CHAPTER I

General Introduction

1.1 Introduction

1.2 Review of Literature

1.2.1 Surfactant-Additive Mixed Systems

1.2.2 Mixed Systems of Surfactants

1.2.3 Mixed Systems of Triblock Polymers

1.2.4 Surfactant-Triblock Polymer Mixed Systems

1.3 Objectives of Present Work

References

1

1.1 Introduction

Surfactants form a unique class of surface active chemicals which possess

ability to radically alter surface and interfacial properties and to self-associate and

solubilize themselves in micelles. These properties enable the use of surfactants in

wettability modification, detergency, stabilization and destabilization of dispersions.

These in turn lead to frequent appearance of surfactants in diverse fields ranging from

routine commercial products: motor oil, pharmaceuticals, detergents, petroleum and

flotation agents for beneficiation of ores, to the high technology products: electronics,

printing, magnetic recording, biotechnology and microelectronics.1-7

Surfactants also

constitute an integral part of industry engaged in the formulations of medicines,

cosmetics and synthesis of nano-materials.8-19

As a consequence, demand for high-

performance surfactants is continuously increasing. Besides the commercial need, high

performance surfactants are also required for environmental protection since a less

amount of surfactant used can contribute in reducing the load on natural purification

systems. Self assembly of surfactant systems is therefore an enthralling arena of

research that implies for extensive research efforts in designing novel surfactant

systems and understanding their self assembly behavior.20-22

Surfactants are the schizophrenic molecules possessing discrete polar and

nonpolar entities that helm the surfactant assembly in aqueous solution. Nature of polar

entity forms the basis for classification of surfactants into anionic, cationic, nonionic

and zwitterionic surfactants. Anionic surfactants possess anionic polar entity like long-

Structure of surfactant monomer

chain fatty acid sulfosuccinates, alkyl sulfates, phosphates and sulfonates. These are the

most widely used class of surfactants in industrial applications due to their relatively

low cost of manufacture, excellent cleaning properties and high sudsing potential.

2

Surfactants with cationic polar entity known as cationic surfactants may be protonated

long-chain amines or long-chain quaternary ammonium compounds. Cationic

surfactants share only a small portion of the surfactant market as ingredients of

antibacterials, liquid crystals, gene transfection agents, phase transfer catalysts and

crystalline mesoporous materials.10,23

Amphoteric surfactants represented by betaines

and lecithins being mild in nature are used in personal care products and household

cleaning formulations. Nonionic surfactants possess non-dissociable polar entity like

polyethylene oxide, alcohol and related polar groups. Most of the cleaning formulations

contain nonionic surfactant as an important ingredient that imparts resistance to the

hardness in the surfactant systems.

Structural duality in surfactant molecules tends to associate them into well

organized supramolecular assemblies called micelles. Organization within these

supramolecular assemblies depends upon a complex interplay of molecular geometry,

amphiphilic character and charge on the molecules. At low concentration, surfactant

monomers are preferentially adsorbed at the air/solution interface that avoids the

energetically unfavorable interaction between nonpolar part of surfactant and water

dipole. Solvation of surfactant monomer occurs in such a way that the polar head

groups are solvated either by dipole-induced dipole or ion-dipole interactions and its

non-polar part lies in the air phase. Presence of these molecules on the surface disrupts

the cohesive energy at the surface and lowers the surface tension. As the concentration

of surfactant increases in the bulk, surface becomes more crowded and eventually gets

Micellization of surfactant

loaded with amphiphiles to such an extent that further increase in surfactant

concentration not leads to decrease in the surface tension. At this concentration,

surfactant monomers orient themselves into intelligent arrangements known as micelles

3

that allow each component to maximally interact with its favored environment. This

typical aggregation process prevalent in surfactant systems is referred to as micellization

and corresponding concentration is named critical micelle concentration (cmc); a highly

temperature-dependent surfactant characteristic.

Micelles are dynamic nanoscopic aggregates possessing variable features in

terms of geometry (globular, cylindrical, spherical etc), physicochemical properties,

polarity and hydrophobicity. Micellar solutions of different surfactants are frequently

used as media in a variety of applications during chemical analysis and synthesis.24-26

Size and stability of the micelles are important features that determine their

physicochemical properties and appliances of a particular surfactant system. Micelles

demonstrate considerable affect on the rate of reaction, its equilibrium and

stoichiometry of the reactants and products.27-30

Exploit of micelles in pharmaceutical

formulations is very common as dissolving agent, vehicle and protecting agent.31-33

Modulation of micellar properties, therefore, receives constant attention from both the

technological as well as ecological viewpoints.

Micellar properties of an aqueous surfactant solution are dependant upon the

identity of surfactant and environmental conditions.34

Surfactant assembly exhibits

more or less fixed physicochemical behavior at ambient conditions of concentration and

temperature and is difficult to modulate. For the modulation of self-assembly and

micellar properties, many approaches have therefore been developed ranging from

molecular modifications to the additive introduction.35-38

Owing to the inherent

difficulty in the preparation of chemically pure surfactants, tailoring of micellar

properties of surfactants by using additives has been the underpinning of a great deal of

scientific and applied research. Although surfactant science is a reasonably mature

discipline, there is still a vast room for designing novel mixed surfactant systems to be

used in specific purposes. As the addition of a judiciously selected and environmentally

benign substance for behavior modulation is a convenient and preferable in practice,

number of external additives like cosolvents, cosurfactants, electrolytes, polar and

nonpolar organics etc39-42

have been employed for the purpose. Addition of different

kinds of additives to surfactant system gives a number of possible surfactant mixtures

with better performance characteristics that has been focused in the thesis work.

4

1.2 Review of Literature

A brief description with detail survey of literature related to different possible

surfactant mixtures is given in the thesis under different headings:

1.2.1 Surfactant-Additive Mixed Systems

Probing the micellar, thermodynamic and interfacial properties of surfactants in

the presence and absence of additives can provide extensive information about solute-

solute and solute-solvent interactions in these systems.43,44

Additive modifies

environment around the micellar self assembly of surfactant molecule and thereby

affects its functionality. Range of possible functional modifications depends upon the

nature of surfactant-additive interactions, which might favor or counter the self

association process and consequently influence the micellar morphology and physico-

chemistry.45-47

Most studies in literature concentrate on the perspective of favorable

association as it provides a way to meet the rising demand of surfactants in industry and

fulfills the requirements regarding stringent environmental policies being implemented.

Need for such type of studies is also increasing on account of the fact that modern

chemical industry engaged in the development of surfactant based formulations often

employs surfactant-additive mixed systems to achieve the optimum properties in terms

of better cleansing power and foaming capacity with lower skin irritation etc.

Most of the additives in aqueous media decrease the solubility of second

component, however, certain additives might exhibit the contrary behavior leading to an

enhanced solubility. Terminology used in surfactant chemistry for reduced solubility is

„salting out‟ and „salting in‟ is the term used for the reverse effect. Additives that

enhance aqueous solubility are called hydrotropes or chaotropes. Phenomenon of

„salting in‟ and „salting out‟ play significant role in numerous practical applications, for

instance in separation processes (protein precipitation, isomer separation), development

of pharmaceutical formulations and modification of cloud point of surfactant solutions.

Assortment of theoretical and experimental efforts aimed at explanation for these

effects leads to more than one possible mechanisms. One of these mechanisms assumes

a strong interaction between additives and surfactant similar to the complex formation

that leads to a higher solubility. Another hypothesis assumes that additive inserts itself

5

into the water structure and modifies it. This hypothesis imparts specific terminology to

the additives depending upon its effect on water structure in terms of structure-breaker

or structure-maker. A third alternative available is based on the self association of

additives to form aggregates similar to micelles that affects the solubility and properties

of other surfactant.48

Considering the wide range of additive‟s diversity, it is reasonable

to ascribe to more than one mechanisms for accounting the changes in the specific

property brought about by different kind of additives. Whatsoever the hypothesis is

employed, enhanced solubility in all the systems results from more favorable solvent-

additive interaction compared to pre-existing surfactant-solvent interaction. Influence of

additives on the interfacial tension and interaction between different composites in

surfactant-additive mixed system has been established in a number of publications.

Zhao and Chen49

have studied the clouding phenomena and phase behavior of

nonionic surfactants, octylphenol ethoxylate surfactants (Tritons-X-114: TX-114,

Triton-X-100: TX-100) in the presence of hydroxyethyl cellulose (HEC) and its

hydrophobically modified counterpart (HMHEC). At lower concentration, HMHEC was

found to have a stronger lowering effect on the cloud point (CP) of these nonionic

surfactants. Difference in clouding behavior observed was attributed to the existence of

different kinds of molecular interactions in these systems. Depletion flocculation was

reported to be the underlying mechanism in the case of HEC-nonionic surfactant

systems, while the chain-bridging effect was responsible for the greater decrease in

cloud point observed for HMHEC-nonionic surfactant systems. Composition analyses

was also carried out for investigating different macroscopic phases formed so as to

support the associative phase separation in HMHEC-nonionic surfactant systems, in

contrast to segregative phase separation reported for HEC-nonionic surfactant systems.

Mahajan et al50

have investigated the effect of glycols oligomers such as

(diethylene glycol: DEG, triethylene glycol: TEG, ethylene glycol monoethyl ether:

EGMEE and ethylene glycol monobutyl ether: EGMBE) on the micellar properties of

nonionic surfactants (Tween 20 and Tween 80) using small angle neutron scattering

(SANS) and turbidity measurements. Shape of Tweens was reported to be oblate

ellipsoidal that do not change predominantly in the presence of DEG and TEG. EGMEE

and EGMBE were reported to be reducing the aggregation number of Tweens due to the

6

solubilization of EGMEE and EGMBE in Tween micelle that provides additional

hydrophobicity. In another study, they have investigated the additive effect of various

glycols: ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG),

ethylene glycol monomethyl ether (EGMME), ethylene glycol monoethyl ether

(EGMEE), ethylene glycol monobutyl ether (EGMBE) and polyethylene glycols (PEG)

with average molecular weight 200, 400, 600, 4,000 and 6,000 on the clouding behavior

of nonionic surfactants: Tween 20 and Tween 80.51

They have reported depression in

the cloud point of Tween 20 and Tween 80 in the presence of these additives. Among

different glycol based oligomeric additives investigated, ethylene glycol monobutyl

ether reduced the cloud point to the maximum extent. An increase in repeating units of

polymeric glycol additives lead to more decrease in cloud point.

Rodrıguez et al52

have investigated the effects of glycols (ethylene glycol, 1,2-

propylene glycol, 1,3-propylene glycol and tetraethylene glycol) on the thermodynamic

and micellar properties of tetradecyltrimethylammonium bromide (TTAB) by

conductivity and fluorescence techniques. They found that addition of these glycols to

aqueous TTAB solutions causes an increase in the critical micelle concentration (cmc)

as well as micellar ionization degree. However, effect on Gibbs energies of

micellization (∆Gm) of TTAB was identical and an increase in the glycol content has

been reported to cause decrease in the interfacial Gibbs energy contribution to ∆Gm and

average aggregation number. Ability of the TTAB micelles present in the water-glycol

mixtures as catalysts has been examined from the study of the reaction of methyl

naphthalene-2-sulfonate + Br- in the water-glycol TTAB micellar solutions and found

that the reaction was faster in water-glycol TTAB micellar solutions than in water.

Sharma et al53

have reported the effect of various electrolytes and

nonelectrolytes on the cloud point of nonionic polyethoxylated alkyl ether surfactants:

C12EO6, C12EO9 and C12EO10. Clouding of C12EO10 exhibited minimum variation with

concentration of additive. Sucrose, glucose, potassium chloride, potassium bromide,

sodium chloride, sodium iodide, tetra methyl ammonium bromide and tetra butyl

ammonium iodide changed the cloud point (CP) to a larger extent. It was reported that

sodium iodide and potassium iodide have different effect on the CP from that of sodium

chloride, sodium bromide, potassium chloride and potassium bromide. Difference in the

7

effect of tetrabutylammonium iodide and tetramethylammonium bromide on the CP was

explained on the basis that the mixed micelle formation of tetra butyl ammonium iodide

with nonionic surfactant predominates over water structure formation. It has been

reported that electrolytes and nonelectrolytes affect the CP of nonionic surfactants by

modifying their water structure and hydrophilicity.

Interaction of amino acids and glycine peptides with sodium dodecyl sulfate

(SDS) and hexadecyltrimethylammonium bromide (HTAB) has been explored by Singh

et al.54

Apparent molar volumes of glycine, alanine, valine, leucine, lysine, diglycine

and triglycine were determined in aqueous solutions of SDS and HTAB using density

measurements. Contribution of charged end groups (-NH3+, -COO

-), CH2 group and

alkyl chains of the amino acids towards partial molar volume has been evaluated for a

homologous series of amino acids. Results indicated that ion-ion, ion-polar and

hydrophobic-hydrophobic group interactions are responsible for the partial molar

volumes of transfer from water to aqueous SDS and HTAB. They have reported that

interactions of the amino acids and peptides are stronger with SDS compared to HTAB.

Comparison between hydration numbers of different amino acids indicated that

hydration numbers are almost same at 1 mol kg-1

level of cosolvent/cosolute. Increasing

molality of cosolvent/cosolute beyond 1 mol kg-1

lowers hydration number of amino

acids due to increased interaction with solvent. Yan et al55

have studied the effect of

glycyl dipeptides: glycylglycine, glycyl-L-valine, and glycyl-L-leucine on the micellar

properties of gemini surfactant pentamethylene-1,5-bis(dodecyldimethyl ammonium

bromide): 12-5-12 by conductivity and fluorescence measurements. Enhancement in

micellization of surfactant has been observed in the presence of glycyl dipeptides.

Negative free energy and positive entropy of micellization have been evaluated using

the mass action model.

Additive effect of ethylene amines (ethylenediamine, diethylenetriamine,

triethylenetetramine, and tetraethylenepentamine) on the surface and mixed

micellization properties of cationic gemini surfactants butanediyl-1,4-bis(dimethyl

cetylammonium bromide): 16-4-16, pentanediyl-1,5-bis(dimethylcetylammonium

bromide): 16-5-16 and hexanediyl-1,6-bis(dimethylcetylammonium bromide): 16-6-16

at 303.15 K has been investigated using surface tension.56

Mixed micellization and

8

synergistic interactions at all mole fractions for the proposed systems were reported.

They observed that increase in concentration of additives decreases the critical micelle

concentration (cmc) and minimum surface area values (Amin) and increases the surface

excess (Γmax). Increase in spacer chain length of gemini surfactants exhibited an

increase in cmc, and Amin values along with a decrease in surface pressure (πcmc) and

surface excess (Γmax). Evaluation of thermodynamic parameters indicated

thermodynamically stability and spontaneity of the proposed mixed system. Attractive

interactions were reported to be increasing with spacer chain length of the gemini

surfactants in the order: 16-6-16 > 16-5-16 ≈ 16-4-16.

Ahmad et al57

have carried out the cloud point study to determine

thermodynamic parameters for tetrabutylammonium dodecyl sulfate (TBADS) in the

presence of quaternary bromides, alcohols, and amines as additives. They found that

clouding of TBADS in presence of quaternary salts and short chain amines was

exothermic and solvated water molecules released from the domain of micellar

aggregation with increasing temperature made entropy values (ΔS◦) negative. Among

investigated additives, higher chain amines and alcohols were reported to be increasing

the micelle size and decreasing the randomness of the system thereby exhibiting the

positive TΔS◦ values and negative Gibbs energy change, ΔG

°. Negative value of Gibbs

free energy, ΔG◦ for all the systems revealed the spontaneity of clouding phenomenon.

Chavda et al58

analysed the partitioning of monohydric alcohol 1-butanol and

dihydric alcohol 1,4-butanediol into the micelles of cationic surfactants by employing

small angle neutron scattering (SANS), conductance and viscosity techniques. From

these studies they concluded that although these alcohols change the micellar properties

and contribute to reduction in electrostatic and hydrophobic interactions yet they behave

in a different manner. SANS measurements were used to explore the affect of alcohols

on the micellization behavior of cationic gemini surfactant (N,N-bis(dimethyldodecyl)-

1,4-butanediammonium dibromide: 12-4-12) and conventional cationic surfactant

tetradecyltrimethylammonium bromide (TTAB). Viscosity measurements indicated

micellar growth as butanol gets partitioned into both surfactant systems, however,

micellar growth in the gemini surfactant system was more pronounced due to enhanced

micellar properties and solubilization. It has been reported that viscosity and micellar

9

size of 100 mM TTAB in the presence of butanol at concentration higher than 5%

decreases. These results were also supported by SANS results. Partitioning of the

alcohols and their locations in the micelles deduced from 2D NMR experiments were

consistent with the number and position of hydroxyl groups making a significant

contribution to both hydrophobic and electrostatic aspects of self-assembly process.

Using conductometric technique, influence of ionic and nonionic hydrotropes

(aniline hydrochloride, 2-methylaniline hydrochloride, 4-methylaniline hydrochloride,

hydroxybenzene, 1,3-benzenediol, benzene-1,2,3-triol) on the micellar behavior of

cationic gemini surfactant, butanediyl-1,4-bis(dimethylcetylammonium bromide):

16-4-16 has been investigated by Khan et al.59

Critical micelle concentrations for

different mixing mole fractions at different temperatures were calculated and found to

increase with increase in temperature. Synergistic interactions observed in all the binary

combinations decrease slightly with increase in temperature due to thermal impediment.

Highly negative micellar interaction parameter and lower activity coefficient value

confirm the existence of synergistic interactions in these systems.

Soni et al60

have studied the effect of 26 non electrolyte additives (including 12

alcohols, 7 glycols and 7 amides) on the association and clouding behavior of silicone

surfactants (SS) in aqueous solutions. They reported that addition of the water soluble

polar organic liquid additives increases the critical micelle concentrations (cmc) and

cloud point of SS, whereas the partially water soluble polar organic liquid additives

decreases the cmc and cloud point of SS. The observed changes of free energy changes

for the process in mixed solvent systems and water revealed the negative values of ΔG◦m

which indicate that presence of additives facilitate the micelle formation at lower

concentrations, ultimately decreasing the cmc as well as cloud point.

Effect of different alkanols viz., ethanol, butanol and hexanol on the micellar

characteristics of alkyltrimethylammonium bromides (dodecyltrimethylammonium

bromide: DTAB, tetradecyltrimethylammonium bromide: TTAB and hexadecyl

trimethylammonium bromide: HTAB) in the presence of 0.1 M salts has been

investigated using viscosity and dynamic light scattering (DLS) techniques by Kuperkar

et al.61

Conductometric measurements were carried out to explore the role of

counterions in binding phenomenon in the presence of salt and alcohol. Viscosity

10

results confirmed the general trend of micellar growth and transitions. They observed

that long chain alcohols (n ≥ 5) act as cosurfactants whereas short chains (n ≤ 3) act as

additives. For butanol, behavior was intermediate with strong dependence on the

concentration of additives. Butanol initially screens the charges resulting into structural

transition and eventually penetrates the micelles to form the mixed aggregates of lower

size and viscosity. Medium chain alcohols in micellar aggregate induce micellar

transition by anchoring at the micellar interface and decreasing the micellar surface

charge density, whereas, their hydrophobic tails intercalate between the surfactant tails

thus modifying the packing structure which in turn changes the rheological properties.

Graciani et al62

have investigated the influence of butanol, pentanol, and

hexanol on the micellization of alkanedyil-α,ω-bis(dimethyldodecylammonium)

bromide type dimeric surfactants 12-s-12,2Br- (s = 3, 4, 6). Influence of nature and

concentration of alcohol on the cmc, micellar ionization degree, average micellar

aggregation number and polarity of the micellar interfacial region was investigated

using conductivity and fluorescence measurements. Presence of alcohol decreased the

average micellar aggregation number that has been attributed to the decrease in

interfacial Gibb‟s energy. Decrease in aggregation number was accompanied by an

increase in the polarity of interfacial region.

Influence of different aliphatic alcohols (methanol, ethanol, propanol, butanol,

iso-butanol, tert-butanol, pentanol, heptanol, decanol and hydrocarbons (n-hexane,

cyclohexane, benzene) on the cloud point of non-ionic surfactant polyoxyethylene(7.5)

nonylphenyl ether (PONPE-7.5) was studied by Fernandez et al.63

They reported that

short-chain alcohols (methanol, ethanol, propanol and butanol) increase the cloud point

by promoting the formation of expanded water structures and favoring the micelle

hydration. This influence increased with the concentration of the additive and was

negligible at low concentrations. Effect of longer-chain alcohols suggests their

preferential solubilization in the inner core of the aggregate that increases the volume-

to-area ratio for the micelle and the cloud point decreases. Effect of branching and

cyclization were also studied by using two series: n-butanol, iso-butanol and tert-

butanol, and n-hexane, cyclohexane and benzene, respectively. For both the series,

addition of compound provoked a marked decrease in the cloud point. These effects can

11

be explained in view of the site of their solubilization in the micelle. Benzene molecule

interacts preferentially with the ethylene groups, leading to strong dehydration, while

the aliphatic compounds are solubilized in the inner core.

Yu et al64

have studied the effect of inorganic and organic salts on the

aggregation behavior of cationic gemini surfactants 12-4-12 and 12-4(OH)2-12. They

have reported that salt effectively reduces the critical micelle concentration of these

gemini surfactants. Ability of salts to promote the surfactant aggregation decreases in

the order: C6H5COONa > p-C6H4(COONa)2 > Na2SO4 > NaCl. Only C6H5COONa

distinctly reduced the cmc and surface tension value at cmc. For 12-4-12 solution,

penetration of C6H5COO- anions and charge neutralization induced a morphology

change from micelles to vesicles, whereas other salts only slightly increased the

micellar size. 12-4(OH)2-12 solution changed from micelle/vesicle coexistence to

vesicles with addition of C6H5COONa, whereas, other salts transfer the 12-4(OH)2-12

solution from micelle/vesicle coexistence to micelles. As compared with 12-4-12, two

hydroxyls in the spacer of 12-4(OH)2-12 promote micellization of 12-4(OH)2-12 and

reduce the amount of C6H5COONa required to induce micelle-to-vesicle transition.

1.2.2 Mixed Systems of Surfactants

Mixing of surfactants has been the subject of intense research in the last decade

owing to fundamental, technological and commercial considerations.65-68

Practical arena

prefers surfactant-surfactant mixed systems in lieu of pure surfactants as these mixed

systems usually exhibit superior performance properties. These mixtures are often

preferred as solublizers, emulsifiers, flow field regulators, membrane mimetic media,

nanoreactors for enzymatic reaction, templates for nanolithography, microelectronics,

microfluidic devices, nanosensors and molecular coverings.69-81

In addition, the

surfactant-surfactant mixtures also carry interest in micellar catalysis.

Micellization in surfactant-surfactant mixtures involves a more complex

interplay of intermolecular forces in comparison to the micelle formation by a single

surfactant. Interactions between surfactants are generally electrostatic and hydrophobic

in nature. Evaluation of different micellar parameters in the mixed state involves the

estimation of the nature and strength of interactions between two surfactants.

12

Surfactant-surfactant mixtures often exhibit nonideal behavior dependent upon

surfactant structure in terms of the size of head as well as tail group. Interactions

leading to non-ideality in surfactant-surfactant mixed solutions may be either favorable

or unfavorable. In case of favorable mixing, these system exhibits enhanced interfacial

properties such as lower critical micelle concentration and higher surface activity

relative to the individual surfactant.82-87

This type of mixing behavior is known as

synergism. Favorable or synergistic interactions make the system more attractive and

useful as it results in reduction of total amount of surfactant to be used in a particular

application thereby reducing its cost and environmental impact. Synergetic effects are

important for a wide range of surfactant-based phenomena such as foaming,

emulsification, solubilization and detergency.88,89

Therefore, investigation of the

surfactants with similar and different nature in aqueous mixtures has attracted a great

deal of attention in literature.

Based on the type of surfactant, numerous types of mixtures are possible viz.

cationic-cationic, cationic-nonionic, anionic-anionic, anionic-nonionic and nonionic-

nonionic. In literature, an extensive study on the surfactant mixed systems has been

performed in order to explore their micellar and structural behavior.

Das et al90

have reported mixed micellization behavior of binary and ternary

mixtures containing cationic surfactants: hexadecylpyridinium chloride (HPyCl),

tetradecyltrimethylammonium bromide (TTAB), and dodecylpyridinium chloride

(DPC) in aqueous medium using tensiometry and conductometry. They have observed

that cmc value increases with decrease in tail length which signifies that self

aggregation behavior in these systems is dictated by dominant effect of hydrophobic

interaction. The cmc values obtained by tensiometry and conductometry for pure and

binary combinations (HPyCl/TTAB and DPC/TTAB) signify concurrence of interfacial

saturation by surfactant monomer and their self aggregation in bulk solution. In

HPyCl/DPC mixture, tensiometric cmc precedes conductometric cmc at lower mole

fraction of HPyCl due to decrease in degree of counterion binding at micellar interface.

Negative deviation in experimental cmc for all the binary combinations infers Van der

Waals attractive interaction among similarly charged surfactant head groups

13

Ray et al91

have studied the interfacial adsorption and micellization behavior of

pure and mixed alkyltrimethylammonium bromides, decyl-, dodecyl-, tetradecyl- and

hexadecyltrimethylammonium bromide (DeTAB, DTAB, TTAB and HTAB) using

tensiometric, conductometric, fluorimetric, viscometric and calorimetric methods.

Critical micelle concentration, aggregation number, micellar polarity counterion binding

and thermodynamics of adsorption were determined. They have reported that binary

components of the mixtures undergo moderate synergistic interactions despite chain

length difference. Contribution of terminal methyl, methylene and hydrophilic

tetramethylammonium groups towards standard Gibbs free energy, enthalpy and

entropy of micellization processes were found to be positive for the systems studied.

Azum et al92

have investigated mixed micellization behavior of alkanediyl-R, ,ω-

bis(tetradecyldimethylammonium bromide): 14-s-14 (s=4,5,6) with cationic surfactants,

hexadecyltrimethylammonium bromide (HTAB) and tetradecyltrimethylammonium

bromide (TTAB) using conductance and fluorescence techniques. Evaluation of critical

micelle concentration, interaction parameters, micellar mole fractions and activity

coefficients for different mole fractions indicate attractive attractions between dimeric and

conventional surfactants. Motomura‟s theory was used to calculate the micellar mole

fractions and contribution of conventional surfactant and was reported to be more in

comparison to the ideal mixing state. Fluorescence quenching results were also found to

be in concurrence with the results obtained from conductometric study.

Jiang et al93

have investigated the exchange dynamics for several mixed

surfactant systems comprising gemini and conventional surfactants (12-2-12/TX-100,

14-2-14/TX-100, SDS/TX-100, 14-2-14/Brij-35, 12-2-12/TTAB, and 14-2-14/TTAB)

using their 1H NMR spectra for comparing the line widths and line shapes of the peaks.

They have reported that for SDS/TX-100 mixed system, addition of SDS fastens the

exchange rate of TX-100 in solution, while, TX-100 slows down exchange rate of SDS.

From the results they have shown that exchange rate of mixed surfactant gets enhanced

in the three cationic/nonionic mixed systems, while the exchange rates were lowered by

each other for the two cationic/cationic mixed solutions.

14

Chakraborty et al94

have investigated the mixed micellization behavior of

hexadecylpyridinium chloride (HPyCl), hexadecyltrimethylammonium bromide

(HTAB) and polyoxyethylene(10)cetyl ether (Brij-56) in different combinations in

aqueous medium by tensiometric, conductometric, calorimetric, spectrophotometric,

and fluorimetric techniques. Different physicochemical properties such as critical

micelle concentration, micellar dissociation, thermodynamic parameters, interfacial

adsorption and micellar aggregation number determined were favorable for

nonspherical micelles in HTAB/Brij-56, HpyCl/Brij-56 and HpyCl/HTAB/Brij-56

mixtures while HPyCl/HTAB system exhibited spherical shape.

Bakshi et al95

studied the mixed micellization of nonionic surfactant

polyoxyethylene alkyl ether (C10EO8, C12EO4 and C14EO8) with twin tail

alkylammonium bromides (12-0-8, 12-0-10, 12-0-12 and 12-0-16) employing

fluorescence, dynamic light scattering and electron spin resonance measurements.

Micelle formation in the mixtures of C10EO8/C12EO4+twin tail surfactants occurred due

to the synergistic interactions and increase in hydrophobicity of the cationic component.

Variation in micelle mole fraction suggested that mixed micelles are rich in nonionic

component. An increase in hydrophobicity of nonionic surfactant from C10EO8 to

C14EO8 induced antagonistic mixing in the mixtures of C14EO8 with twin tail surfactants

of longer hydrophobic tail.

Zhu et al96

carried out surface tension and dilational viscoelastic studies of water

in presence of surfactants, Tyloxapol and Triton-X-100 (TX-100) with hexadecyl

trimethylammonium bromide (HTAB). Results predicted nonideal mixing and attractive

interactions between constituent surfactants in the mixed micelles and monolayer.

Using Maeda theory, it was suggested that chain-chain interaction among surfactants

were not too high. Interpretation of surface dilational viscoelasticity results illustrated that

maximum value of dilational modulus |ε| for these surfactant mixtures lies between the

values exhibited by the individual surfactant. Higher value of |ε| for Tyloxapol/ HTAB

system in comparison to the TX-100/HTAB system infers that surfactant with high

surface activity has higher mole fraction in the mixed monolayer.

Ruiz and Aguiar97

have examined the aggregation behavior of binary system of

octaoxyethylene monododecyl ether (C12EO8) with dodecyltrimethylammonium

15

bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and

hexadecyltrimethylammonium bromide (HTAB) by fluorescence. Study established that

in addition to electrostatic interactions between head groups, secondary effects of steric

character due to varying alkyl chain length in alkyltrimethylammonium bromide

accounts for the existence of synergism. Increase in micellar micropolarity on the

incorporation of ionic component was consistent with the corresponding change in

micellization entropy due to formation of open micelles with more hydrated structure.

Study also concluded that micellar hydration is predominant factor in the formation of

proposed mixed micelles.

Saien and Asadabadi98

used drop-weight method to study the influence of octyl

phenol ethoxylate (Triton-X-100) and dodecyltrimethylammonium chloride (DTMAC)

surfactants on the interfacial tension of the individual and binary mixed systems.

Results indicated that addition of Triton-X-100 with bulk mole fraction less than 0.01

enhances the influence of DTMAC in lowering interfacial tension. For bulk mole

fractions up to 0.2, interfacial composition and attractive interactions in binary mixtures

calculated from the theory of nonideal interactions exhibited a different trend. Attractive

interaction parameter has maximum absolute value at the bulk mole fraction of 0.01 and

lower values at bulk mole fraction between 0.05-0.1. Experimental and ideal area per

molecule at interface exhibited variation in agreement with the interaction parameter.

Sehgal et al99

have studied the properties of mixed monolayers and micelles in

the mixture of dodecyltrimethylammonium bromide (DTAB) and dimethyldodecyl

ammoniopropane sulfonate (DPS) by interfacial tension, fluorescence, dynamic light

scattering (DLS) and viscosity. Authors have explained the deviation from ideality on

the basis of Clint theory. Interaction parameter and composition of the mixed micelles

and monolayers were obtained with the help of regular solution theory. Microviscosity

of the pure and mixed micelles was monitored by fluorescence polarization

measurements using rhodamine B as a fluorescence probe. Hydrodynamic radii of the

micelles were obtained in mixtures at different mole fractions of surfactant from DLS

measurements. Change in viscosity at high concentration of DTAB revealed that

minima got shifted to a higher value in the mixed state. This minima suggested

structural change in the mixed micelles.

16

Organization of amphiphilic nonionic surfactants, polyoxyethylated octyl

phenols (OP-10, OP-30) in mixtures with ionic surfactants, hexadecyltrimethyl

ammonium bromide (HTAB) and sodium dodecyl sulphate (SDS) have been

investigated by Dash and Misra.100

Employing conductometric method in conjunction

with fluorescence, surface tension, zeta potential and DLS measurements, they have

observed antagonistic interactions for the systems studied (OP-10/OP-30+HTAB/SDS).

Antagonism was found to be more prominent in presence of OP-10 as compared to OP-

30. Observed antagonistic effect was attributed to formation of hydrophobic complex

and was supported by fluorescence and surface tension measurements. Zeta potential

and DLS studies suggested the association of ionic surfactants to nonionic micelles.

Ray et al101

studied the micellization of mixed binary surfactant systems of

sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) by

conductometry, tensiometry, fluorimetry and microcalorimetry at different mole

fractions. Counter-ion binding, micellar aggregation number, thermodynamics of

micellization, interaction of components in mixed micelles and their composition

therein alongwith amphiphile packing in micelles indicated antagonistic interaction in

mixed micelles. Difference in head groups of SDS and SDBS has manifested interesting

solution and interfacial behaviors. Packing parameter of the amphiphiles in the pure and

mixed micelles advocated spherical geometry of the assemblies.

Janczuk et al102

have carried out surface tension studies on systems comprising

sodium dodecyl sulphate and sodium hexadecyl sulphonate. Evaluation of molecular

parameters for the mixed micelles and monolayers indicated that these surfactants do

not exhibit synergism in the adsorbed monolayers and micelles, however, a small

maxima in surface concentration excess at water/air interface and a minima in the

surface tension corresponding to cmc was observed at α = 0.4 and 0.2.

Mahajan et al103

have carried out tensiometric, fluorescence and viscometric

measurements on the interfacial and bulk properties of mixtures comprising anionic

(Sodium bis(2-ethylhexyl)sulfosuccinate: AOT) and zwitterionic surfactants {3-(N,N-

dimethyldodecylammonio)propane sulfonate: DPS, 3-(N,N-dimethyltetradecyl

ammonio)propane sulfonate: TPS, 3-(N,N-dimethylhexadecylammonio)propane

sulfonate: HPS} in aqueous media. These mixtures have been reported to be exhibiting

17

synergism and synergistic interactions increase with elongation of hydrocarbon chain of

the zwitterionic surfactant. Among all the systems reported highest synergism was

observed for AOT+HPS mixtures. This study demonstrated that the AOT+DPS/TPS/

HPS mixtures behave nonideally, whereas binary mixtures of SDS with same

zwitterionic surfactants show nearly ideal behavior. These observations confirmed that

twin-tail surfactant (AOT) introduces synergism more efficiently than single chain

surfactant (SDS) in the mixture.

Parekh et al104

have studied the mixed surfactant systems of anionic (sodium

dodecyltrioxyethylene sulfate: SDES) and gemini surfactants {N,N‟-bis-(dimethyldo

decyl)-α,ω-dialkanediammonium dibromide: 12-s-12, s = 2, 4, 6} at different molar

ratios by surface tension measurements. Results indicated that synergism and vesicle

formation on mixing leads to rich phase behavior, where mixture of vesicles and non

spherical micelles coexists. Negative interaction parameters for the micelles as well as

interface indicate strong synergism. Microstructure of mixed surfactant system was

reported to vary with mixing ratio and asymmetry of the two surfactants that enlarges

phase region. Strong interactions, low cmc and negative interaction parameter values

were reported to be due to the weakening electrostatic head group repulsions which

favor the mixed micelle formation.

Surface properties of binary mixed systems of decyl- and dodecylpyridinium

chloride or bromide, sodium pentyl- and heptylsulfonate have been investigated by

Goralczyk et al.105

Analysis of results concerning the composition of mixed monolayers

indicated that monolayers are rich in more surface active alkylpyridinium ions (RnPy+).

Properties of mixed films depend on both the ionic strength and the nature of inorganic

electrolyte added. With increase in electrolyte concentration, content of more surface

active ion in the adsorption film gets enhanced and this enhancement was highest in the

presence of sodium iodide and smallest for sodium chloride. Mutual interactions in the

mixed adsorbed films were attractive, however, strength of these attractive interactions

weakened with the increase in ionic strength. These interactions were also found to be

dependent upon the nature of inorganic ions with the order: Cl- > Br

- > I

-.

Li et al106

have measured the synergistic interactions between sodium dodecyl

sulfonate (SDS) and zwitterionic surfactants of varying alkyl chain length (alkyl

18

dimethylammoniopropane sulfonates: C12DPAS, C10DPAS and C8DPAS) at 40.0°C in

aqueous 0.1M NaCl medium. They have reported that synergism in these mixtures

exhibits maxima at 7:3 molar ratio. Fluorescence studies on SDS+C12DPAS mixed

system showed that addition of a small amount of anionic surfactant reduces the mixed

micellar aggregation number to a value lower than pure zwitterionics. However,

addition of C12DPAS to sodium dodecyl sulfonate enhances the mixed micellar

aggregation number to a value much larger than sodium dodecyl sulfonate. Study

reports that mixed micellar aggregation number is highest at the composition point

where efficiency of mixed micelle formation is maximum.

Murphy and Taggart107

have explored the surface and electrochemical properties

of cationic surfactants, octyl-, decyl-, dodecyl-, tetradecyl- and hexadecyl

trimethylammonium bromide (OTAB, DeTAB, DTAB, TTAB and HTAB respectively)

and sodium octyl-, decyl- dodecyl-, tetradecyl- and hexadecyl sulphates (SOS, SDeS,

SDS, STDS, SHDS respectively) in their ternary mixed states employing surface

tension, conductivity and fluorimetry. The cmc values obtained using molecular-

thermodynamic theory were of the same order as the pseudophase separation

approximation and experimental cmc values. They have demonstrated the suitability of

molecular-thermodynamic theory for the prediction of cmc values with individual

species concentrations and molar ratios.

Gharibi et al108

have studied the interaction of cationic surfactants alkyltrimethyl

ammonium bromides (octadecyl, dodecyl, tetradecyl, hexadecyltrimethyl ammonium

bromide) with nonionic surfactant (Triton-X-100: TX-100) using potentiometeric and

surface tension techniques. They have reported new methods for the evaluation of

physicochemical parameters, concentration of free monomer of ionic and nonionic

surfactants, degree of dissociation of ionic surfactants and mole fraction of surfactants

in mixed micelles. Comparison of new methods and previous methods indicated

noticeable difference between values of calculated synergetic parameters. Comparison

between cmc values indicates synergistic effect for octadecyltrimethylammonium

bromide with Triton: TX-100, whereas, cosurfactant effect has been observed between

dodecyl, tetradecyl and hexadecyltrimethylammonium bromides and Triton: TX-100.

19

Minimum structural effects on the magnitude of interaction parameters were also

observed for these systems.

Singh and Ismail109

have studied the mixtures of sodium bis(2-ethylhexyl)

sulfosuccinate (AOT) and sodium dodecyl sulfate (SDS) in water using surface tension,

conductance, emf and fluorescence emission methods. They found that system exhibits

synergism at mole fraction of AOT less than 0.7. Molal conductance versus

concentration behavior of AOT was found to be different from other ionic surfactants.

Values of counter ion binding from emf data show that counterion binding behavior of

the mixed micelles is controlled entirely by AOT. Aggregation number determined by

fluorescence quenching method indicated that as mole fraction of first component α1

increases, number of AOT molecules remains constant and that of sodium dodecyl

sulfate decreases in the mixed micelles.

Hierrezuelo et al110

have carried out fluorescence study on the micellization of

binary surfactant system formed by n-octyl-d-thioglucopyranoside (OTG) and sodium

dodecylsulphate (SDS) in 0.1 M NaCl aqueous solution. Data reported by treatment

based on Gibbs-Duhem equation exhibited reasonable agreement with regular solution

theory suggesting that mixed system behaves as a regular solution. Mixing

thermodynamic function values reveal that electrostatic repulsions between sulphate

groups of sodium dodecyl sulfate controls the stability of mixed micelles. Variation in

micellar aggregation number with solution composition infers that micellar size is

determined by delicate balance between two opposing contributions namely the

repulsive interactions between headgroups of ionic surfactant and steric interactions

favoured by size of headgroup of sodium dodecylsulfate in comparison with that of

OTG. Micellar micropolarity show little variation and is in accordance with the trend

observed for the aggregation number.

Bakshi and Kaur111

have characterized binary mixtures of polyoxyethylene alkyl

ethers (C10EO8, C12EO4 and C14EO8) with series of monomeric cationic, zwitterionic

and phosphonium cationic surfactants from steady state fluorescence measurements.

They have observed that the hydrophobicity of both nonionic as well as cosurfactant

components strongly influences the nature of mixed micelles. Study demonstrates that

the presence of bulky head groups in phosphonium surfactants significantly contributes

20

towards unfavorable mixing and increase the polarity of palisade layer of mixed

micelles in comparison to ammonium head groups. Such increase in the polarity of stern

layer has been reported to be reducing the hydrophobic environment of mixed micelles

and decreases the mean micelle aggregation number. This effect further increases with

hydrophobicity of the cosurfactant and was attributed to the screening of polar head

group repulsions.

Ohta et al112

using surface tension measurements have examined the influence of

polar head group size on the interaction and miscibility of binary surfactant mixtures in

adsorbed films and micelles. Mixtures studied comprised of polyoxyethylene alkyl ether

(C10EO5, C8EO5, C10EO5, C8EO4), dodecyltrimethylammonium bromide (DTAB) and

dodecylammonium chloride (DAC) surfactants. They have reported that lack of balance

in hydrophilic group size makes packing of C10EO5 and C8EO4 molecules less favorable

than that of C10EO5 and C8EO5 molecules both in adsorbed film and micelle. However,

interactions between cationic surfactants (DTAB or DAC) and C8EO4 is stronger than

between same species both in the adsorbed film and the micelle. Results indicated that

DAC molecule with smaller head group and higher surface charge density interacts

more strongly with C8EO4 molecules than DTAB molecules and these interactions are

stronger in micelles than the adsorbed films.

Formation of viscoelastic solution of worm-like micelles and one-dimensional

micellar growth on addition of lipophilic polyoxyethylene dodecyl ether (C12EO3) to

hexadecyltrimethylammonium bromide (HTAB) and sucrose monohexadecanoate

(P1695) in aqueous media has been reported by Engelskirchen et al.113

Steady and

oscillatory shear rheological measurements show that in the aqueous HTAB system,

extent of micellar growth decreases with increasing dispersion of polyethylene oxide

(EO) chain in the mixture in order: C12EO3 > C12EO2+C12EO4 > C12EO0+C12EO4 >

C12EO1+C12EO5. In the C12EO0+C12EO6 mixed system, micellar growth does not occur

but phase separation was observed. In P1695+C12EOn system, viscoelastic solution is

formed at relatively lower concentration of C12EOn and micellar growth decreases in the

order: C12EO3 ≈ C12EO2+C12EO4 > C12EO1+C12EO5 > C12EO0+C12EO6.

Hierrezuelo et al114

used conductance and fluorescence spectroscopic techniques

to describe interactional behavior in mixtures of MEGA-10 with n-alkyltrimethyl

21

ammonium bromide based cationic surfactants (DTAB, TTAB, HTAB). All the systems

exhibited negative deviation from ideal behavior and the combined effect of

electrostatic and short range interactions was responsible for nonideality in these

systems. By using static quenching method, they observed that in the mixed systems of

DTAB and TTAB with MEGA-10, aggregation number gets initially reduced and

afterwards becomes almost constant with value closer to the aggregation number of

pure ionic micelle. However, in the systems involving HTAB, size of micelles initially

increases and then decreases slightly for mixtures with a high content of the ionic

component. Fluorescence polarization data suggests that increasing participation of

cationic component induces formation of less compact micelles and this effect becomes

more pronounced as the alkyl chain length of ionic component decreases. Greater

interactions imply enhanced packing of hydrocarbon chains and consequent formation

of micelles with more ordered structures.

Synergism in zwitterionic/anionic surfactant systems of N-dodecyl-N,N

dimethyl-3-ammonio-1-propane sulfonate (ZW3-12)/sodium dodecyl sulfate (SDS), N-

dodecyl-N,N-(dimethylammonio)butyrate (DDMAB)/SDS, N-octyl-N,N-dimethyl-3-

ammonio-1-propane sulfonate (ZW3-08)/sodium octyl sulfate (SOS) and zwitterionic/

cationic systems of ZW3-12/dodecyltrimethylammonium bromide (DTAB), DDMAB/

DTAB has been witnessed by McLachlan and Marangoni.115

Conductivity and NMR

techniques were employed for cmc determination. ZW3-08+SOS and DDMAB+SDS

systems behaved synergistically at all mole fractions studied, while the ZW3-12+SDS

system exhibited synergistic behavior above mole fractions 0.30. Greater negative

deviations from ideal behavior were demonstrated in DDMAB+SDS system than other

zwitterionic/anionic systems. Zwitterionic/cationic systems of ZW3-12+DTAB and

ZW3-08+DTAB displayed only slight deviations from ideal behavior, therefore

indicating near ideal mixing.

1.2.3 Mixed Systems of Triblock Polymers

Polymeric aggregates of triblock polymers (TBPs) offer good candidature to

accomplish the rising demands of emerging fields in nanotechnology and biomedical

science research due to associated advantages like low toxicity, high stability and small

22

size.116-132

Unique solution behavior of these polymeric aggregates is also useful for the

emulsion stabilization, viscosity regulation, catalyst support and surface

modifications.133-136

All these applications make micelles of triblock polymer an

interesting and engaging subject of research. Triblock polymers constitute an important

class of water soluble surface active EO-PO-EO block copolymer nonionic surfactants

which are commercially available under the trade name Pluronics (BASF) with a wide

range of molecular compositions. Chemically triblock polymers are fabricated of

hydrophobic blocks polypropylene oxide units (PO)y and hydrophilic blocks

polyethylene oxide units (EO)x and are formulated as EOxPOyEOx, where x and y

represent the number of EO and PO units respectively.

These polymers exhibit unique morphological architecture due to their

amphiphilic character. In water or organic solvents, these polymers get self-assembled

into wide range of morphologies of micro to nanometer dimensions depending upon

their chemical nature. These micellar aggregates possess core-shell morphology with

compact core of water insoluble polypropylene oxide (PO) blocks surrounded by highly

swollen corona of water soluble polyethylene oxide (EO) blocks. Application of these

polymeric micellar aggregates is controlled by the characteristic properties of its core

and shell.135

For property modification and fabrication of novel aggregates with desired

structures, two aspects are usually adopted namely the alteration in synthetic parameters

or modification in surroundings. Alteration in synthetic parameters is carried out by

changing the polymer architecture, block nature or length and molecular weight of the

polymer, whereas, modification in surroundings is brought about by changing the

environment of block copolymer in terms of temperature, ionic strength, solvent type

and presence of cosolutes. As per needs, one or both of these aspects can be used,

however, alteration in synthetic parameters is time consuming and tedious to implement

in comparison to the modification of surroundings. As a consequence, past decade has

witnessed great efforts being devoted to the investigation of physicochemical properties

of polyethylene oxide and polypropylene oxide based block copolymers under different

solution conditions. Resulting nanosize micellar aggregates of triblock polymers have

been efficiently applied in pharmacology, cosmetology and encapsulation technologies.

These applications have been based on their tendency to act as nanocontainers for

23

solubilization and targeted release of hydrophobic drugs. Rapidly increasing interest in

triblock polymers has also been due to their biodegradability and biocompatibility.136-149

Mixing of two or more triblock polymers provides us a means for altering the

surroundings and modification of surfactant properties as mixing of structurally and

morphologically different polymers exhibits complementary effect on the system during its

adjustment to external environmental changes such as temperature and concentration.

Mixed micelles from cooperative self assembly of different triblock polymers exhibit

significantly improved stability and enhance their application potential.150

Mixed micelles

thus formed can be feasible candidate for fine tuning the micellar properties and matching

up with the constantly rising demands of diverse industrial fields.

Zhuang et al151

have investigated self-assembly behavior of AB/AC amphiphilic

diblock polymer mixtures in dilute solution by real-space-implemented self-consistent

field theory in three dimensions. They have reported that two diblock polymers

cooperatively self-assemble into hybrid aggregates and their morphologies were

dependent on the mixture ratio and interaction between their hydrophilic blocks.

Bakshi et al152

have reported fluorescence and viscometric study on the mixed

micellization of binary triblock polymer (TBP) mixtures P103 (EO17PO60EO17)+F127

(EO97PO69EO97)/P84 (EO19PO43EO19)/L64 (EO13PO30EO13)/P104 (EO18PO58EO18)/

P123 (EO20PO70EO20) in aqueous media at 30 oC. They observed that binary mixtures

P103+F127/P84/L64 undergo mixed micelle formation due to synergistic interactions

while P103+P104/P123 show antagonistic behavior. Nonideal behavior of the proposed

combinations has been found to be significantly dependent upon variation in the PO/EO

ratio of unlike TBP components.

Yang et al153

have studied the formation of mixed micelles of BO8EO41 and

BO12EO76BO12 and found that BO12EO76BO12 (BO: polybutylene oxide) being highly

hydrophobic could not be dissolved in water even at room temperature. Addition of

BO8EO41 into the solution induces formation of mixed micelles and thus increases the

solubility of BO12EO76BO12. Presence of BO12EO76BO12 in the solution decreases the

cmc value of BO8EO41 to 0.9 mg/mL at 298.1 K. They also reported that aggregation

number of the mixed micelles were determined mainly by BO8EO41 (around 60 at 25 °C)

24

and was not affected by the presence of BO12EO76BO12 even though BO12EO76BO12

was responsible for the very low cmc value. This conclusion is a little bit different from

the general understanding of micellization process whereby the cmc and the aggregation

number are intercorrelated and are determined mainly by the more solvent phobic

block. More importantly, cmc of BO12EO76BO12 was always much smaller than that of

BO8EO41 and was even too small to be measured precisely.

Trong et al154

have studied micelle formation of triblock polymer F127 and

Pluronic EG56 (branched molecule with three branches of F127) by calorimetric and

rheological measurements. Micellization in these systems was reported to be

endothermic with enthalpy of formation/melting for both polymers proportional to the

total concentration. Rheological analysis before gelation for F127 revealed that initially

solubility of monomers exhibit progressive changes like decease in relative solution

viscosity which was followed by micelle formation. Enthalpy measurements revealed

the dependence of micelle concentration on temperature as well as volume fractions.

Portnaya et al155

have reported the micellization characteristics, structure and

morphology for the binary mixture of β-casein and triblock polymer, Lutrol

(EO101PO56EO101). Isothermal titration calorimetry experiments revealed that addition

of lutrol to monomeric and assembled β-casein results into mixed micelle formation.

Above cmc of β-casein, strong perturbations caused by penetration of hydrophobic

oxypropylene sections of lutrol into the protein micellar core lead to disintegration of

the micelles and reformation of the mixed lutrol/β-casein micelles. Negative enthalpy of

micelle formation and cooperativity increases with increasing β-casein concentration in

solution. Zeta-potential measurements show that lutrol interacts with protein micelles

even below its critical micellization temperature. Lutrol effectively masks the protein

charges probably by forming a coating layer of ethylene oxide rich chains. Small-angle

X-ray scattering and cryogenic-transmission electron microscopy (cryo-TEM) indicate

swelling of β-casein micelles in the presence of lutrol along the small axis that confirms

the formation of mixed micelles.

Han and Jiang156

have studied self assembly of AB/BC diblock polymer mixture

based on hydrogen bonding by Monte Carlo simulation. They observed that micellar

structure is a function of intensity of associative interactions, directional dependency of

25

H-bonding and intrinsic repulsive interaction between A and C blocks. Decreasing

directional dependency and increasing H-bonding interaction has an equivalent effect

on micelle structure and H-bonding association for AB/BC diblock polymer mixture.

Tian et al157

investigated the mixed micellization of diblock polymer

polystyrene-blockpoly(methacrylic acid) with triblock polymer poly-(methacrylic acid)

-block-polystyrene-block-poly(methacrylic acid), depending upon the system have

found single and two-peaked distributions of sedimentation velocity after prolonged

equilibration. Honda et al158

have used a temperature jump method to initiate the

micellization and noted the effect of micellization conditions on the formation of mixed

micelles in the mixture of diblock poly(methylstyrene)-block-poly(p-vinylphenethyl)

alcohol: KT-326 and KT-327 with slightly different block lengths in benzyl alcohol by

means of static and dynamic light scattering techniques.

1.1.4 Surfactant-Polymer Mixed Systems

Surfactant-polymer mixtures belong to a special category of soft materials

possessing superior characteristic features (surface activity, viscosity, wetting, foaming,

solubilization etc.) in comparison to either of the pure component.159,160

As self

assemblies of these mixtures range from micro to nanoscale, it consequently opens

doors for current and potential applications in diverse fields such as data storage,

photonics, biomimetics and catalysis.161,162

Other important technical applications based

on interfacial and rheological properties of these complex fluids and colloid

formulations are in the fields of nanomedicine and nanotemplating. Scientific view

behind these mammoth applications is vast variety of intermolecular interactions

resulting from multiple probable combinations.163-177

Surfactant polymer mixtures also

carry importance in research aimed at the basic understanding of biological behavioral

motifs as nature too extensively employs the analogous systems (i.e. charged

biomacromolecules and phospholipids) to create functional self-assembled

nanostructures.178-182

Understanding of the core principals of physics involved in the self

association of surfactant-polymer mixture may thus be helpful in manipulation of these

systems to suit the needs of academia as well as industry.

26

Several plausible scenarios for exploring the interactions between surfactant and

polymer components in surfactant-polymer mixtures have been proposed. According to

these, surfactant-polymer interactions are mainly dependant upon tail length and head

group of surfactant along with hydrophobicity and flexibility of the polymer

component. In the mixed state, surfactant binds noncovalently to the polymer and

induces cooperative self association at a given surfactant concentration known as

critical micelle concentration. This self association results in formation of “bead-like”

structures wherein surfactant micelles are bound to the polymer backbones.179

Besides

bead like structures, other types of complexes have also been reported wherein

surfactant monomers are bound to the polymer in a non-aggregated form. Because,

mode of interaction in these complexes is dependant on the type of surfactant and

polymer involved therefore ionic surfactants interact strongly with nonionic polymers

than non-ionic surfactants. Similarly, interactions between ionic surfactant and

oppositely charged polymer are coulombic in nature at lower surfactant concentration

and hydrophobic at higher surfactant concentration. However, interaction of non-ionic

polymers with surfactants is usually hydrophobic in nature.

Triblock polymers constitute an important class of nonionic amphiphilic block

copolymers among numerous commercially available water soluble polymers. Triblock

polymers and their mixtures with surfactants are widely employed on account of their

adaptive properties to various environmental conditions that arise from their vast

possible architectural and chemical compositions.183-193

Though carrying inherent

importance, yet, interactions in surfactant neutral polymer systems have comparatively

been less understood than surfactant-polyelectrolyte interactions.

Barbosa et al194

have investigated complex formation between triblock polymer

EO71G7EO71 (G: phenyl glycidyl ether), surfactant sodium dodecyl sulfate (SDS) and

various amphiphiles (penicillins/cloxacillin/dicloxacillin) by dynamic light scattering

and isothermal titration calorimetry. Different polymer/amphiphilic systems were

studied at constant polymer concentration with varying amphiphile concentrations. For

all these systems, relaxation time distributions show a well-defined single mode with a

shift toward slightly faster times, which indicate that electrostatic interactions between

complexes is minimum, so, it is possible to estimate size in terms of the apparent

27

hydrodynamic radii. Isothermal titration calorimetric data reveals interaction between

surfactant, hydrotropes and triblock polymer even at lowest SDS concentration.

Punjabi et al195

explored interactions between sodium dodoecyl sulfate (SDS),

dodecyltrimethylammonium bromide (DTAB), Triton-X-100 (TX-100) with diblock

and triblock polymers (EO18BO9, EO13BO10EO13 where BO stands for polybutylene

oxide). They observed that addition of ionic surfactant to the polymer solution induces

change in shape from spherical to prolate ellipsoids, destabilize them and suppresses

micelle formation at high surfactant loading. Resultant polymer-surfactant complexes are

hydrophilic in nature and characterized by high turbid and cloud points. It has been

observed that triblock polymer micelles are easily destabilized than diblock polymer

indicating importance of interaction between hydrophilic chains and surfactants.

Wettig and Verall196

have reported interaction between triblock polymers F68

(EO80PO30EO80), F108 (EO141PO44EO141), P123 (EO13PO30EO13) and N,N-bis(dimethyl-

α,ω-alkanediammonium) bromide type dimeric surfactants (12-3-12, 12-6-12) using

conductivity, fluorescence and equilibrium dialysis techniques. They have reported that

interactions observed in these systems were markedly different from those generally

observed in surfactant polymer systems. It has also been observed that increased

surfactant concentration in the polymer coil region results in gradual change from

clusters of monomer surfactant polymers to the formation of regular micelles.

Li et al197

have carried out isothermal titration calorimetry and dynamic light

scattering (DLS) studies on the interactions between gemini surfactants of m-s-m type

(m = 8, 10, 12, 18 and s = 3, 6, 12) and triblock polymers (P103: EO17PO60EO17, F108:

EO141PO44EO141). Study revealed that addition of gemini surfactant to the triblock

polymer is accompanied by large endothermic enthalpy changes. One broad peak in the

enthalpograms of F108 and two broad peaks for P103 solutions indicated interactions

between surfactant and triblock polymer monomers. DLS results revealed that lower

concentrations of gemini surfactant can also induce stabilization of loose aggregates of

monomers (F108) or triblock polymer micelles (P103). Transitions observed in the

hydrodynamic radii were consistent with breakdown of micelle clusters on addition of

gemini surfactant followed by mixed micelle formation and de-aggregation into

monomer P103.

28

Ge et al198

have studied structural transition during complex formation between

triblock polymer F127 (EO97PO69EO97) and nonionic surfactant TX-100 using 1H NMR

spectroscopy, dynamic light scattering and isothermal titration calorimetry. Three

concentration regions of TX-100 were identified wherein TX-100/F127 complex

undergoes temperature-induced structure transitions. Between 5-10 ˚C, F127 monomers

were mainly adsorbed on the surface of TX-100 micelles. Between 15-25 ˚C, TX-100

micelles prompted F127 micelle formation, whereas, within 30-40 ˚C, TX-100 gets

inserted into F127 micelles. At higher concentration of TX-100, it leads to the

breakdown of F127 aggregates and thus obtained monomers thread through TX-100

micelles to form complex with TX-100 micelle.

Ortona et al199

have studied interactions of triblock polymer PE6200

(EO11PO28EO11), 25R4 (PO19EO33PO19) with sodium decyl sulfate (C10OS), decyltri

methylammonium bromide (C10TAB) and pentaethylene glycol monodecyl ether

(C10EO5). Investigation of aggregation behavior of these systems by surface tension

measurements illustrate that triblock polymers in their non-aggregated state interact

more with anionic surfactant (C10OS) than cationic (C10TAB) and nonionic (C10EO5)

ones. Similar lowering in aggregation number of C10OS was shown by fluorescence

quenching measurements. Number of polymer chains necessary to bind each C10OS

aggregate was 6 for PE6200 and 2 for 25R4. Furthermore, this surfactant also induced

same increment in the gyration radius of the polymers as revealed by viscosimetry.

Calorimetric results were also applied by a simple equilibrium model to the aggregation

processes.

Niemiec and Loh200

used calorimetric technique to follow the interaction of random

copolymers (EO19PO15, EO34PO25, EO116PO48, EO43PO11, EO106PO27, EO204PO52) and

triblock polymers (L35: EO11PO16EO11, L64: EO13PO30EO13, F77: EO52PO35EO52, F88:

EO103PO39EO103 and F127: EO97PO69EO97) with ionic surfactant, sodium dodecyl

sulphate. Profile of isothermal titration calorimetry (ITC) curves indicated that

interaction increases with increase in the polymer hydrophobicity revealing the

intermediate behavior of these triblock polymers. For non-aggregated triblock polymers

with large EO blocks, ITC curves revealed that sodium dodecyl sulfate interacts with

PO and EO blocks almost independently, being more favorable with PO block, which

29

controls the critical micelle concentration value. Due to this fact the interaction of

surfactant with triblock polymer is more intense (as measured by a smaller cmc value)

than that with a random copolymer of similar composition

Kelarakis et al201

investigated polymer-surfactant vesicular complex formation

in aqueous medium. They have reported that introduction of ionic single-tailed

surfactant to aqueous solution of EO18BO10 [EO: poly(ethylene oxide), BO: poly(1,2-

butylene oxide)] leads to the formation of vesicles with dispersed particles having

hydrodynamic radius of few hundred nanometers. Dynamic light scattering showed that

at initial stages, dimensions of these aggregates do not depend on the sign of charge on

the surfactant head group. Small angle X-ray scattering (SAXS) analysis indicated the

coexistence of smaller micelles of different sizes and varying polymer content. In strong

contrast to the dramatic increase in size of dispersed particles induced by surfactants in

dilute solution, d-spacing of corresponding mesophases gets reduced monotonically

upon increase in surfactant loading. This effect has been attributed to the stabilization of

smaller aggregates at the expense of vesicular structures within gel phase.

Ghosh et al202

have explored structural features of mixed micelles comprising

hydrophobic triblock polymer L121 (EO5PO68EO5) and anionic surfactant sodium

bis(2-ethylhexyl)sulfosuccinate (AOT) in water by small angle neutron scattering

measurements. They have observed that L121 or AOT alone form vesicles in water

whereas in the mixed state and at appropriate ratio of the two components, a

thermodynamically stable isotropic solution containing micelles of prolate ellipsoidal

shape is formed. Size and fractional charge of these ellipsoids decrease as AOT/L121

ratio increases that imply an increase in associated charge of aggregates and coulombic

repulsions between surfactant molecules.

Effect of surfactants on the self-assembly of triblock polymer F108 in the

aqueous solution by measuring the percolation transition temperature, micellar size, zeta

potential and rheological properties has been studied by Nambam and Philip.203

Different kind of surfactants such as anionic sodium dodecyl sulfate (SDS), cationic

cetyltrimethylammonium bromide (CTAB) and nonionic nonylphenol ethoxylate (NP9)

were employed for the investigations. Addition of SDS to TBP solution resulted in a

dramatic reduction of the viscoelastic properties, while no significant reduction was

30

obtained with CTAB and NP9. The magnitude of decrease in the elastic modulus in the

presence of SDS indicated formation of a soft solid-like microstructure due to self-

assembled triblock polymer. This observation confirm the fact that strong electrostatic

barriers imparted to the micellar interface by SDS molecules at the core-corona

interface restrict the self-assembly and growth of micelles. Hydrodynamic diameter of

F108+SDS mixed micelles at 313.15 K decreases from 14 to 5 nm as SDS

concentration increases from 0.06 to 4 mM. With increasing surfactant concentration,

zeta potential of the triblock polymer micelles is found to decrease. These results

suggest that microstructure and elastic properties of triblock polymer micelles can be

tuned by varying the concentrations of ionic surfactant that enhances their application

potential as nanocarrier for drug delivery.

Kelarakis et al204

have studied the interaction of sodium dodecyl sulfate (SDS)

with poly(ethylene oxide) and poly(alkylene oxide) block copolymers. They observed

that addition of SDS induces fundamentally different effects to the self-assembly

behavior of block copolymer solutions with respect to its specific compositional

characteristics. For EO18BO10+SDS system, development of large surfactant-polymer

aggregates was observed while for BO20EO610+SDS, BO12EO227BO12+SDS,

EO40BO10EO40+SDS, EO19PO43EO19+SDS systems, formation of smaller particles

compared to pure polymeric micelles was observed. It was ascribed to the physical

binding between hydrophobic block of unassociated macromolecules and nonpolar tail

of the surfactant. Analysis of critical micelle concentrations of polymer-surfactant

aqueous solutions revealed negative deviation for EO40BO10EO40+SDS and

EO19PO43EO19+SDS systems but positive deviations for EO18BO10+SDS system.

Ultrasonic study on EO19PO43EO19+SDS system enabled identification of three regions

corresponding to three main steps of complexation namely SDS absorption to the

hydrophobic backbone of polymer, development of the polymer-surfactant complex and

gradual breakdown of the mixed aggregates.

Thummar et al205

have studied the effect of sodium dodecyl sulfate (SDS),

dodecyltrimethylammonium bromide (DTAB) and octylphenol ethoxylate surfactant

(TX-100) as additives on the association characteristics of amphiphilic triblock polymer

P105 (EO36PO57EO36). Surface tension, dynamic light scattering and viscosity

31

measurements were carried out for these systems at 303.15 K in presence of 200 mM

sodium chloride solutions. Successive addition of surfactant resulted in the increase of

critical micelle concentration of the triblock polymer (TBP) and adsorption tendency of

the polymer-surfactant complexes at water/air interface. Proposed mixtures exhibited

synergistic interaction behavior and drastic decrease in apparent hydrodynamic radius

of the TBP micelles with addition of surfactants. This decrease was attributed to the

shift of equilibrium between triblock polymer micelles and monomers in favor of

monomers. They have reported that drug release and drug diffusion coefficient of the

sparingly water soluble drugs increases upon the surfactant addition.

Bakshi et al206

have investigated mixed micelle formation of hexadecyltri

phenylphosphonium bromide (HTPB) and tetradecyltriphenylphosphonium bromide

(TTPB) with triblock polymers: EO18PO31EO18, EO2PO15EO2, EO2PO31EO2 and F68

(EO77PO29EO77) using fluorescence, relative viscosity and cloud point measurements.

They have observed that mixtures of HTPB+EO18PO31EO18/EO2PO15EO2 and

TTPB+EO18PO31EO18/EO2PO15EO2 undergo mixed micellization due to synergistic

interactions, whereas mixtures of HTPB/TTPB+EO2PO31EO2/EO77PO29EO77

demonstrate antagonistic or ideal mixing. Relative viscosity and cloud point

measurements also fully support these results. Results suggest that favorable mixed

micellization with triblock polymers can be achieved if the difference in number of PO

and EO groups is not very large as in case of mixtures of EO18PO31EO18 and

EO2PO15EO2 in comparison to that of EO2PO31EO2 and EO77PO29EO77.

Jansson et al207

have studied complex formation of nonionic triblock polymer

P123 (EO20PO70EO20) with cationic surfactant hexadecyltrimethylammonium chloride

(HTAC) in dilute aqueous solution using small-angle X-ray scattering, static and

dynamic light scattering and self diffusion NMR. P123/HTAC system has been

investigated between 1 wt% and 5 wt% P123 and with varying surfactant concentration

up to 170 mM HTAC. On mixing P123 and HTAC, two different types of complexes

were observed at various HTAC concentrations. At low molar ratios 0.5, P123 micelle-

HTAC complex was obtained as HTAC monomers associate non-cooperatively with

P123 micelle forming a spherical complex. Interactions investigated by calculating the

structure factor using the generalized indirect fourier transformation (GIFT) method

32

revealed that between molar ratios 1.9 to 9, two coexisting complexes were found, one

P123 micelle-HTAC complex and one “HTAC-P123” complex. As HTAC

concentration increases above molar ratio 9, P123 micelles were broken up and only

HTAC-P123 complex that is slightly smaller than HTAC micelle exists due to stronger

electrostatic repulsion.

Kaur et al208

have studied the aggregation behavior of twin tail cationic

surfactants, didodecyldimethylammonium bromide, ditetradecyldimethylammonium

bromide, and dihexadecyldimethylammonium bromide with triblock polymer L64

(EO13PO30EO13) in aqueous solution by employing small angle neutron scattering

(SANS), fluorescence, conductivity and surface tension techniques. SANS data analysis

has been revealed the vesicle formation in case of pure twin tail cationic surfactants and

spherical micelles of pure TBP: L64. Mixed systems of these surfactants with L64 have

been reported to be spherical in shape as shape of mixed micelles is predominantly

controlled by triblock polymer L64. Surface tension study indicated that the studied

mixed system exhibited nonideal and antagonistic behavior.

Lof et al209

carried out the rheological study on mixed micelles of triblock

polymer P123 and nonionic surfactant (C12EO6) in aqueous solutions. Purpose of the

study was to investigate the time dependence of shape transition of the mixed micelles

and characterizing the shape before and after the transition. Rheology results give clear

evidence that the P123-C12EO6 mixed micelles grow and change gradually in shape

from spherical to elongated (rod like) geometry with increasing temperature.

Castro et al210

have characterized the interactions between triblock polymer

EO67SO15EO67 (SO: styrene oxide) and surfactant sodium dodecyl sulfate (SDS) by

static light scattering, dynamic light scattering (DLS) and isothermal titration

calorimetry (ITC). They have observed that addition of surfactant changes the

physicochemical properties of the micellized triblock polymer. An abrupt change in the

hydrodynamic radius of the triblock polymer micelles at low SDS concentrations was

reported to be due to the association of SDS molecules with triblock polymer micelles.

Thus formed surfactant-polymer complex becomes more charged as more surfactant is

added due to interactions between the surfactant and triblock polymer. DLS data

suggested that size of the mixed micelles changes depending on the amount of SDS.

33

These results were also confirmed by ITC measurements. From the ITC data,

dehydration and rehydration of triblock polymer chains following formation of triblock

polymer rich-surfactant mixed micelles and their distortion were obtained.

1.3 Objectives of Present Work

Visualizing the tremendous use and unique properties of surfactant mixtures, a

detailed study of their physicochemical properties is required so as to meet their

constantly rising demand in academic as well as commercial sectors. For this purpose, a

systematic and comprehensive investigation on the physicochemical properties of

various surfactants in the presence and absence of different types of additives has been

carried out in the present study. Selection of different surfactants and additives to be

used in this work was carried out keeping in view the following objectives:

To find new surfactant mixtures with improved properties in comparison to the pure

surfactants to meet the ever increasing demand of diverse fields more efficiently.

To identify the mixing range of surfactants where synergistic interactions attain

maxima and the mixture exhibits better properties for surfactant based applications.

To investigate the shape and size of the mixed micelles of surfactant mixtures to

control the morphology of mixed micellar aggregates.

To study the phase separation behavior of surfactants in pure as well as the mixed

state so as to have novel surfactant mixtures with improved shelf life.

To understand the effect of presence of additives on the physicochemical properties

of surfactants in order to have an idea about the working potential of surfactants

under different conditions.

To accomplish the aforementioned objectives, mixing behavior of surfactant-

additive, triblock polymer-triblock polymer, surfactant-triblock polymer, cationic-

nonionic and anionic-nonionic surfactant mixtures was investigated using techniques

like cyclic voltammetry (CV), small angle neutron scattering (SANS), dynamic light

scattering (DLS), surface tension (ST), fluorescence (Flu), viscometry and cloud point

(CP) measurement. Investigation involved study of physicochemical aspects of these

mixtures in terms of micellization and diffusion behavior, microstructural parameters,

34

interfacial properties and phase separation behavior. Thesis work has been divided into

seven chapters:

Chapter I deals with introduction of surfactant and their mixtures alongwith a

review of literature. Detailed description of materials and experimental techniques

employed for the aforesaid investigations has been described in Chapter II.

Chapter III incorporates cyclic voltammetric and cloud point study on the

micellization and phase separation behavior of triblock polymer mixed systems

comprising of F127: EO97PO69EO97, F88: EO104PO39EO104, P123: EO20PO70EO20, P85:

EO26PO40EO26. For cyclic voltammetric investigations, 2,2,6,6-Tetramethyl-1-

piperidinyloxy (TEMPO) has been employed as an electroactive probe. Presence of

probe helps to explore the micellar environment of triblock polymer mixtures

(F127/F88+P123/P85). Triblock polymers used in this study have different

hydrophobicity due to difference in their PO/EO ratio. Influence of hydrophobocity on

the mixed micellization behavior of these mixtures has been thoroughly investigated. To

determine the shelf life and working temperature range, phase separation behavior of

these triblock polymer mixtures has also been studied and correlated to the micellization

behavior.

A systematic investigation on the microstructure and mixing behavior of cationic

surfactant (benzalkonium chloride) with nonionic triblock polymers (F127:

EO97PO69EO70, P123: EO20PO70EO20, P105: EO36PO57EO36, L64: EO13PO30EO13) has

been carried out using surface tension, small angle neutron scattering (SANS), dynamic

light scattering (DLS) and viscometric measurements. Surface tension measurements

were used to study those micellar parameters which determine surface active and

interfacial behavior of these mixed systems. DLS measurements were used for

measuring average dimensions of mixed micelles like diffusion coefficient and

hydrodynamic diameter. SANS measurements were used to determine the micellar

shape and size of mixed micelles. Detail of this study has been given in Chapter IV.

Clouding is an important property of nonionic surfactants which determines

their working range for a particular surfactant based application. It is very sensitive to

the presence of third component i.e. additive, which affects the interactional behavior of

35

surfactant. In order to investigate the influence of additives of biological origin on the

clouding behavior of triblock polymers: P84 (EO19PO43EO19) and L64 (EO13PO30EO13), a

study has been carried out using different biomolecules as additives. Triblock polymers

P84 and L64 used for this study were of same hydrophobicity and biomolecules

employed belonged to category of amino acids, dipeptides, amino alcohols, sugars,

hydroxy acids and dicarboxylic acids. Detail of this study is given in Chapter V.

Anionic-nonionic surfactant mixtures are important constituents of cleaning

formulations as anionic component maximizes the solublization capacity and nonionic

surfactant component maximizes the tolerance towards water hardness. In view of this,

micellization and phase behavior of anionic surfactants: double alkyl chain surfactant,

sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and single alkyl chain surfactant,

sodium alkylphenol ether sulphate (APES) in the presence of nonionic polyoxyethylene

alkyl ethers (POE: C10EO8/C10EO10/C13EO10) has been investigated using surface

tension, fluorescence and cloud point techniques. Detailed description of the surfactant

mixtures investigated is given in Chapter VI.

Micellar and interfacial behavior of the mixed systems containing cationic

benzalkonium chloride and nonionic polyoxyethylene alkyl ether surfactants (C10EO7,

C10EO8, C10EO9, C10EO10) has been examined by surface tension, fluorescence and

dynamic light scattering techniques. Surface tension and fluorescence techniques were

used to measure the micellar parameters, while, dynamic light scattering illustrates

micellar growth in these mixed systems. Detail of this study is included in Chapter VII.

36

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