Chapter-I

General Introduction

Among natural forces, hydrophobic-lipophilic effect is one of the most

important and necessary force for the formation of spatially ordered

semi-microscopic assemblies of amphiphilic molecules. Many assemblies and

supramolecular aggregates such as association colloids, biological membranes,

vesicles, monolayers, proteins, DNA, polyelectrolytes and ion-exchange resins all

share an important structural feature, an interfacial region of moderate polarity

juxtaposed to a highly polar aqueous region [1-5]. Dynamic aggregates of

amphiphillic molecules known as association colloids are formed which also have

a substantial non-polar region adjacent to the interfacial region composed of

aggregated hydrocarbon chains.

Increasing demand for newer materials with improved properties in most of

applications, has given emphasis in surfactant research to study new phenomenon

in interesting world of surfactants with highly controlled molecular architectures

[6,7]. Many structures of the aggregates are formed by the association of

amphiphillic molecules such as surfactant monomers.

Surfactants and Their Classification

Surfactants are Amphiphilic

SURFace ACTive AgeNTS (referred to as SURFACTANTS) are

amphipathic molecules that, when present at low concentration in a system, has

the property of adsorbing onto surfaces or interfaces of the system and of altering

to a marked degree the surface or interfacial free energies of those surfaces

(or interfaces). The name amphiphile is sometimes used synonymously with

surfactant. The word is derived from the Greek word amphi, meaning both, and

the term relates to the fact that all surfactant molecules consist of at least two

parts, one which is soluble in a specific fluid (the lyophilic part) and one which is

insoluble (the lyophobic part). When the fluid is water one usually talks about the

hydrophilic and hydrophobic parts, respectively. The hydrophilic part is referred to

as head group and hydrophobic part as the tail (Fig. 1.1).

Hydrophilic(Head Group)

Hydrophobic (Tail)

Fig. 1.1: Schematic representation of a surfactant.

Today, surfactants find application in almost every field of life, they are

widely used in both industrial and domestic fields. The first surface-active product

was prepared commercially by C. Scholar in Germany in 1930 [8]. Now-a-days

these materials have impact on almost all aspects of our daily life, either directly

in household detergents and personal care products or indirectly in the production

and processing of materials which surround us [9-11]. Surfactants have been the

subject of investigation into the origin of life, meteorites containing lipid-like

compounds and may be an interstellar prebiotic earth source of cell membrane

materials [12]. In biological systems the surface active agents are used in very

much the same way as surfactants are employed in technical systems: to overcome

solubility problems, as emulsifiers, as dispersants, to modify surfaces, etc. There

are many good examples of this in biological systems: bile salts are extremely

efficient solubilizers of hydrophobic components in the blood, while mixtures of

phospholipids pack in ordered bilayers of the surfactant liquid crystal type and

such structures constitute the membrane of cells.

Classification of Surfactants

Depending upon the nature (charge) of hydrophilic group, surfactants are

classified as:

(I) Ionic Surfactants

(a) Cationic surfactants: The surface active portion has a positive charge.

Examples: Hexadecyltrimethylammonium chloride [CH3(CH2)15N+(CH3)3 Cl

–],

Dodecylpyridinium chloride [CH3(CH2)11C6H4N+Cl

]

(b) Anionic surfactants: The anion is the surface active species.

Examples: Sodium dodecyl sulfate [CH3(CH2)11OSO3– Na

+],

Sodium dodecylbenzene sulfonate [CH3(CH2)11C6H4SO3

Na+]

(c) Zwitterionic or ampholytic surfactants: Both positive and negative charges are

present in the surface active portion, and can behave as either an anionic, nonionic,

or cationic species, depending upon the pH of the solution.

Examples: N-Dodecyl-N, N-dimethylglycine [CH3(CH2)11N+(CH3)2CH2COO

],

3-(Dimethyldodecylammonio)-propane-1-sulfonate

[CH3(CH2)11N+(CH3)2CH2CH2CH2SO3

]

(II) Nonionic Surfactants

The surface active portion bears no apparent ionic charge, but has a polar

head group (containing hydroxyl groups or polyoxyethylene chains).

Examples:Poly(ethyleneglycol)-t-octylphenylether

[t-C8H17-C6H4-(OCH2CH2)nOH],

Polyoxyethylene(6)dodecanol [CH3(CH2)11(CH2CH2O)6OH]

(III) Polymeric Surfactants

Association of one or several macromolecular structures exhibiting

hydrophilic and lipophilic characters forms these.

Example: Polystyrene-block-poly (vinyl acetate)

–CHm

CH2

–

Br

Br

CC

O

)n

(CH2CH2OCH3

(IV) Dimeric Surfactants

Over the past decade the emergence of the dimeric (gemini) surfactants has

been one of the most exciting developments in surfactant chemistry. Gemini

surfactants have generated interest in colloid chemistry due to their superior

performance over conventional surfactants in various industrial applications.

Menger [13] coined the term “gemini” for describing dimeric surfactants, that is,

surfactant molecules containing two hydrophobic groups (sometimes three) and

two hydrophilic groups in the molecule, connected by a linkage (spacer) close to

hydrophilic groups [6,13,14]. The interest in this field was generated more due to

the report of Rosen [14] which pointed out that these surfactants could be more

surface active by orders of magnitude than comparable conventional surfactants

containing a similar single hydrophobic tail and a single hydrophilic group. A

schematic representation of a gemini surfactant is shown in Fig. 1.2.

HeadHead

Tail Tail

Spacer

Fig. 1.2: Schematic representation of a gemini surfactant.

Geminis were known long before to Bunton et al. [15], who studied

catalysis of nucleophilic substitutions by “dicationic detergents” and to Devinsky

et al. [16] who reported the surface activity and micelle formation of some new

geminis “bisquaternary ammonium salts”. Later, Okahara et al. [17] prepared and

examined amphiphatic compounds with two sulphate groups and two lipophilic

alkyl chains.

Menger and Littau [6] assigned the name “gemini” to bis-surfactants with a

rigid spacer (i.e., benzene, stilbenzene), but name is then extended to other bis or

double tailed surfactants, irrespective of the nature of the spacers (Fig. 1.2). The

great majority of geminis have symmetrical structures. Among these

gemini surfactants, the cationic bis(alkyldimethylammonium)alkane dibromide

type, with two tails and spacer separating the quaternary nitrogen atom

can be represented as m-s-m (where m is the number of carbon atoms in alkyl

chain and s is the number of carbon atoms in spacer) has received more attention.

A great deal of variation exists in the nature of the spacer, which can be short or

long; rigid or flexible; and polar or nonpolar. The polar group can be positive

(ammonium), negative (phosphate, carboxylate), or nonionic (polyether, sugar)

[18]. Some unsymmetrical geminis and geminis with three or more polar groups or

tails have recently been reported [19,20]. The three structural elements-

hydrophilic head group, a hydrophobic tail group, and the spacer may be varied to

change the properties of the gemini surfactants. The interest in academic circles

and among scientists at surfactant-producing companies is due to the following

reasons:

(i) Their CMC, on a weight percent basis, is at least one order of magnitude lower

than for the corresponding single tail – single head surfactants.

(ii) They are 10–100 times more efficient at reducing the surface tension of water

and the interfacial tension at an oil/water interface than conventional surfactants.

(iii) They appear to have better solubilizing, wetting, foaming, and lime-soap

dispersing ability than the conventional surfactants. Some cationic gemini

surfactants possess interesting biological properties.

(iv) The aqueous solutions of some gemini surfactants with a short spacer show

special rheological properties (viscoelasticity, shear-thickening) at relatively low

concentration.

(v) Gemini surfactants can be synthesized with an enormous variety of structures.

In principle, it is possible to connect any two identical or different surfactants

among the available ones by a spacer group that can be hydrophilic or

hydrophobic, flexible or rigid, heteroatomic, aromatic, etc. Therefore, the

structures and properties of gemini surfactants can be more finely tuned for a

given application than for conventional surfactants.

Although majority of the geminis have symmetrical structures (identical

heads and tails on both end of the spacer), unsymmetrical geminis [19,21] and

geminis with three or more polar groups or tails are also known [22,23].

(IV) Bolaform Surfactants

Bolaform surfactants or bolaamphiphiles (also known as bolaphiles or

alpha-omega-type surfactants) are amphiphilic molecules which consist of two

hydrophilic head groups, connected by a long, linear polymethylene chain.

Compared to single-headed amphiphiles, the introduction of a second head-group

generally induces a higher solubility in water, an increase in the critical micelle

concentration (CMC), and a decrease in aggregation number (Fig. 1.3).

Head Head

Long polymethylene chain

Fig. 1.3: Schematic representation of a bolaform surfactant.

Micelle Formation

One of the most characteristic properties of amphiphilic molecules is their

capacity to aggregate in solutions. The aggregation process depends, of course, on

the amphiphilic species and the condition of the system in which they are

dissolved. The narrow concentration range over which surfactant solutions show

an abrupt change in physicochemical properties is called the critical concentration

for the formation of micelle or „critical micelle concentration‟ (CMC) [24,25].

The word micelle is a Latin term meaning “small bit” and was coined by

J. W. McBain [26] in 1920 to describe colloidal sized particles of detergents and

soaps, and the phenomenon of self-association of monomers into micelles was

called micellization. Micelle formation or micellization is an important

phenomenon not only because a number of important interfacial phenomena, such

as detergency and solubilization, depend on the existence of micelles in solution,

but also it affects other interfacial phenomena, such as surface or interfacial

tension reduction, that do not directly involve micelles. The driving force behind

micellization – the hydrophobic effect- was proposed by G.S. Hartley [27] in

1936. He also suggested the roughly spherical model for the micelles, a suggestion

that gained general favor later.

The term CMC was established by Davis and Bury [28] in 1930, defining it

as the threshold concentration at which micelles first appear in solution is termed

as critical micelle concentration. CMC is an important property of the surfactants

which reflects its micellization ability. The physico-chemical properties of

surfactants vary markedly above and below the CMC value [29-32]. Below the

CMC value, the physico-chemical properties of ionic surfactants (e.g.,

conductivities, electromotive force) resemble those of strong electrolytes. Above

the CMC value, these properties change drastically, indicating a highly

cooperative association process is taking place. This is illustrated by Preston‟s [24]

classic graphs (Fig. 1.4).

The CMC value of a surfactant micelle can be obtained by plotting an

appropriate physico-chemical property versus the surfactant concentration and

observe the break point in the plot. The aggregation of surfactants/amphiphilic

compounds can be demonstrated by measuring solution properties such as surface

tension [33,34], dye solubilization [35], 1H-NMR [36-38], light scattering [39],

fluorimetry [40,41], osmotic pressure [42], electrical conductivity [43], ultrasound

velocity [44], against the surfactant concentration.

Fig. 1.4: Changes in the concentration dependence of wide range of physico-

chemical changes around the critical micelle concentration.

As concentrations of surfactant or salt (or both) in water are increased,

globular micelles gradually turn into larger, rodlike micelles. Under some

experimental conditions, spherical and rodlike micelles coexist in the same

solution, such systems containing two (or more) distinct distributions of micellar

sizes are called polydisperse. At higher concentrations of surfactant or salt, rodlike

micelles begin to predominate. Finally, at very high surfactant concentrations,

lamellar liquid crystal phases may be formed [3]. In a micellar solution, there is

always a dynamic equilibrium between the surfactant monomers, monolayers and

micelles (Fig. 1.5).

Fig. 1.5: Surfactant existence in different phases, dependent on surfactant

concentration.

Types of Micelles

Although the exact structure of the micelle is still somewhat controversial,

just above the CMC it is considered to be roughly globular or spherical [3,45]. The

radius of the micelle cannot be greater than the stretched-out length of the

surfactant molecules.

Three types of micelles are formed in surfactant solutions. Micelles formed

in polar solvents are called normal micelles and those formed in nonpolar solvents

are called reverse micelles. Another type is mixed micelle which is formed upon

mixing of two or more surfactants. All the three are briefly discussed below:

(I) Normal Micelle

The structure of normal micelle just above the CMC can be considered as

roughly spherical (Fig. 1.6) [46,47]. When the hydrophobic portion of the

surfactant is a hydrocarbon chain, the micelle will consist of liquid-like

hydrocarbon core. The radius of this core is roughly equal to the length of fully

extended hydrocarbon chain (~12-30 Å).

Menger has proposed that water can penetrate inside the micelle up to a

certain level [48], the idea gets support from fluorescence and 1H NMR

measurements. Partial molar volume determinations indicate that the alkyl chains

in the core are more expanded than those in the normal liquid state. An ionic

micelle formed in polar solvents such as water generally consists of three regions

(Fig. 1.6): (i) The interior or core of the micelle which is hydrocarbon like as it

consists of hydrocarbon chains of the ionic surfactant molecules, (ii) Surrounding

the core is an aqueous layer known as the Stern layer. The Stern layer constitutes

the inner part of the electrical double layer. It contains the regularly spaced

charged head groups and 60-90% of the counterions (the bound counterions). The

head groups are hydrated by a number of water molecules. One or more methylene

groups attached to the head group may be wet. The core and the Stern layer form

the kinetic micelle.

(iii) The outer layer is a diffuse layer and contains the remaining counterions and

is called the Gouy-Chapman layer that extends further into aqueous phase. The

thickness of this layer is determined by the (effective) ionic strength of the

solution.

Small-angle neutron scattering (SANS) experiments on the SDS and ionic

micelles support the basic Hartley model of spherical micelle.

For nonionic micelles the structure is essentially the same, except that the

outer region contains no counterions, but includes coils of hydrated

polyoxyethylene chains. Water molecules appear to be trapped on the oxyethylene

sites [49].

Fig. 1.6: Schematic representation of the regions of spherical micelle.

(II) Reverse Micelle

In a reverse micelle, head group of surfactant molecules locate inside to

form a polar core and hydrocarbon tails are directed towards the bulk solvent to

form the outside shell of the micelle [50-53]. At a very low concentration of

surfactant, the reverse micelles are very close to spherical in which water

molecules occupy the central part of the sphere, thus forming a so-called micro

water-pool, and these water molecules are in contact with head groups of reverse

micelle-forming surfactant molecules. The tails of these surfactant molecules are

extended toward bulk nonpolar solvent phase (Fig.1.7). The most often used

reverse micellar system is the Aerosol OT (AOT)/H2O/isooctane system. AOT has

a negatively charged polar head and two nonpolar tails (AOT, sodium bis

(2-ethylhexyl) sulfosuccinate).

Fig. 1.7: A two dimensional schematic representation of reverse micelle.

Isooctane (2, 2, 4-trimethylpentane) has a structure similar to the tail structure of

AOT and thus has the best penetration into AOT tail [54]. Micelles with

different sizes and properties can be made by changing the water/surfactant

ratio in the solution. Reverse micelles of this type have been studied widely,

primarily because of their usefulness as micro reactors for

chemical and biochemical reactions [55].

(III) Mixed Micelle

In most practical applications, mixtures of surfactants, rather than

individual surfactants, are used or purposely mixed to improve the properties of

the final product. Mixing of two or more surfactants in an aqueous solution leads

to the formation of mixed micelles (Fig. 1.8).

Monomers Mixed micelle

Fig. 1.8: Schematic representation of formation of mixed micelle by the

monomers.

From the application point of view, mixed micelles are of great importance

in biological, technological, pharmaceutical and medicinal formulation, enhanced

oil recovery process for the purpose of solubilization, suspension, dispersion etc.

[56]. Due to numerous applications of such systems, a lot of attention has been

devoted for the understanding of mixing behavior using various techniques such as

conductivity, surface tension, viscosity, NMR, calorimetry, potentiometry,

fluorimetry, density, SANS, etc. [57-71]. The theoretical approaches are found to

be most successful in describing the micellar behavior of anionic-anionic

surfactant solutions. A generalized multicomponent nonideal mixed micelle model

based on the pseudophase separation approach is presented by Holland and

Rubingh [72]. Surfactant-surfactant interactions in mixed micelles and monolayer

formation were extensively studied by Rosen [73].

Factors Affecting the Value of the Critical Micelle Concentration

Since the properties of solutions of amphiphiles change markedly when

micelle formation commences, a great deal of work has been done on elucidating

the various factors that determine the concentration at which micelle formation

becomes significant (i.e., CMC), especially in aqueous media.

The factors known to affect the CMC markedly in aqueous micellar solutions

are: (i) structure of amphiphiles, (ii) the presence of various additives in the

solution, (iii) experimental conditions such as temperature, pH, pressure,

solvent, etc.

(I) Structure of Amphiphiles

In general, the CMC decreases as the hydrophobic character of the

surfactant increases. The reduction in the CMC with the increase in the tail length

of the surfactant molecule is due to the enhancement in hydrophobicity. However,

when the number of carbon atoms in a straight chain hydrophobic tail exceeds 16,

the CMC no longer decreases rapidly with the increase in the chain length. After

exceeding 18 carbon atoms, the CMC values remain substantially unchanged with

further increase in the chain length due to coiling of these long chains in

water [74].

The carbon atoms in a branched hydrophobic group appear to have about

one half the effects of carbon atoms in straight chains. Hydrophobic groups having

unsaturated bonds usually show a higher CMC than the corresponding saturated

groups. Surfactants with bulky hydrophobic or hydrophilic groups show higher

CMC than those with less bulky groups.

In aqueous medium, ionic surfactants have much higher CMCs than

nonionic surfactants containing equivalent groups. Zwitterionic surfactants appear

to have about the same CMCs as ionics with the same number of carbon atoms in

the hydrophobic groups. The CMC of the ionic surfactant decreases as the

hydrated radius of the counterion decreases. For usual type of polyoxyethylenated

nonionic surfactants, the CMC decreases with the decrease in the number of

oxyethylene units in the polyoxyethylene chain, since this makes the surfactant

more hydrophobic.

(II) Presence of Various Additives in the Solutions

Addition of electrolytes to aqueous surfactant solutions may result in a

modification of both intramicellar and intermicellar interactions. Decrease in the

CMC in presence of electrolytes is due to reduced repulsion between the

electrostatic headgroups in the micelles enabling micelles to form more easily, i.e.,

at lower concentration. An increase in size of counterions decreases the CMC due

to the increase in the hydrophobic character. This is the reason why N)HC( 473 is

more efficient in reducing the CMC than N)HC( 452 , which is more efficient

than N)(CH 43 .

There have been attempts to examine the salts effect on micelle formation

in the light of Hofmeister (lyotropic) series [75,76]. The series plays a notable role

in a wide range of biological and physicochemical phenomena. However,

depending on the system and type, there may be changes in order in the series. A

very recent study carried out by Moulik and coworkers [77] shows that, for a

given anionic surfactant, the order of effectiveness in reducing the CMC decreases

in the order 2Mg > Cs > K >

4NH > Na > Li . The same authors have reported

two CMC values for a given cationic surfactant in presence of anions like

salicylate )OH(C 357

, benzoate

)OH(C 257

, oxalate

)O(C

2

42

, tartrate

)OH(C2

644

.

For a given nonionic surfactant, the effect of anions on the CMC follows the order

F > Cl > 2

4SO > Br > 3

4PO > 3

353 O(COO)HC > I > SCN and the effect of

cations follows the order K > Na > Rb > Li > 2Ca > 3Al [78].

Small amount of organic materials significantly influences the CMC of

aqueous micellar solutions. Additives like urea have been shown to increase the

CMC of ionic [79,80] and nonionic surfactants [81,82]. For fluorocarbon

surfactants, addition of urea slightly decreases the CMC [83]. Addition of alcohols

produces both increase and decrease in CMC of surfactants [84-86]. A decrease in

the CMC has been observed with the increase in the carbon number of the linear

alcohols (heptyl to decyl) in nonaqueous dimethylformamide [87]. Introduction of

sugars has been known to decrease the CMC of the system [88,89]. For an ionic

surfactant solution, decrease [90] as well as increase [91] in CMC has been

reported with different concentrations of acetamide. Amines are more surface

active than alcohols at air-water interface [92]. Addition of n-alkylamines (butyl to

decyl) has been found to be solubilized in micellar phase, leaving the amine group

on the surface of the micelles [93]. These solubilized amines have been reported to

form mixed micelles with ionic surfactants [94-96].

(III) Experimental Conditions

(a) Temperature. Temperature increase causes decreased hydration of the

hydrophilic group, which favors micellization. However, temperature increase also

causes disruption of the structured water surrounding the hydrophobic group, an

effect that disfavors micellization. The relative magnitude of these two opposing

effects, therefore, determines whether the CMC increases or decreases over a

particular temperature range. From the data available, the minimum in the CMC –

temperature curve appears to be around 25 °C for ionics [97] and around 50 °C for

nonionics [98]. For bivalent metal alkyl sulphates, the CMC appears to be

practically independent of the temperature [99].

(b) pH. When amphiphile molecules contain ionizable groups such as –NH2,

–(CH3)2NO and –COOH, the degree of dissociation of the polar group will be

dependent on pH [100]. In general, the CMC will be high at pH values where the

group is charged (low pH for –NH2 and – (CH3)2NO, high pH for –COOH) and

low when uncharged. Some zwitterionic surfactants become cationic at low pH, a

change that can be accompanied by a rapid rise in the CMC [101], or a more

modest rise [102] depending on the structure and hence hydrophilicity of the

zwitterionic form.

(c) Pressure. Many reports have appeared on the effect of pressure on micelle

formation of ionic [103-105] and non-ionic amphiphiles [106]. With pressure,

CMC of ionic surfactants increases up to 1000 atm followed by a decrease above

this pressure [107-112]. Such behavior has been rationalized in terms of

solidification of the micellar interior, increased dielectric constant of water [108],

and other aspects related to water structure [109]. For nonionic amphiphiles, the

CMC value increases monotonously and then levels off with increasing pressure.

(d) Polar Nonaqueous Solvent. For micelle formation in polar nonaqueous

solvents, the term “solvophobic interaction” has been coined, in analogy with

“hydrophobic interactions” which causes micellization in aqueous medium [25].

Aggregation Number and Micellar Morphology

Aggregation Number (N)

Micelle aggregation number (N) which is the number of monomers making

up a micelle is a fundamental parameter concerning the micelle. It gives an idea

about the size of the micelle and is vital in determining the stability and practical

applications of the investigated systems [32,74]. It is affected by different factors

such as concentration of surfactant [110,113,114], temperature [5,32,115,116],

concentration of added electrolyte [113,117-120], organic additives [121-124], etc.

Various experimental techniques like dynamic light scattering (DLS), small-angle

neutron scattering (SANS), steady-state fluorescence quenching (SSFQ), and time-

resolved fluorescence quenching (TRFQ), etc. may be used for the determination

of aggregation number [110,111,113,125-129].

In aqueous medium, the greater the„dissimilarity‟between surfactant and

solvent, the greater the aggregation number. An increase in the temperature

appears to cause a decrease in the aggregation number of the ionic surfactants

[130-132]. For nonionic surfactants, it increases fairly rapidly [133]. In a micellar

solution, all micelles may not have the same aggregation and polydispersity exists

[134].

Micellar Morphology

The concept of molecular packing parameter has been widely cited in the

chemistry, physics, and biology literature because it allows a simple and intuitive

insight into the self-assembly phenomenon [135]. The packing parameter approach

permits indeed to relate the shape of the surfactant monomer to the aggregate

morphology [5,30,136]. The shape of micelles depends strongly upon the actual

packing parameters in the micellar assembly [137,138]. The shapes of the micelles

produced in aqueous media are of importance in determining various properties of

the surfactant solution, such as its viscosity, its capacity to solubilize

water-insoluble materials, and its cloud point. At the present time, the major types

of micelles appear to be (1) relatively small, spherical structures (aggregation

number < 100), (2) elongated cylindrical, rodlike micelles with hemispherical ends

(prolate ellipsoids), (3) large, flat lamellar micelles (disklike extended oblate

spheroids), and (4) vesicles-more or less spherical structures consisting of bilayer

lamellar micelles arranged in one or more concentric spheres.

The surfactant packing parameter, introduced by Israelachvili, Mitchell, and

Ninham [30] developed a general theoretical frame work from which the

secondary structures formed by a surfactant may be deduced based on its

molecular geometry. They showed that many surfactants can be generalized into

certain shape categories, which are likely to produce specific secondary aggregates

in aqueous solution. The packing parameter (p) determines which aggregate the

surfactant is most likely to form [139].

It is calculated by dividing the volume of hydrocarbon chains (v)

(Table 1.1) by the cross-sectional surface area (a0) of the head groups and length

of the alkyl chain (lc), so that the non-dimensional packing parameter (p) is

cola

vp (1.1)

where, the tail and volume of the hydrocarbon chain of nc carbon atoms can be

approximated by correlations of experimental data as:

ccnl 265.154.1 (Å) (1.2)

c

nv 9.264.27 (Å) (1.3)

As shown in Table 1.1, spherical micelles are formed when p is lower than

1/3; wormlike micelles are formed when p has a value in between 1/3 to 1/2;

vesicles or bilayers are formed when 1/2 < p < 1. When the volume of the

hydrocarbon part is large relative to the head group area (p > 1), reverse micelles

are formed. However, it is to be noted that the Eq. (1.1) and its implications listed

in Table 1.1 present only general guidelines for surfactant structures. Solution

parameters such as concentration, pH, temperature and solvent polarity may

heavily modify the specific structures formed.

Table 1.1: Aggregate structures with their corresponding packing

parameters.

Effective shape of the

surfactant molecule

Packing

parameter (p)

Type of aggregation

Cone

<1/3

spherical micelles

truncated cone

1/3–1/2

wormlike micelles

cylinder

1/2–1

bilayers

Vesicles

inverted cone

>1

reverse micelles

Effect of Additives on Structural Transitions

Amphiphilic molecules self-assemble in aqueous solution into a variety of

structures such as e.g. spherical or cylindrical micelles, vesicles, etc., depending

on the molecular design and on the conditions under which aggregates are formed

[140]. Single-tail surfactants usually form spherical micelles in aqueous solution

above their critical micellar concentration (CMC) [141], which eventually grow to

other shapes with an increase in surfactant concentration. The growth of spherical

micelles to cylinders can also be achieved by the addition of co-surfactants

[142,143], of inorganic salts [144,145], or of strongly binding organic salts

[146,147]. Other ways towards micellar growth consist in using special surfactant

structures, e.g. dimeric surfactants with a very short spacer group (namely an

ethylene group linking covalently the head groups) [148], hetero-gemini

surfactants [149], or mixtures of cationic and anionic surfactants [150-153]. The

growth of micelles can be explained in terms of change in the surfactant packing

parameter [154] due to decreased electrostatic repulsions and/or increased

hydrophobic interactions, which results in a reduction of the spontaneous

curvature of the surfactant assemblies.

The properties of micellar solutions such as CMC, aggregation number,

micelle size and shape, etc., depend on the balance between “hydrophobic” and

“hydrophilic” interactions [5,155]. For ionic surfactants this balance can be

modified in several ways, e.g., salt addition, counterion complexation, addition of

alcohols or other substances (that can be solubilized into the micelle), change of

the solvent, or change of the “structure” of the solvent itself. Amphiphilic

substances are capable of forming supramolecular systems [156], from

thermotropic-lyotropic liquid crystals and manifold micellar systems upto the

highly ordered membranes in liposomes and cells. At low surfactant

concentrations they assume rod or disk like shapes [157]. Micelles transform to

lyotropic liquid crystalline structures [158] at very high surfactant concentrations.

The sphere-to-rod transition is important from both a theoritical and a practical

point of view. Theoritically, because (i) it implies a micellar growth which seems

to be related to the classical Derjaguin-Landau-Vewey-Overbeek theory (DLVO

theory); (ii) the more structuralized rod micelles, as compared to spherical

micelles, can be related more closely to the formation of biological structures

(membranes, for example); (iii) if the sphere-to-rod transition can be predicted

from theoritical models it would promote a better understanding of micellar and

related organized structures.

From a practical point of view, the presence of rod shaped micelles gives

solution a very high viscosity, which might be of importance in industrial

formulations of surfactant solutions.

(I) Effect of Salts

Generally, in the absence of salts at moderate concentrations, the

surfactants (pure and mixed) aggregate exist in the form of spherical micelles in

aqueous solutions. In the presence of salts, with its increasing concentration, the

spherical aggregates tend to transform into nonspherical ones (viz., rod, branched

or wormlike micelles). Among various factors acting on salt addition, the

formation and growth of micelles are mainly favored by the screening of

electrostatic repulsion among the polar head groups and movement of the

hydrophobic alkyl chains away from the aqueous environment. This is evidenced

by a decrease in CMC and an increase of the micelle aggregation number

[120,159]. Addition of salt to a surfactant often gives rise to a salting-out

phenomenon, which is the result of the movement of water molecules (which are

not playing the role of a solvent) from coordination shells of surfactant molecules

to those of salts. The effect of inorganic salts on ionic surfactant solutions have

been discussed in terms of electrostatic interactions, changes in the water

structure, ionic hydratability, etc. [160]. Rod-like micelles are produced with

cationic surfactants in presence of inorganic salts [161].

When salt is added to aqueous ionic surfactant solution rod-like micelles

are formed [120,162-164] as its concentration reaches a threshold value, because

the presence of salt ions near the polar heads of surfactant molecules decreases the

repulsion force between the headgroups. Due to this reduction in the repulsion, the

surfactant molecules approach eachother more closely, and as a result larger

aggregates are formed which require much more space for hydrophobic chains. As

the spherical micelle has a small volume, it must change into the rod-like micelle

to increase the volume/surface ratio. In the transition from sphere to rod, micelles

change their aggregation number dramatically and grow linearly, keeping their

radii constant.

Inorganic salts are used as thickening agents for concentrated surfactant

solutions. The effects of inorganic salts on ionic surfactant solutions have been

discussed in terms of electrostatic interactions, changes in the structure of water,

ionic hydratability, etc. [5,160,165,166]. Two main factors are responsible for

structural transition in presence of salts are – (a) electrostatic effect of simple salts

due to the counterion binding on ionic micelles, (b) hydrophobic interaction

between surfactant molecules or ions caused by the change in the

hydrogen-bonded structure of water.

Micellar sphere to rod transition is highly dependent upon the nature of

counterions. „Counterions‟ are bound primarily by the strong electrical field

created by the head groups but also by a specific interaction that depends upon

head groups and counterion type. The micellar transition is promoted by strong

counterion binding, which can be shown by highly increase in the relative

viscosities [167-174]. There has been numerous studies of dilute and moderately

concentrated aqueous cationic surfactant solutions [163,165-188], with aqueous

salt solutions using different techniques such as light scattering [177,178,185]

flow birefringence [184,186], viscosity [177], solubilization [181,183], 1H NMR

[179], SANS [182,188], electron microscopy [187].

Ikeda et al. [171] measured light scattering from aqueous solutions of SDS

in the presence of 0.8 M NaX (X = F–, Cl

–, Br

–, I

–, or SCN

–) at 35°C and found

that the molecular weight of the rod like micelles depends on the co-ion species of

added salt and changes in the order of the lyotropic series of halide ion except for

SCN– ion: NaSCN < NaF < NaCl < NaBr < NaI. The difference in the micelle size

caused by the effect of co-ion species on hydrophobic interaction in the micelle

formation or the extent of destruction of the hydrogen–bonded structure of water.

They [162,163] showed that for sodium dodecyl sulphate (SDS) and for a series of

cationic surfactants in NaCl solutions a sharp break in the apparent micelle

molecular weight is observed when the NaCl concentration reaches a value of 0.45

M and the breakpoint correspond to the sphere-to-rod transition.

Symmetrical quaternary ammonium ions (R4N+) are essentially less

hydrated and, therefore, binding with the micelle will be favorable. On the other

hand, R4N+

has a low charge density and may also try to intercalate between head

groups of anionic micelles. This will decrease the electrostatic interactions in

addition to increased hydrophobic interactions. All these factors contribute

towards micellar growth [189].

Several reports indicate that change from Li+ to Cs

+ induces micellar

growth, which is related to hydration of specific counterion [190]. The formation

of rod like micelles can be strongly enhanced in anionic surfactant solutions in

presence of multivalent counterions (Ca2+

, Al3+

) [191,192]. Al3+

can bind together

three surfactant head groups at the micelle surface, thus causing a decrease of the

area per head group [191]. This induces a transition from spherical to cylindrical

micelles.

Usually, spherical micelles are formed in combination with halide

counterions, whereas aromatic counterions often induce the formation of rod-like

micelles at relatively low surfactant and counterion concentrations [193]. The

formation of such rod-like micelles is attributed to the strong binding of organic

counterions on surfactant micelles (at the level of the head groups of surfactants)

to minimize the contact of their bulky hydrophobic part with water (see Fig. 1.9).

This results in a screening of charges. Hence, electrostatic repulsion between the

ionic hydrophilic groups decreases, while hydrophobic interactions simultaneously

increase in the palisade layer of the micelle, leading to a tighter packing of the

surfactant–counterion mixed system (reduction of the spontaneous curvature of the

surfactant assemblies). This will therefore drive the micelle to change its

microstructure.

Water

Micelle core

Fig. 1.9: Schematic representation of the binding of organic anions at the micellar

interface of cationic surfactants.

The organic counterions have strong tendency to affect the organized

assemblies as compared to the inorganic counterions, as, besides the electrostatic

interaction, they have additional hydrophobic interaction [194,195] (which also

plays a role in promoting aggregate morphology). Organic counterions, having

central benzene ring, penetrate into micelles by inducing strong hydrophobic

interaction and hence reducing electrostatic repulsion between the hydrophilic

headgroups, which give rise to tight packing and possible reduced curvature of

surfactant aggregates. They are capable of producing strong viscoelasticity in the

conventional cationic surfactants [196-198], which confirms the formation of

rod- and worm-like micelles in ionic surfactants with organic counterions

[194,195]. Also, the position of substituent group present on the benzene ring of

organic salt determines the extent of hydrophobic interaction between organic

counterions and surfactant aggregates [194,199]. The transition of different

aggregate morphologies by addition of salts may be applied in bio-engineering,

surface chemistry, and natural sciences.

Anions such as salicylate are known to promote very efficiently the growth

of cationic micelles. Solutions of worm-like micelles so formed have interesting

rheological properties [200-202] and the theories of the structure and dynamics of

these complex systems have been well developed [203,204]. Worm-like micelle

containing systems [205] are discussed intensely as drag reducing agents (DRA) in

recirculation systems [206-208] and in fracturing fluids in oil production

[205,209].

Different organic salts, also often called hydrotropes, are commonly short

amphiphilic molecules (often with a bulky “hydrophobic” part) that, without

forming micelles at high concentrations, enhance the solubility of a variety of

hydrophobic compounds in water [210]. Many salts with hydrophobic counterions,

such as sodium salicylate (NaSal), sodium benzoate (NaBenz) and sodium tosylate

(NaTos), are particularly effective in inducing micellar growth even at low

concentrations. Variations occur in the rheological properties with increasing salt

content complex [198,211,212]. The classic examples are solutions of CTAB and

NaSal [213], for which the zero-shear viscosity (η0) goes through a maximum at

low NaSal concentrtions, then a minimum, and subsequently a second maximum

around 1M NaSal. NMR studies on the cetyltrimethylammonium salicylate system

reveal that the 1H lines for the N

+ (CH3) group are shifted to higher fields, and the

signals are broadened [193,213-215]. The salicylate anion orientates in such a way

that the negatively charged site (COO– group) stands perpendicular to the micellar

surface [193].

Some surfactant molecules in aqueous solution are spontaneously

transformed from micelles into a lamellar array in the presence of high salt

concentration. This morphological change is facilitated by an increase in

counterion binding and dehydration of the surfactant head groups and bound

counterions. This salt-induced lamellar arrangement of surfactant molecules is

commercially utilized in liquid laundry detergents.

(II) Effect of Organic Additives

Both dynamic and structural properties of micellar solutions can be altered

by the addition of third component in the solution. This last substance can act

through two different mechanisms: by interactions with the surfactant molecules

or by changing the solvent nature. The effect of organic additives on the micellar

size and shape has been explained in terms of their effects on water structure and

on their role inside the micelle. Aqueous micellar solutions are known to

solubilize water insoluble or slightly soluble organic compounds.

Surfactant solutions have a general tendency to solubilize a certain amount

of hydrocarbons. Micelles are well known for presenting structural aspects

consisting of a non-polar inner core and a polar outer surface. This structure

allows micellar aggregates to enhance the solubility of hydrophobic materials and

to modify environmental features. Systems with rod-like micelles can actually

solubilize rather large amount of hydrocarbons [216].

The environment of solubilization of different compounds in or around

micellar systems can be correlated with the structural organization of micellar

aggregates and their mutual interactions [217-220]. Interfacial partioning of

organic additives causes micellar growth while interior solubilization produces

swollen micelles [221,222]. These two types of micelles impart different viscosity

behavior to micellar solutions. The interior (core) solubilization of organics

provides swelling to the already grown micelle and releases the requirement of the

surfactant chain to reach the center of the core [223]. These factors may increase

the smaller dimension of such anisotropic micelles with a resultant decrease in

axial ratio (more spherical). This increased spherecity will cause micelles to flow

easily with an eventual drop in viscosity. The term electrostatic repulsion

originating from intermicellar and intramicellar Coulombic interactions favors

micelles with a higher surface area per head group. On the other hand,

hydrophobic interaction between the hydrocarbon part of the micelles/monomers

tries to achieve aggregates with closely packed monomer chains.

Mukerjee [223] had proposed that an additive which is surface active to a

hydrocarbon–water interface is mainly solubilized at the micellar surface and

promotes micellar growth. The greater partioning of the additive to the core was

shown to retard micellar growth by virtue of relaxing the requirement of the

monomer tails to reach the center of the aggregates which maintain the micellar

shape with higher surface area that is spherical micelles [223]. There has been

considerable discussion about the location of aromatic solutes such as benzene and

toulene in ionic surfactant micelles. Benzene solubilizes mainly in the surface

region of the micelles [224], or primarily within the micellar interior [225,226], or

in both states [227]. However, extensive and precise solubilization studies do not

indicate a strong preference of these compounds in either the headgroup region or

the interior [228,229]. The aromatic hydrocarbons seem to be intermediate

between highly polar solutes, clearly embedded in the headgroup region, and

aliphatic hydrocarbons, which usually solubilize in the micellar interior [223].

It is suggested that the lower chain length alcohols shows marginal effect

on viscosity changes in comparison to higher chain length alcohols [230]; hence

the change in ηr values shows the dependence on the alkyl chain length of

alcohols. It has been suggested that the short chain alcohols are localized mainly in

the aqueous phase, which therefore changes the micellar structure by altering the

organization of solvent molecules. Medium chain length alcohols are distributed

between the two phases (i.e., micelle and bulk water) and long chain length

alcohols are localized in the micellar phase [231,232].

n-Alkylamines (C4-C10) have been earlier found to be solubilized in ionic

micelles by electrostatic and hydrophobic effects with –RNH2 group left on the

micellar surface [93]. Their partial dissociation into –RNH3+ and –OH

¯ may

influence electrostatic interactions with cationic gemini head groups. As a result,

the partitioning content of –RNH2 content at the head group region is decreased.

Thus, the decrease in effective –RNH2 content at the micellar surface hinders the

micellar growth, which is indeed reflected by the lower ηr values in presence of

amines as compared to alcohols. Therefore, amines are found to be more surface

active in comparision to alcohols, at the air-water interface [92].

Wormuth and Kaler [233] ranked three classes of additives on the basis of

their hydrophilicity. Primary amines were found more hydrophilic than either

alcohols or carboxylic acids. But, when coupled with anionic surfactant, the

hydrophilicity of amine was lower than expected. Lindemuth and Bertrand [234]

observed that on comparison, amines have been found to be more effective in SDS

than in TTAB. This is due to the interaction between the amines and the anionic

surfactant headgroups at the micellar interface. In addition to this, amine

headgroup has the ability to reside deeper in the SDS micelle, which relieves the

requirement of the tails of the surfactant to reach the center of the micelle at a

shorter alkyl chain length of additive. On interaction of cationic surfactants with

carboxylic acids similar effects were seen. Thus, a cosurfactant with the ability to

bear an opposite charge to that of the surfactant headgroup is more effective at

promoting sphere –to– rod transition and has the ability to better peneterate the

surfactant rich film, separating the micellar and aqueous pseudophases [235].

The micellar tails must be reachable to the center of the micelle to maintain

a spherical form. On addition, an aliphatic hydrocarbon generally resides in the

micellar core. Now the association structure can maintain spherical form

containing solubilized oil at a radius which was previously prohibitive. In this way

the presence of aliphatic hydrocarbons retard the structural transition. On the other

hand, the presence of aromatic hydrocarbons stimulate rod growth in case of

cationic surfactants, which may rise from the interaction of the delocalized π-

electron cloud of the benzene ring with the positive charges of the surfactant

headgroups; a behavior very similar to that of a cosurfactant or counterion. The

resulting reduction of head group repulsion favors transition to rods by shrinking

the surface area occupied per amphiphile, thus increasing the aggregation number.

Kandori et al. [236] studied the effect of phenol and benzene additives on

micellar structures in aqueous solutions of dodecyltrimethylammonium bromide

by additive solubilization, tracer diffusion coefficients, electrical conductivity,

viscosity, and ultraviolet absorbance. The solubilization of phenol and benzene in

the system causes the micelle to swell and it was observed that phenol addition

leads to a greater increase in the size of aggregates than addition of benzene.

Ultraviolet absorbance measurments revealed that the site of solubilization within

the micelles is different for two additives. Benzene solubilizes in the central core,

while at low concentrations phenol is taken up in the outer palisade layer.

(III) Synergism in presence of Salts + Organic Additives

Due to their importance and complicated aggregation behavior [120,162-

164,190-192,210-223,225-230,237-243] a wide attention has been given to the

studies of size and structure of micelles. For most aqueous ionic surfactant

solutions just above the CMC, the micelles are regarded as spherical in shape. The

micellar structure can be influenced by the addition of neutral additives

(e.g., alcohols and amines etc.). This effect is dependent on the types of surfactants

used, their concentration, the salt content, and the additives.

Incremental calorimetric technique was used by Nguyen and Bertrand

[244] to study the effect of low concentrations of alcohols on solutions of SDS

with added electrolytes at 25°C. These measurments reveal a discontinuity in the

slope of partial molar enthalpy of solution versus concentration of alcohol curves.

The authors assert that this break corresponds to the micellar sphere-to-rod

transition.

Stephany et al. [245] studied the same system with varying concentration of

the electrolyte (NaCl). They varied the concentration of 1-pentanol too for each

NaCl concentration. Their data show characteristics of a continuous sphere-to-rod

transition. From static and quasielastic light scattering methods they concluded

that the micelles could be modeled as flexible worm-like objects.

The effect of addition of n-alcohols on the viscosity of CTAB was studied

by capillary viscometry method. Prasad and Singh [246] found that the lower

alcohols (C2-C3OH) decreased the viscosity of CTAB solution in presence of

0.1 M KBr right from the beginning, while C4, C5 and C6OH in low concentration

were found to increase the viscosities. Depending on the nature of alcohol, further

addition made the solution either turbid or lowered the viscosity of the solution.

The result was interperated in terms of the possible micellar transition from rod to

sphere or elongated rods in presence of added alcohols. It is known that rod-

shaped micelles are formed in aqueous solutions of 0.1 M CTAB+0.1 M KBr

[247]. The effects of added aliphatic n-amines (C4, C5, C6, C7 and C8NH2) and

temperature on the above system show that transition of rod-shaped micelles to

larger aggregates is induced by addition of higher amines (>C6NH2) and that too

upto a certain concentration only: a further increase in concentration produced the

opposite effect. Addition of C4NH2 amine was reported to induce only a rod-to-

sphere transition. Kumar et al. [221], interpreted the data in terms of

solubilization/incorporation (decrease of micellar surface charge density) of

amines inside in the micelles and nature of the effective solvent (water+amine).

The latter effect dominated the change from larger aggregates to smaller micelles

at higher concentrations of the added amine.

Mixed Micellization

The amphiphilic molecules have a tendency to collect at any interface

where the hydrophobic groups can be partially or completely removed from the

contact of water and the hydrophilic groups remain wetted. This dual tendency of

the molecules, as we know, results in the formation of the micelles.

The formation of micelles from more than one chemical species gives rise

to what is known as mixed micelles. Micelles may be formed from the compounds

which are either heterodisperse or polydisperse, and Gibbons [248] had identified

important difference between the two types of compounds. Nemethy and Ray

[249] have taken advantage of this important property of polydisperse compounds

in a thermodynamic study of micelle formation by nonionic surfactants in ethylene

glycol-water mixtures. Another class of mixed micelles results when low

molecular weight molecules are solubilized by micelles formed from surfactants

containing a relatively larger nonpolar side chain. The solubilized substance, also

called a penetrating additive [250], may be located in the hydrocarbon core [251]

or hydrophilic mantle [252]. Several studies have been concerned with this aspect

of micelle formation [253]. The surface properties of ionic micelles have been

shown to be altered by mixed micelle formation. Tokiwa and Ohki [254] have

shown that the addition of an anionic surfactant increases the apparent dissociation

constant of micelles of a cationic-nonionic surfactant, and the addition of a

cationic surfactant produces the opposite effect. The degree of counterion binding

by mixed micelles formed from anionic and nonionic surfactants was found to

decrease as the proportion of the nonionic component increased. It was also found

to depend upon the length of the nonionic polyoxyethylene head groups [255].

These observations can be understood in terms of an altered charge density at the

micelle surface as a result of mixed micelle formation, and possible interaction

between the anionic and nonionic head groups. The latter has been demonstrated

by NMR studies [256,257] in which the aromatic portion of the anionic surfactant

shifts the proton resonance signal of the polyoxyethylene group upfield.

From application point of view, mixed micelles are often used in technical,

pharmaceutical, and biological fields, since they work better than pure micelles

[258,259]. They have importance in industrial preparation, pharmaceutical and

medicinal formulations, enhanced oil recovery process, and so forth, by way of

efficient solubilization, suspension, dispersion, and transportation influenced by

temperature, pressure, pH, nature of solvent and additives, etc. [56].

Theories of Mixed Micellization

Many theoretical models have been put forward for dealing with the mixed

binary systems to evaluate the composition and interaction parameter among the

components at the air/water interface and in the micellar phase. The first model,

given by Lange [260] and used by Clint [261,262], is a phase separation model

which relates the mole fraction and the critical micellar concentration of the ith

components (i = 1, 2) in an ideal mixture, which is successfully applicable to

systems of mixed surfactants of similar structure, but hardly applicable to

combinations with dissimilar structures. Rubingh‟s model is the first model

developed for nonideal mixed system [72]. It is based on a regular solution

approach for the treatment of nonideal mixing, and due to its simplicity, it has

been mostly used, even after the development of more complex models. Although

Rubingh‟s treatment found to be reasonably satisfactory in many cases, the theory

was criticized on thermodynamic grounds. Rosen et al. [263] have extended the

nonideal solution treatment of Rubingh for mixed micelle formation by binary

surfactant systems to estimate, from surface tension data, the surfactant molecular

interactions and also the composition in the adsorbed mixed monolayer at

air/water interface. Motomura et al. [264] developed a model that is an attempt to

overcome the limitations of the Rubingh‟s model and improve the predictions of

the phase separation model. Basically, it is a thermodynamic method which

considers the micelles as a macroscopic bulk phase, the thermodynamic quantities

associated with the mixed micelle formation process being expressed as a function

of the excess thermodynamic quantities. More recently, Rodenas et al. [70] used a

simple theoretical treatment, based on Lange‟s model that utilizes the

Gibbs-Duhem equation to relate the activity coefficients of the surfactants in the

mixed micelles.

Based on the CMC, the chemical structures of the hydrophobic and

hydrophilic moieties of the individual components, surfactant concentration in

solution, and other solution conditions such as temperature, concentration, salt

effect, etc., a molecular thermodynamic model applicable to the binary mixtures of

nonideal surfactant solutions has been predicted by Blankschtein et al. [265,266].

Strictly, this theory is applicable to mixed micellar systems when at least one

component is ionic. This theory helps us to find out the CMC of the binary

mixture, size, and shape of the micelle and the phase behavior of the solution. The

Maeda [267] model is applicable to ionic/nonionic mixed systems with moderately

high ionic strength where the short range electric interaction is no longer

negligible. Therein lies a difference of this model from the RST where only long-

range electric interaction plays an important role in the mixed system. The model

assumes the decrease in repulsion among the ionic head groups in an

ionic/nonionic mixed micelle is due to the presence of nonionic surfactant

molecules in the micellar phase.

Relevance of the Research Problem

Over the past decade emergence of the „gemini surfactants‟ made up of two

amphiphilic monomers linked at the level of polar head group of each monomer

by a spacer has been one of the most exciting developments in surfactant

chemistry. They have attracted wide attention by virtue of their appealing

properties which include high surface activity and low CMC values, unusual

viscosity changes with an increase in surfactant concentration, greater efficiency

in lowering the interfacial tension, better wetting and solubilizing abilities, unusual

micellar structure, etc. [14,268,269]. Besides this, surfactant and/or added salt

concentrations are other relevant factors affecting the aggregation of gemini

surfactants.

Increasing demand for newer materials with improved properties in most of

applications, has given emphasis to the use of surfactants in presence of additives.

„Synergy‟ is the best way to improve the surface or interfacial properties of a

surfactant. Usually, the additives can improve the desired properties of surfactant

solutions. The most widely used additives are alcohols, amines and salts.

In particular, the choice of cationic gemini surfactants was made in an

effort because of the low toxicity of this type of quaternary ammonium

surfactants. Also, they exhibit broad spectrum of antibacterial/antifungal activity.

As such, the gemini surfactants have already been shown to have germicidal

properties greater than those of traditional monoquaternary surfactants [270]. This

fact, coupled with the (probable) low toxicity of such compounds and the observed

low critical micelle concentrations, suggest that the gemini surfactants may be an

appropriate alternative to traditional quaternary ammonium compounds. The

implications of the results obtained of gemini micellization may also be useful in

micellar catalysis. Survey of the available literature reveals that no seriuos attempt

has been made to study the effect of addition of organic additives: alcohols and

amines in the presence of salt on the micellization phenomenon of dicationic

gemini surfactants. In the present study we have find out the effect of various

amines/alcohols additives on the gemini surfactants, which can further be used for

drug encapsulation and delivery. Thus, the experimental results of my present

study may be useful in understanding and predicting the surfactants selection for

controlled drug release targeted delivery.

Generally, in the absence of salts at moderate concentrations, the surfactant

(pure and mixed) aggregates exist in the form of spherical micelles in aqueous

solutions. In the presence of salts, with its increasing concentration, the spherical

aggregates tend to transform into nonspherical ones (viz., rod, branched or

wormlike micelles). Among various factors acting on salt addition, the formation

and growth of micelles are mainly favored by the screening of electrostatic

repulsion among the polar head groups and movement of the hydrophobic alkyl

chains away from the aqueous environment. This is evidenced by a decrease in

CMC and an increase of the micelle aggregation number [120,159].

A vast majority of experimental data are available on solution/aggregational

behavior of conventional surfactants in presence of different classes of additives.

Many studies on the influence of a variety of additives (organic/inorganic

compounds, non-electrolytes, surfactants, etc.) by Kabir-ud-Din and coworkers

have yielded important results including structural transitions and growth of

micelles in gemini solutions [271-277]. Except for some earlier studies done by

others [159,194,278-290] and Kabir-ud-Din and coworkers [271,272,277] on the

interaction of salts (inorganic and organic) with cationic gemini micelles, studies

on the gemini–salt systems is still scarce. Herein, we report for the first time the

interaction of inorganic and organic salts with the cationic gemini surfactants

systematically on the aggregation of gemini surfactant systems where we have

covered details of the following: (1) the influence of inorganic and organic

counterions on the morphological transition of seven cationic gemini surfactants,

(2) evaluation of the spacer chain length effect on the micellar morphology of

gemini surfactants, and (3) assessment of the solubilization sites of added salts in

the gemini micellar systems.

The interaction between gemini surfactants and salts is mainly related to the

nature of added counterions and structure of the gemini surfactant including polar

headgroup and spacer moiety. Gemini surfactant, with a short spacer having a

relatively large molecular packing parameter and small surface area accupied per

head group (a0), is more efficient for producing rich aggregate morphologies.

Thus, a system causing a decrease in the surface area occupied per head group (a0)

of gemini surfactant is responsible for inducing growth of gemini micelles.

Therefore, various types of aggregate morphologies could be achieved by using

such type of molecular structure in a fixed system. The transition of different

aggregate morphologies by addition of salts may be applied in bio-engineering,

surface chemistry, and natural sciences.

Layout of the thesis

This thesis consists of the following five chapters:

Chapter-I General Introduction

Chapter-II Experimental

Chapter-III Micellization and Interfacial Properties of Gemini Surfactant

(12-4-12) in the Presence of Additives: A Tensiometric Study

(A) Effect of Inorganic and Organic Salts

(B) Effect of Organic Additives (Alcohols and Amines) in the Absence and

Presence of Potassium Nitrate

Chapter-IV Micellar Growth of Gemini Surfactants (12-4-12, 14-s-14 and

16-s-16; s = 4, 5, 6) in the Presence of Additives: A Viscometric Study.

(A) Effect of Inorganic and Organic Salts

(B) Effect of Organic Additives (Alcohols and Amines) in the Absence and

Presence of Potassium Nitrate

Chapter-V 1H NMR Study of Gemini Surfactants (12-4-12 and 14-s-14;

s = 4, 5, 6) in the Absence and Presence of Inorganic and Organic Salts

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