia

ns

ð7Þ

Kd can easily be evaluated if I, S and a are known. For

higher alkanols than n-butanol naw = 0 and a = I, then [27]

Kd ¼Ið1þ SÞSð1þ IÞ ð8Þ

The standard Gibbs free energy of transfer (DGto) of

alkanol from the oil phase to the interfacial phase is as

follows:

DGot ¼ �RT ln Kd ð9Þ

where, ideal behavior is assumed and the concentration is

expressed on the mole fraction scale.

Standard enthalpy changes of transfer (DHto) and entropy

changes of transfer (DSto) are given by the relations,

oðDGot =TÞ

oð1=TÞ

� �p

¼ DHot ð10Þ

and DSot ¼

DHot � DGo

t

Tð11Þ

However, for a nonlinear dependence of DGto on T, a

two degree polynomial equation (shown below) can be

used to get DHto [27]:

DGot ¼ aþ bT þ cT2 ð12Þ

where a, b and c are the polynomial coefficients.

The differential form of Eq. 12 helps us to evaluate

DHto. Thus,

oðDGot =TÞ

oð1=TÞ

� �p

¼ a� cT2 ¼ DHot ð13Þ

Evaluation of Structural Parameters

The structural parameters of the nanodispersion of water in

a w/o microemulsion can be determined using a simplified

structural model assuming monodispersion of the droplets

[18, 26, 37, 38].

The total volume of the dispersed microemulsion

droplets (Vd) is related in the following way:

Vd ¼4

3pR3

eNd ð14Þ

with Nd (total number of droplets) and Re (their effective

radius). Vd also follows the relation:

Vd ¼ VH2O þ VS þ V ia ð15Þ

where VH2O, Vs and Vai are the volumes of water, surfactant

and cosurfactant at the interface, respectively. These vol-

umes are related to their masses with their respective

densities [20, 39].

The total droplet surface area (Ad) is expressed as:

Ad ¼ 4pR2eNd ¼ ðnsAs þ ni

aAaÞNA ð16Þ

where AS and Aa are the polar head group areas of the

surfactant and the alkanol, respectively, and NA is the

Avogadro constant.

The values of Re and Nd are calculated from the fol-

lowing equations:

Re ¼3Vd

Ad

ð17Þ

and Nd ¼3Vd

4pR3e

ð18Þ

In a reversed micelle, the average aggregation number

of the surfactant (Ns) and the cosurfactant (Na) are obtained

from

Ns ¼nsNA

Nd

ð19Þ

Na ¼ni

aNA

Nd

ð20Þ

The effective radius of the water droplet (Rw) of

the dispersed phase including the contributions of the

amphiphile and the cosurfactant head groups is the

following,

J Surfact Deterg (2010) 13:475–484 477

123

Rw ¼VH2O þ Vh

s þ Vha

Vd

� �1=3

Re ð21Þ

where Vsh and Va

h are the volumes of the head groups of the

surfactant and the alkanol, respectively. They obey the

following relations [40]:

Vhs ¼

4

3p1=2A3=2

s Ns ð22Þ

Vha ¼

4

3p1=2A3=2

a Na ð23Þ

Results and Discussion

Thermodynamics of Dilution

Figure 1 depicts the representative linear plots ofnt

a

nsversus

no

nsfor the dilution experiments with C14TAB (Fig. 1a), and

PS40 (Fig. 1b) in terms of Eq. 4. For C14TAB the slopes

and the intercepts increase with x, whereas for PS40 the

slopes remain nonvariant but the intercepts progressively

increase with x. With increasing water pool size, more

cosurfactant is required to form a stable microemulsion

[26, 27]. Higher pool size also requires more naw for the

reason of solubility. The slopes representno

a

noi.e., the dis-

tribution of alkanol between the oil/water interface and oil

which is a constant (c.f. Eq. 2). The observed results sug-

gest increased solubility requirement of the alkanol in the

oil as x increases when C14TAB is the surfactant. For PS40

the solubility requirement remains unchanged. Nonionics

have relatively bulkier head groups (polyoxyethylene sor-

bitan moieties). With the increase in water pool size the

head groups can ‘‘open up’’ or uncoil or unfold which

requires a higher number of n-alkanols at the interface, as

the unfolded hydrophilic head group cannot provide better

coverage at the oil–water interface. On the other hand,

cationic surfactants (CnTAB) having a specific head group

area could not undergo such processes as is the case in

polysorbates. Hence Kd values for cationic surfactants do

not change significantly with the increase in x. In one of

our previous studies, we explored the dilution behavior on

different surfactants having a similar chain length with

different head groups [27]. Results are reflected through the

Kd values as represented in Table 1.

It can be seen (Fig. 2a, b) that, at all temperatures, Kd

declines faster with x for C14TAB than for PS40. But at all x,

the Kd increases faster with temperature for PS40 than for

C14TAB, where the dependence is very mild. These phe-

nomena are the same for all the cationic and nonionic repre-

sentatives. For a fixed x, Kd, on the whole, increases with

temperature (Table 1) except for the system containing

C14TAB, where Kd mildly declines with temperature. This out

of trend phenomenon needs corroboration by further studies.

For both categories of surfactants, the spontaneity of the

transfer increases with the surfactant chain length. This is

related to HLB, which declines with increasing chain

length [41]. The hydration/dehydration of the nonionic

surfactant head groups is fairly susceptible to temperature

in comparison with the cationics, which causes a difference

between the two. The observed DGto (kJ mol-1) values

vary in the range of—(2.5–7.6). Similar variations were

also reported in the ranges of—(3.1–4.5) by Bansal et al.

[18],—(5.5–9.9) by Gerbacia and Rosano [21],—(4.5–5.1)

by Kumar et al. [22],—(5.0–6.0) by Birdi [23], and—(4.0–

8.0) by Singh et al. [38]. A non-linear (two degree poly-

nomial) variation of DGto with surfactant chain length was

followed with both the cationic and the nonionic surfac-

tants. The presence of unsaturation in PS80 has little effect

on this trend. For both classes of surfactants, the enthalpy

values are found to be positive; the processes are endo-

thermic in nature (excepting for C10TAB at 303 K) while

the changes in entropy are largely positive. The processes

are fairly controlled by entropy, particularly for the non-

ionic surfactants-derived formulations. Digout et al. [26]

have reported that the presence of the surfactant at the oil–

water interface can make an entropic contribution to reduce

the energy of transfer of alkanols between the bulk oil and

the interfacial region. For cationic surfactants, the extent of

endothermicity decreases with x. However, temperature

does not produce a trend in their variation for the cationic

surfactants. For nonionic surfactants, DGto weakly varies

with temperature whereas both DHto and DSt

o increase

constantly with temperature.

Figure 3 presents the dependencies of DHto and DSt

o on

surfactant chain length (i.e., the number of carbon atoms or

Cn) at various temperatures at a constant x = 15. Both

Fig. 1 Plot of nat /ns versus no/ns according to Eq. 4 for water/

(surfactant?n-butanol)/heptane w/o microemulsion systems at 313 K

and at different x. Surfactants: a C14TAB; b PS40. x: open circle, 5;

open triangles, 10; open squares, 15; inverted triangles, 20 and opendiamonds, 25

478 J Surfact Deterg (2010) 13:475–484

123

DHto and DSt

o curves cross at C14; the sequences for the

cationics just become reversed after C14, which is the

crossing point at all x values (the results other than

x = 15 not shown). It would be interesting if results with

homologues like C10 and C18 were available. This remains

to be examined in a future study.

Table 1 Thermodynamic parameters for the transfer of n-butanol from the oil phase to the interface in water/(surfactant?n-butanol)/heptane w/

o microemulsions at different temperatures

Surfactant Temp./K Kd -DGtO/kJ mol-1 DHt

O/kJ mol-1 DStO/Jmol-1K-1

C10TAB 303 4.86 3.98 -0.44 11.68

308 4.89 4.06 0.74 15.58

313 4.87 4.12 1.95 19.40

318 5.03 4.27 3.18 23.41

323 5.09 4.37 4.43 27.22

C14TAB 303 9.47 5.66 2.82 9.39

308 9.13 5.66 2.82 9.24

313 9.02 5.72 2.82 9.28

318 9.01 5.81 2.82 9.42

323 8.74 5.82 2.82 9.30

C16TAB 303 11.41 6.13 5.08 37.00

308 11.95 6.35 3.98 33.54

313 11.71 6.40 2.86 29.59

318 12.43 6.66 1.72 26.36

323 12.23 6.72 0.56 22.54

PS40 303 2.85 2.64 0.82 11.40

308 2.75 2.59 3.55 19.94

313 3.07 2.92 6.32 29.52

318 3.03 2.93 9.14 37.98

323 3.29 3.20 12.01 47.08

PS60 303 3.74 3.32 3.69 23.16

308 3.97 3.53 3.95 24.29

313 4.04 3.63 4.21 25.07

318 3.95 3.63 4.48 25.49

323 4.28 3.90 4.75 26.78

PS80 303 3.51 3.16 8.90 39.80

308 3.60 3.28 10.37 44.31

313 4.01 3.61 11.86 49.43

318 4.36 3.89 13.37 54.29

323 4.45 4.01 14.91 58.60

[Water]/[surfactant] mole ratio, x = 15

Fig. 2 Interdependence of

-DGto on x and T for water/

(surfactant?n-butanol)/heptane

w/o microemulsion systems.

Surfactants: a C14TAB; b PS40.

Temp. (K): open squares, 303;

filled squares, 308; open circles,

313; filled triangles, 318 and

open triangles, 323

J Surfact Deterg (2010) 13:475–484 479

123

According to a previous report [27], DSto decreased with

increasing x both for ionic and nonionic surfactants for the

formation of microemulsions using alkanol as cosurfac-

tants. The comparative increase in DSto when the rise in

temperature was low. In this work, we have observed

almost unchanged DSto with temperature for C14TAB, its

lowering for C16TAB and increasing for C10TAB. The

observed trends remain unaccountable. The DHto values for

the cationics also follow the same trend as with DSto. For all

the nonionics DSto values clearly increase with temperature.

The DHto and DSt

o produce compensations with good cor-

relation for both classes of surfactants (Fig. 4), which was

also reported by others [42, 43]. The compensation tem-

peratures were 280 and 308 K obtained for the cationic and

the nonionic surfactant systems, respectively. The linear

compensation features comprising results at all x values

are extra thermodynamic correlation phenomenon.

Structural Parameters

The structural parameters computed from the experimental

data at x = 15 using Eqs. 14–23 are presented in Table 2

and Figs. 5, 6, 7. For both kinds of surfactants, Re increases

linearly with the surfactant chain length, also with x (as

expected). On the overall basis Re decreases with increas-

ing temperature (as shown in Fig. 5); the values range

between 1.66 and 3.30 nm in the temperature range of

303–323 K. They are comparable with the earlier reported

values for other systems [35, 37, 42]. The ratio of Re/Rw is

found to be independent of temperature, x, and surfactant

chain length (as revealed from Table 2). On average, the

Re/Rw ratio is found to be 1.55 for the ionic surfactants, and

2.60 for the nonionics. They were higher than in our earlier

report [27]. The types of surfactants used produce this

difference. For ionic surfactants a 50% reduction in droplet

size was noted while for nonionic surfactants it was only

17%.

The effective radii of the droplets decrease with the rise

in temperature and hence their number density, Nd,

increases. The Nd values decrease with increasing x for the

cationic systems but increase for the nonionic formulations

(Fig. 6). The latter results were not expected. This matter

requires further examination through experiments such as

dynamic light scattering measurements.

The population of surfactant (Ns) and cosurfactant (Na)

per droplet at the oil–water interface is found to decrease

with a temperature rise, whereas the Nd values increase.

Na–x–T profiles for C14TAB and PS40-containing micro-

emulsion are shown in Fig. 7 as representative plots.

Results are also summarized in Table 2. The results agree

with the literature reports [35, 37, 42]. For stabilization, in

the cationic surfactant-derived microemulsion systems,

Na [ Ns whereas Na � Ns in the nonionic-derived systems

(Table 2). These findings corroborate the existing literature

reports [35, 37, 42]. Like Re/Rw, temperature does not

affects the ratio Na/Ns either as shown in Table 2. For ionic

surfactants, however, Na/Ns decreases from 5.0 to 3.0 with

the variation of the surfactant from C10TAB to C16TAB.

Longer tail surfactants thus produce a better surface cov-

erage compared with the shorter analogues. However, for

the nonionic surfactants, such dependence was not found,

and Na/Ns remains almost constant at 35. Their bulkier

head group was considered to be responsible for producing

such an effect. For a clear elucidation further investigation

is warranted.

Conclusions

Dilution experiments for water/(surfactant?n-butanol)/

n-heptane w/o microemulsion at various temperatures and

[water]/[surfactant] mole ratios x, were performed. Two

sets of surfactants of the homologous series, alkyltrimethyl

ammonium bromides and polysorbates were used in the

Fig. 3 Variation of a DHto with surfactant carbon number on its

chain(Cn) and b DSto with the surfactant carbon number on its

chain(Cn) at different temperatures (K) at x = 15 for water/

(CnTAB(n = 10,14,16) ? n-butanol)/heptane w/o microemulsion

systems. Temp. (K): open squares, 303; filled squares, 308; opencircles, 313; filled circles, 318 and open triangles, 323

Fig. 4 DHto and DSt

o profiles for two different water/(surfactants?

n-butanol)/heptane w/o microemulsion systems. All x values (5, 10,

15, 20 and 25) have been used in the plots. a open circles, C10TAB;

open triangles, C14TAB; open squares, C16TAB; b inverted triangles,

PS40; diamonds, PS60 and filled circles, PS80. Compensation

temperatures for surfactants (K): Cationic, 292 and nonionic, 309

480 J Surfact Deterg (2010) 13:475–484

123

Table 2 Structural parameters of water/(CnTAB?n-butanol)/heptane w/o microemulsions under varied conditions

Surfactant Temp./K Re(Rw)/nm Re/Rw (Re-Rw)/nm 10-18 Nd (per mL) Na(Ns) (per droplet) Na/Ns

A. Cationic surfactants

C10TAB 303 2.92(1.86) 1.56 1.06 6.65 310(65) 4.8

308 2.58(1.63) 1.58 0.95 9.93 222(43) 5.2

313 2.23(1.41) 1.58 0.82 15.29 142(28) 5.1

318 1.90(1.19) 1.60 0.71 25.77 93(17) 5.5

323 1.60(1.01) 1.58 0.59 42.30 54(10) 5.4

C14TAB 303 3.29(2.18) 1.51 1.11 3.69 343(97) 3.5

308 2.87(1.91) 1.50 0.96 5.50 223(65) 3.4

313 2.41(1.59) 1.51 0.82 9.52 137(38) 3.6

318 1.99(1.32) 1.51 0.67 16.54 75(22) 3.4

323 1.64(1.09) 1.50 0.55 29.71 43(12) 3.6

C16TAB 303 3.45(2.29) 1.50 1.16 2.99 362(110) 3.3

308 3.00(2.00) 1.50 1.00 4.47 231(74) 3.1

313 2.50(1.66) 1.50 0.84 7.79 136(42) 3.2

318 2.05(1.37) 1.49 0.68 13.88 73(24) 3.0

323 1.68(1.12) 1.50 0.56 25.31 39(13) 3.0

B. Nonionic surfactants

PS40 303 2.65(1.01) 2.62 1.64 9.86 354(10) 35.4

308 2.53(0.96) 2.63 1.57 11.34 307(8) 38.4

313 2.42(0.91) 2.65 1.51 13.21 269(7) 38.4

318 2.32(0.89) 2.61 1.43 14.28 233(7) 33.3

323 2.22(0.84) 2.64 1.38 16.94 210(6) 35.0

PS60 303 2.69(1.03) 2.61 1.66 8.84 358(10) 35.8

308 2.54(0.97) 2.61 1.57 10.89 310(8) 38.8

313 2.44(0.95) 2.56 1.49 11.34 265(8) 33.1

318 2.34(0.93) 2.52 1.41 12.33 224(7) 32.0

323 2.23(0.88) 2.53 1.35 14.23 195(6) 32.5

PS80 303 2.67(1.02) 2.62 1.65 9.34 357(10) 35.7

308 2.55(0.97) 2.63 1.58 10.62 309(9) 34.3

313 2.43(0.92) 2.64 1.51 12.58 271(7) 38.7

318 2.33(0.89) 2.62 1.44 13.96 236(7) 33.7

323 2.23(0.86) 2.50 1.37 15.21 203(6) 33.8

At x = 15

Fig. 5 Dependence of Re on

x and T for water/(surfactants?

n-butanol)/heptane w/o

microemulsion systems at all

x values (5,10,15,20 and 25).

Surfactants: a C14TAB; b PS40.

Temp. (K): open squares, 303;

filled squares, 308; open circles,

313; filled triangles, 318 and

open triangles, 323

J Surfact Deterg (2010) 13:475–484 481

123

study. From the results, thermodynamic parameters on

the formation of w/o microemulsions were evaluated, and

the structural features of the nanodroplets of water formed

as dispersions in heptane were estimated. The results led to

the following conclusions:

1. Nonionic surfactants having bulkier head groups

require a higher amount of n-butanol to obtain a

microemulsion compared with the cationic surfactants.

2. The process is endothermic with a positive entropy

change. Enthalpy and entropy changes nicely com-

pensate for each other.

3. For the cationic surfactants the effective size of the

droplets increases with an increase in water pool size,

and decreases with a temperature rise. However, for

the nonionic surfactants, both water pool size and

temperature result in a lowering of the effective size of

the droplets.

4. The average aggregation number of surfactants per

droplet is lower for the nonionic surfactants because of

their larger head group dimension which is compen-

sated for by the accumulation of a larger number of

alkanol molecules. The reverse effect was noticed for

the cationic surfactants.

Acknowledgments Financial assistance from the University Grants

Commission, New Delhi, India, is thankfully acknowledged, and MD

acknowledges with appreciation the receipt of a fellowship from UGC

to execute the work. SPM acknowledges the support from the Indian

National Science Academy in the form of an Honorary Scientist

position and an Emeritus Professorship at the Jadavpur University.

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Author Biographies

Mrinmoy De received his M.Sc. degree in Chemistry from University

of Kalyani in 2003. He is doing his Ph.D. work under the supervision

of Dr Amiya Kr. Panda and Prof. Satya P. Moulik from Jadavpur

University. His area of research is in the field of surface science,

especially in microemulsions.

Subhash C. Bhattacharya is Professor of Chemistry and recently

Dean, Faculty of Science, Jadavpur University. He received his M.Sc.

and Ph.D. in Chemistry from the University of Burdwan and Jadavpur

University, respectively. His area of research includes surface science

and spectroscopic analysis.

Satya P. Moulik is an Emeritus Professor of Chemistry at Jadavpur

University, and Honorary Scientist of the Indian National Science

Academy at the Centre for Surface Science, Jadavpur University. He

is the Editorial Adviser of the Journal of Surface Science and

Technology, and the President of the Indian Society for Surface

Science and Technology. His research interests are surface and

biophysical chemistry with special reference to microheterogenous

systems, polymer (biopolymer)-surfactant interactions, synthesis, and

the characterization of nanoparticles and drug encapsulation and

delivery. He has published 300 original research papers.

Amiya K. Panda obtained his M.Sc. and Ph.D. degree in Chemistry

from Tripura University. He is an Associate Professor in Chemistry,

University of North Bengal. His area of research is in the fields of

polymer and surface science, nanomaterials and membrane mimetic

systems. He has so far published 40 papers in various journals.

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