COUNTER-ION EFFECTS ON THE KRAFFT TEMPERATURE AND MICELLE FORMATION OF IONIC SURFACTANTS IN AQUEOUS SOLUTION

Transcription

1 COUNTER-ION EFFECTS ON THE KRAFFT TEMPERATURE AND MICELLE FORMATION OF IONIC SURFACTANTS IN AQUEOUS SOLUTION A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN THE PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER OF PHILOSOPHY IN CHEMISTRY (PHYSICAL-INORGANIC) M.Phil Thesis SUBMITTED BY Komol Kanta Sharker St. ID F Session-April, 2013 Department of Chemistry Bangladesh University of Engineering and Technology Dhaka-1000, Bangladesh September, 2016

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4 ABSTRACT In this work the effect of some sodium and chloride salts on the Krafft temperature (T K ) and critical micelle concentration (CMC) of two classical ionic surfactants, Octadecyltrimethylammonium bromide (OTAB) and Sodium dodecyl sulfate (SDS) have been investigated by conductometric and tensiometric method. Sodium salts of different monovalent and divalent anions belonging to the Hofmeister series found to decrease or increase the T K of OTAB. In terms of decreasing the T K the propensity follows the order: 2 C 7 H 5 O 3 > C 7 H 5 O 2 > C 6 H 5 SO 3 > SO 4 > Cl > NO 3 > F > Br > SCN > I. The results show that hydrotropic and kosmotropic counter-ions decrease while chaotropic counter-ions increase the T K of the surfactant. Chloride salts of monovalent cation such as Li +, Na +, Cs +, K + affect the solubility of SDS and hence T K of the surfactant. Some salts increase while some decrease the T K of the system. In terms of deceasing the T K the ions follows the trend: Li + > Na + > Cs + > K +. Added counter-ions screen the charge of the micelle head group and facilitate closer packing of the surfactant. Thus added salts always decrease the CMC of the surfactant. Different salts interact differently with surfactant and thus decrease the CMC differently. For SDS the effectiveness in lowering the CMC the ions follows the order: Cs + > K + > Na + > Li +. On the other hand, in terms of OTAB the ions follow the following trends in 2 decreasing the CMC: C 6 H 5 SO 3 > C 7 H 5 O 2 > C 7 H 5 O 3 > SO 4 > NO 3 > Br > Cl > F. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy changes) of micellization and adsorption were calculated from the specific conductance and surface tension data. The negative value of free energy change indicates the process to be spontaneous. The enthalpy and entropy terms are found to compensate each other for both micellization and adsorption. For most of the cases surface excess concentration (Г) was found to be higher in presence of salts than pure surfactant showing lower equilibrium surface tension of the system. The solubilization behavior of a water insoluble dye, Sudan Red B (SRB), in the micellar system was studied by the UV visible spectrophotometric technique. The solubilization of SRB in OTAB in the presence of Na 2 SO 4 was found to be about 1.33 times higher than that in pure water. In the case of SDS the value was found to be 1.07 times in the presence of NaCl. This indicates that the solubilization of SRB in the surfactant micelles significantly increases in the presence of added salts. III

5 Dedicated To My Ever Loving Parents, Sweet Brother And Sisters IV

6 ACKNOWLEDGEMENTS In extreme humbleness I bow my head before supreme personality of Godhead Vagaban Shree Krishna who created mankind in a most splendid manner and bestowed upon him a distinguished honor in the form of knowledge. I venture to get inspiration from an adage that knowledge is an ornament amongst friends and armor against enemy and adore the historical day when I joined the august institution to acquire knowledge. I feel elated in having successfully to accomplish my studies with the keep support and guidance of many personages to whom I owe a depth of gratitude. I fumble for the appreciate words to offer thanks and pay gratitude to my respectable and worthy supervisor Prof. Dr. Md. Nazrul Islam, Department of Chemistry who always exhibited commendable alacrity in providing me proper guidance combined with educative discussions and suggestions whereby I was encouraged to complete my research work confidently. I would like to thank him for always keeping his door open for me. My respectable faculty teachers deserve praise and thanks for their educative and constructive corrective suggestions whenever I needed. I also would like to extend my heartfelt thanks to the Board of Examiners: Dr. Md. Nazrul Islam (Chairman), Dr. Md. Rafique Ullah (Member, Ex-Officio), Dr. Md. Shakhawat Hossain Firoz (Member), Dr. Mahbub Kabir (Member, External) for their corrective suggestions. I reciprocate the respect and regards shown to me by lab fellows, the technical staff and office bearers of Department and the unforgettable cooperation exhibited by them during my research work. I am grateful and thankful to my friends, roommates, nears and dears who extended all possible moral support and encouragement during my strenuous study period and prayed for me. Without you, I couldn t have such a joyful life in BUET. The biggest of all of my acknowledgements goes to my family for getting me here. Your sacrifices and encouragement has allowed me to be who I am. Without your constant support this arduous task would never have met the fateful and fruitful end. Therefore, I would like to thank my parents Modhu Shudon Sharker and Srimotee Sharker and my brother Mithun Chandra Sharker for their support, encouragement, unselfish love and faith. I love you and am glad to forever have your support. K. K. Sharker September, 2016 V

7 CONTENTS Title DECLARATION CERTIFICATION OF THESIS ABSTRACT DEDICATION ACKNOWLEDGEMENTS TABLE OF CONTENTS LAYOUT OF THIS PAPER Page No. I II III IV V VI XIII CHAPTER ONE: INTRODUCTION 1.1 SURFACTANTS AND ITS BULK AND INTERFACIAL PHENOMENA TYPES OF SURFACTANTS Anionic Surfactants Cationic Surfactants Nonionic Surfactants Zwitterionic Surfactants PHYSICAL STATE PROPERTIES OF SURFACTANTS Adsorption of Surfactants Micellization Micelle Micellization Process Critical Micelle Concentration Factors affecting CMC in aqueous solution Cooperative association process in Surfactants Thermodynamics of micellization Micellar Solubilization 20 VI

8 Solubilization Theory Factors affecting solubilization Reasons for self-aggregation of surfactant molecules SURFACTANT SOLUBILITY The Krafft temperature The Cloud point APPLICATION OF SURFACTANTS Consumer Products Detergents and dishwashing Cosmetics and Personal Care Products Industrial Applications Food products Pharmaceutical industry Insecticides and herbicides Agriculture Textiles and fibers Chemical industry Plastics industry Paints and lacquers Cellulose and paper Leather and furs Photographic industry Metal processing Electroplating Adhesives Road construction and building materials Firefighting Mining and flotation Oilfield chemicals 33 VII

9 Cleaning agents Other: Surfactants in biological systems THE SCOPE AND OBJECTIVES OF THE PRESENT THESIS 34 REFERENCES 37 CHAPTER TWO: THEORY AND EXPERIMENTS 2.1 MATERIALS Surfactants Salts Dye METHOD Measurement of Krafft Temperature Measurement of Critical Micelle Concentration Solubilization 47 REFERENCES 50 CHAPTER THREE: RESULTS AND DISCUSSION 3.1 EFFECT OF ELECTROLYTES ON KRAFFT TEMPERATURE EFFECT OF ADDED SALTS ON SURFACE ADSORPTION AND MICELLIZATION SURFACE EXCESS CONCENTRATION THERMODYNAMICS OF MICELLIZATION THERMODYNAMICS OF SURFACE ADSORPTION SOLUBILIZATION STUDY OF SUDAN RED B (SRB) 87 REFERENCES 94 CONCLUSIONS 99 APPENDIX 101 LISTS OF PUBLISHED PAPER 124 VIII

10 LIST OF FIGURES No. Title Page No. Figure 1.1: Typical surfactant structure 1 Figure 1.2: Figure 1.3: Figure 1.4: Figure 1.5: Figure 1.6: Figure 1.7: Figure 1.8: Figure 2.1: Figure 2.2: Figure 2.3: Adsorption of amphiphiles at the air/water interface and micelle as formed by self-assembly of the monomer units 3 Different phase structure of self association of surfactant monomer 10 Changes in the concentration dependence of a wide range of physico-chemical changes around the critical micelle concentration (CMC) 12 Effect of "N" on fraction of added surfactant that goes to micelle 17 Relation between the solubilized material and concentration of surfactant 21 The chemical and physical solubilization (incorporation) of drugs within micelle 22 The Krafft temperature (T K ) is the point at which surfactant solubility equals the critical micelle concentration. Above T K, surfactant molecules form a dispersed phase; below T K, hydrated crystals are formed 28 Hydrated crystal in the beaker (left side) and arrangement for Krafft temperature measurement (right side: EUTECH CON 510 conductivity meter and Froilabo RE 5 refrigerated bath circulator) 44 Surface tension measurement: Surface tensiometer (Kruss K9) and refrigerated bath circulator (JSRC-13C) 46 Shaking of the surfactant solution with dye (Top: Stuart Orbital shakers, SSL1) and solution after shaking (Below) 48 IX

11 Figure 2.4: Figure 3.1: Figure 3.2: Jenway UV-spectrophotometer, model 7315 (Top) and a spectrophotogram of SRB (Below) 49 Specific conductance vs. temperature plots of SDS in pure water and in the presence of different electrolytes at ionic strength. (i) Pure SDS, (ii) LiCl, (iii) KCl, (iv) CsCl, (v) NaCl. The sharp break point in the plot indicates the Krafft Temperature 52 Specific conductance vs. temperature plots of OTAB in pure water and in the presence of different electrolytes at ionic strength. (i) Pure OTAB, (ii) Na 2 SO 4, (iii) NaBr, (iv) NaF, (v) C 6 H 5 SO 3 Na, (vi) C 7 H 5 O 2 Na, (vii) NaNO 3, (viii) C 7 H 5 O 3 Na, (ix) NaCl. The sharp break point in the plot indicates the Krafft Temperature 53 Figure 3.3: Figure 3.4: Effect of ionic strength of electrolytes on the Krafft Temperature of SDS. (i) LiCl, (ii) NaCl, (iii) CsCl, (iv) KCl 55 Effect of ionic strength of electrolytes on the Krafft Temperature of OTAB. (i) C 7 H 5 O 3 Na, (ii) C 7 H 5 O 2 Na, (iii) Na 2 SO 4, (iv) C 6 H 5 SO 3 Na, (v) NaF, (vi) NaNO 3, (vii) NaCl, (viii) NaBr, (ix) NaSCN, (x) NaI 56 Figure 3.5: Figure 3.6: Figure 3.7: Figure 3.8: Figure 3.9: Conductometric determination of CMC of SDS in pure water at 30 C 62 Conductometric determination of CMC of SDS in the presence of 0.005M NaCl solution at 30 C 62 Conductance vs. surfactant concentration plot for OTAB in aqueous solution at 40 C 63 Conductance vs. surfactant concentration plot for OTAB in the presence of 0.005M NaCl solution at 40 C 63 Surface tensiometric determination of CMC of SDS in pure water at 30 C 65 X

12 Figure 3.10: Figure 3.11: Figure 3.12: Figure 3.13: Figure 3.14: Figure 3.15: Figure 3.16: Figure 3.17: Figure 3.18: Figure 3.19: Figure 3.20: Figure 3.21: Surface tensiometric determination of CMC of SDS in the presence of 0.005M NaCl solution at 30 C 65 Surface tension vs. Log 10 C plot for OTAB in aqueous solution at 40 C 66 Surface tension vs. Log 10 C plot for OTAB in the presence of 0.005M NaCl solution at 40 C 66 surface excess concentration of SDS (i) in pure and (ii) in 0.005M aqueous solution of NaCl 75 surface excess concentration of OTAB (i) in pure and (ii) in 0.005M NaCl solution. 75 Enthalpy-Entropy compensation plot for (a) Micellization (b) surface Adsorption of SDS in aqueous solution 84 Enthalpy-Entropy compensation plot for (a) Micellization (b) surface Adsorption of OTAB in aqueous solution 85 Effect of surfactant concentration on the absorption spectra of SRB: i 0.4, ii 0.6, iii 1.0, iv 1.5 and v 2.0 mm OTAB solutions in pure water 88 Effect of surfactant concentration on the absorption spectra of SRB: i 0.06, ii 0.1, iii 0.2, iv 0.4 and v 0.8 mm OTAB solutions in ionic strength Na 2 SO 4 88 Effect of surfactant concentration on the absorption spectra of SRB: i 8, ii 9, iii 10, iv 15, v 20 and vi 30 mm SDS solutions in pure water 89 Effect of surfactant concentration on the absorption spectra of SRB: i 6, ii 7, iii 8, iv 9, v 10 and vi 20 mm SDS solutions in 0.005M NaCl 89 Solubilization of SRB in OTAB solution in (a) pure water and (b) ionic strength aqueous Na 2 SO 4 solution. The break point in the curve shows the CMC below which no significant XI

13 absorbance was observed. This indicates the SRB solubilized only when OTAB forms micelles 90 Figure 3.22: Solubilization of SRB in SDS solution in (a) pure water and (b) 0.005M NaCl solution. The break point in the curve shows the CMC below which no significant absorbance was observed. This indicates the SRB solubilized only when SDS forms micelles 91 LIST OF TABLES No. Title Page No. Table 1.1: Some representative examples of surfactant 6 Table 3.1: CMC values of OTAB at different temperatures in pure water and in the presence of ionic strength solutions of several electrolytes 70 Table 3.2: CMC values of SDS at different temperatures in pure water and in the presence of ionic strength solutions of some electrolytes 71 Table 3.3: Thermodynamic parameters of adsorption and micellization* of the SDS surfactants solution. 78 Table 3.4: Thermodynamic parameters of adsorption and micellization* of the SDS 0.005M NaCl surfactants solution. 78 Table 3.5: Thermodynamic parameters of adsorption and micellization* of the OTAB surfactants solution. 79 Table 3.6: Thermodynamic parameters of adsorption and micellization* of the OTAB 0.005M NaCl surfactants solution. 79 Table 3.7: T c value for OTAB and SDS in water and 0.005M NaCl solution 86 Table 3.8: Molar Solubilization Ratio (MSR) values of SRB in SDS 93 Table 3.9: Molar Solubilization Ratio (MSR) values of SRB in OTAB 93 XII

14 LAYOUT OF THIS DISSERTATION This thesis paper has been divided into three chapters- Chapter one presents a general introduction. Here review of some earlier research works is given for present investigation. Objectives of the present work are also described in this chapter. Theory and experimental procedures are explained in chapter two. Experimental results and discussions are presented in chapter three. The conclusions of this research work have also been discussed here. References are added at the end of the respective chapter. Appendix was also included at the end of this thesis paper. List of publications related to the present work have also been mentioned at the very end of this thesis paper. XIII

15 Education is the most powerful weapon which you can use to change the world.. Nelson Mandela XIV

16 Chapter One Introduction

17 Introduction 1.1 SURFACTANTS AND ITS BULK AND INTERFACIAL PHENOMENA Surfactants are compounds that lower the surface tension of the liquid, the interfacial tension between two liquids or interfacial tension between a liquid and solid. Surfactants can act as wetting agents, emulsifiers, foaming agents and dispersants. For this reason they are used in vast amounts in domestic and industrial applications such as in soaps, detergents, paints, dyestuffs, paper coatings, inks, plastics and fibers, personal care and cosmetics, agrochemicals, pharmaceuticals, food processing, oil industry, etc. [1-3]. They are amphiphilic molecules and carry in the same molecule two moieties of completely different properties: one moiety is polar and hydrophilic; the other is nonpolar and hydrophobic (Figure 1.1). Therefore, these molecules contain both a water soluble and water insoluble (or oil soluble) component. Soap molecules made up of long hydrocarbon chain (hydrophobic) ending with a carboxyl group (polar) is a good example of an amphiphile molecule. Because of its dual affinity, an amphiphilic molecule does not feel "at ease" in any solvent, be it polar or non-polar, since there is always one of the groups which "does not like" the solvent environment. This is why amphiphilic molecules exhibit a very strong tendency to migrate to interfaces or surfaces and to orientate so that the polar group lies in water and the non-polar group is placed out of it, and eventually at the air-water or oil-water interface [4-6]. (Head Group) (Tail Group) Figure 1.1: Typical surfactant structure 1

18 Introduction When surfactants are dissolved in water less work is required to bring a surfactant molecule to the surface than a water molecule, as migration of the surfactant to the surface is a spontaneous process. So these molecules are strongly attracted to and accumulate (adsorb) at the air/water interface or the particle (assumed hydrophobic)/water interface. As a result of their molecular structure, the molecules orientate themselves with the hydrophilic part pointed toward water (polar) and the hydrophobic part away from it. This results in the formation of an oriented monolayer of the amphiphiles at the interface as shown in Figure 2. This strong tendency of the amphiphiles to adsorb at an interface is termed surface activity and amphiphiles are also known as surface active agents (SAA) or surfactants [5, 7]. However, high density condensed phase formation in adsorbed monolayer sometimes becomes difficult due to electrostatic repulsion, bulkiness as well as strong hydration of the polar head group. In such a case, hydrophobic interactions among the alkyl chains make it more favorable to remain in the bulk of the aqueous solution by forming colloidal sized clusters in solution, known as micelles and the concentration of monomeric amphiphile at which micelles appear is called the critical micelle concentration (CMC). The CMC is an important characteristic of a surfactant [8]. Below this concentration surfactant molecules remain as single molecule but above this concentration they aggregate as micelles [9]. Thus, the CMC represents a phase separation between single molecules of surfactant and surfactant aggregates in dynamic equilibrium [10]. Below the CMC micelles are not present and adsorption is a dynamic equilibrium with surfactant molecules perpetually arriving at, and leaving the surface. Above the CMC, the concentration of unaggregated surfactant will stay constant and the number of micelles will increase as the total surfactant concentration increases and the system then consists of an adsorbed monomolecular layer, free monomers and micellised surfactant in the bulk, with all these three states in equilibrium [11]. [Figure1.2] A typical micelle in aqueous solution forms with the hydrophilic head regions in contact with the water and the hydrophobic aliphatic tail regions buried in the inner portion of the micelle [11]. It is believed that surfactant molecules or ions are associated in micelles because the forces that act between polar water molecules exceed the forces that act 2

19 Introduction between hydrocarbon chains and water. Therefore, the transfer of hydrocarbon chains from water into a phase close to them in polarity is energetically favorable [12]. Figure 1.2: Adsorption of amphiphiles at the air/water interface and micelle as formed by self-assembly of the monomer units An immediate consequence of the adsorption of surfactant molecules at an interface is that its interfacial energy is reduced. For a water surface covered with a monolayer of surfactant molecules, its surface tension is very much lower than that of clean water surface [13]. Surface tension occurs when water molecules on a surface bond very tightly to other water molecules both next to and below them. When surfactants are dissolved in water they form a monolayer upon spontaneous adsorption at the air-water interface [14] and do not completely mix with water but are able to bond to water and prevent water molecules from binding as tightly to one another, thus lowering the tension or strength of the surface. 3

20 Introduction Below the CMC surfactants tend to accumulate at the interface, reducing surface tension. At CMC, the surface tension of the solution does not change but remains constant, as the gas-liquid interface is already fully packed with the surfactant molecules. Above the CMC, most of the surfactant molecules are inside the bulk aggregating into micelles. When this occurs, the addition of surfactants just increases the number of micelles and the surface tension becomes independent of surfactant concentration [15]. 1.2 TYPES OF SURFACTANTS Surfactants may be classified according to their applications (emulsifiers, foaming agents, wetting agents, dispersants etc.), some physical characteristics (water and oil solubility and stability) and chemical structure of both the head and tail group of surfactants. The head group can be charged or neutral, small and compact in size, or a polymeric chain. The tail group is usually a single or double, straight or branched hydrocarbon chain, but may also be a fluorocarbon, or a siloxane, or contain aromatic group(s). Since the hydrophilic part normally achieves its solubility either by ionic interactions or by hydrogen bonding, the simplest classification is based on surfactant head group type, with further subgroups according to the nature of the lyophobic moiety. Four basic classes therefore emerge as: Anionic, Cationic, Nonionic and Zwitterionic [16-19] Anionic Surfactant Anionic surfactants are dissociated in water into two oppositely charged species anion (the surfactant ion) and cation (counter ion). Carboxylate, sulfate, sulfonate and phosphate are the polar groups found in anionic surfactants. The counterions most commonly used are sodium, potassium, ammonium, calcium and various protonated alkyl amines. One main reason for their popularity is the ease and low cost of manufacture. Anionics are used in most detergent formulations and the best detergency is obtained by alkyl chains in the C12-C18 range. They are by far the largest surfactants class. They are generally sensitive to hard water. Sensitivity decreases in the order carboxylate > phosphate > sulfate sulfonate. 4

21 Introduction Cationic Surfactant Cationic surfactants are dissociated in water into an amphiphilic cation and an anion, most often of the halogen type. A very large proportion of this class corresponds to nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or several long chain of the alkyl type, often coming from natural fatty acids. These surfactants are in general more expensive than anionics and are only used in which there is no cheaper substitute. They are the third largest surfactants class. They adsorb strongly to most surfaces and their main uses are related to in situ surface modification Nonionic Surfactants Nonionic surfactants do not ionize in aqueous solution, because their hydrophilic group is of a non-dissociable type, such as alcohol, phenol, ether, ester, or amide. A large proportion of these nonionic surfactants are made hydrophilic by the presence of a polyethylene glycol chain, obtained by the polycondensation of ethylene oxide. They are called polyethoxylated nonionics. The polycondensation of propylene oxide produce a polyether which (in opposition to polyethylene oxide) is slightly hydrophobic. This polyether chain is used as the lipophilic group in the so-called polyeopolypo block copolymers, which are most often included in a different class, e.g. polymeric surfactants. They are the second largest surfactant class. They are normally compatible with all other types of surfactants. They are not sensitive to hard water. Their physicochemical properties are not markedly affected by electrolytes. Contrary to ionic compounds they become less water soluble-more hydrophobic Zwitterionic Surfactant When the headgroup of a surfactant molecule contain both a negative and a positive charge it is called amphoteric or zwitterionic. Whereas the positive charge is almost invariably ammonium, the source of negative charge may vary, although carboxylate is by far the most common. Some amphoteric surfactants are insensitive to ph, whereas others are cationic at low ph and anionic at high ph, with an amphoteric behavior at intermediate ph. Amphoteric surfactants are generally quite expensive, and consequently, 5

22 Introduction their use is limited to very special applications such as cosmetics where their high biological compatibility and low toxicity is of primary importance. They are the smallest surfactant class. They are compatible with all other classes of surfactants. They are not sensitive to hard water. Most types show very low eye and skin irritation. They are therefore well suited for shampoos and other personal care products. The past two decades have seen the introduction of a new class of surface active substance, so-called polymeric surfactants or surface active polymers, which result from the association of one or several macromolecular structures exhibiting hydrophilic and lipophilic characters, either as separated blocks or as grafts. They are now very commonly used in formulating products as different as cosmetics, paints, foodstuffs, and petroleum production additives. Recently, there has been considerable research interest in certain dimeric surfactants, containing two hydrphobic tails and two head groups known as gemini surfactants, which have efficiency in lowering surface tension and very low CMC. Some representative surfactants along with their chemical formulae are listed in Table 1.1. Table 1.1: Some representative examples of surfactant Class Examples Molecular structure Anionic Sodium stearate CH 3 (CH 2 ) 16 - COO Na + Sodium dodecyl sulfate CH 3 (CH 2 ) 11 - SO 4 Na + Sodium dodecyl benzene sulphonate CH 3 (CH 2 ) 10 C 6 H 4 - SO 3 Na + Cationic Laurylamine hydrochloride CH 3 (CH 2 ) 11 NH + 3 Cl Hexadecyltrimethylammonium bromide CH 3 (CH 2 ) 15 N + (CH 3 ) 3 Cl Tetradecyltrimethylammonium bromide CH 3 (CH 2 ) 13 N + (CH 3 ) 3 Cl Non-ionic Polyoxyethylene(4)dodecanol CH 3 (CH 2 ) 11 -O-(CH 2 CH 2 O) 4 H Polyoxyethylene(9)hexadecanol CH 3 (CH 2 ) 15 -O-(CH 2 CH 2 O) 9 H Zwitterionic Dodecyl betaine C 12 H 25 N + (CH 3 ) 2 CH 2 COO Dodecyldimethylammonium acetate CH 3 (CH 2 ) 11 (CH 3 ) 2 N + CH 2 COO Gemini Bis (quaternary ammonium bromide) C 12 H 25 N + (CH 3 ) 2 -(CH 2 ) 8 - N + (CH 3 ) 2 C 12 H 25 2Br 6

23 Introduction 1.3 PHYSICAL STATE Ionic surfactants are generally amorphous or crystalline solids and nonionic surfactants are liquid or solid. Crystalline surfactants can be prepared relatively purely. They can be polymorphic, if their structures have different unit cell, or polytypic if their structures have one dimensional polymorphism. Amorphous solids are surfactants that have one or more chiral centres and exist in multiple optical isomers. Liquid crystalline surfactants exhibit properties common to crystalline and to liquid physical state. Liquid surfactants are fundamentally amorphous with no long range order and are typically isotropics. 1.4 PROPERTIES OF SURFACTANTS Surfactants distort water structure and raise free energy of solution. The system, however, has natural tendency to minimize its free energy. To satisfy this natural desire the system may undergo- (A) Adsorption (B) Micellization Adsorption of Surfactants Adsorption is an entropically driven process by which molecules diffuse preferentially from a bulk phase to an interface. Due to the affinity that a surfactant molecule encounters towards both polar and non-polar phases, thermodynamic stability (i.e. a minimum in free energy or maximum in entropy of the system) occurs when these surfactants are adsorbed at a polar/non-polar (e.g. oil/water or air/water) interface. Due to its amphiphilic structure, the surfactant can adsorb onto interfaces and lower the tension (γ) of the interfaces. The adsorption dynamics, i.e. the time-dependent adsorption process of surfactant molecules onto interfaces, is of significant importance in lots of applications including foaming, emulsifying and coating processes, in which bubbles, drops or films are rapidly formed [20-22]. The surfactant adsorption process from the bulk to the air/water interface can be divided into two: the motion of the surfactant molecules from the bulk to the sub-surface and the transfer of molecules from the sub-surface to the air/water interface [23-25] Due to the different environment of molecules located at an interface compared to those from either bulk phase, an interface is associated with a surface free energy. At the air- 7

24 Introduction water surface for example, water molecules are subjected to unequal short-range attractive forces and thus, undergo a net inward pull to the bulk phase. Minimisation of the contact area with the gas phase is therefore a spontaneous process, explaining why drops and bubbles are round. The surface free energy per unit area, defined as the surface tension (γ o ), is then the minimum amount of work (W min ) required to create new unit area of that interface ( A), so W min = γ o A. Another, but less intuitive, definition of surface tension is given as the force acting normal to the liquid-gas interface per unit length of the resulting thin film on the surface [15]. A surface-active agent is therefore a substance that at low concentrations adsorbs thereby changing the amount of work required to expand that interface. In particular surfactants can significantly reduce interfacial tension due to their dual chemical nature. Considering the air-water boundary, the force driving adsorption is unfavourable hydrophobic interactions within the bulk phase. There, water molecules interact with one another through hydrogen bonding, so the presence of hydrocarbon groups in dissolved amphiphilic molecules causes distortion of the solvent structure apparently increasing the free energy of the system. This is known as the hydrophobic effect [26]. Less work is required to bring a surfactant molecule to the surface than a water molecule, so that migration of the surfactant to the surface is a spontaneous process. At the gasliquid interface, the result is the formation of an oriented suractant monolayer with the hydrophobic tails pointing out of, and the head group inside, the water phase. The balance against the tendency of the surface to contract under normal surface tension forces causes an increase in the surface (or expanding) pressure π, and therefore a decrease in surface tension γ of the solution. The surface pressure is defined as π = γ o γ, where γ o is the surface tension of a clean air-water surface. Depending on the surfactant molecular structure, adsorption takes place over various concentration ranges and rates, but typically, above a well-defined concentration the critical micelle concentration (CMC) micellisation or aggregation takes place. At the CMC, the interface is at (near) maximum coverage and to minimise further free energy, molecules begin to aggregate in the bulk phase. Above the CMC, the system then consists 8

25 Introduction of an adsorbed mono-molecular layer, free monomers and micellised surfactant molecules in the bulk, with all these three states in equilibrium Micellization Micelle The solubility pattern with respect to solvent properties of a non-polar compound like alkane is in sharp contrast to that of a charged or otherwise strongly polar chemical species. If these two features occur simultaneously in the same chemical entity, an interesting phenomenon is observed. For aqueous solutions, one well known situation is that the polar group is located in the solution while the nonpolar part seeks to avoid the aqueous environment by stretching into the gas phase or into an adjacent non-polar liquid phase. Except for this adsorption at gas liquid, liquid-liquid or liquid-solid interfaces there is an alternative possibility to avoid the unfavorable contact between non-polar groups and water and between polar groups and non-polar solvent, i.e. by self-association into various types of aggregates (Figure 1.3). The term micelle is introduced by the pioneer in the field J.W. McBain in 1913 to describe the formation of colloidal properties by detergents and soaps [27]. The word micelle has also been used in biology and in colloid chemistry for other phenomena. Important features of the micelle are the high aggregation number and effective separation of hydrophilic and hydrophobic part. It was established at an early stage that micelle formation displays peculiar concentration dependence. Thus at low concentration an aqueous ionic surfactant solution behaves essentially as a strong electrolyte. On the other hand, an increased amphiphile concentration leads to a corresponding increase in the amount of micelles while the monomer concentration stays roughly independent of the total amphiphile concentration. Under these circumstances, pronounced changes in the concentration dependence of a large number of properties occur at the CMC. The existence of micelles in a solution is an important parameter due to a number of important interfacial phenomena, such as detergency and solubilization. Furthermore, micelles have become a subject of great interest in the fields of organic chemistry and the 9

26 Introduction biochemistry because of their unusual catalysis of organic reactions and their similarity to biological membranes and globular proteins. Aggregation is not, however, just limited to aqueous solution; it is sometimes observed in non-aqueous polar solvents such as ethylene glycol and non-polar solvents such as hexane [15]. Figure 1.3: Different phase structure of self association of surfactant monomer Micellization Process: The aggregation phenomenon of amphiphilic molecules involves contributions from both repulsive and attractive interactions. Especially, in ionic surfactants, the repulsive forces originated primarily from electrostatic repulsion between the polar head groups [28], whereas attractive interactions have generally been attributed to hydrophobic interactions between the non-polar tails of the surfactant monomers [29]. However, in this context a considerable emphasis has been given to the London dispersion interactions [30-31]. These interactions depend on various factors such as temperature, dielectric constant of the medium, length of the alkyl chain, presence of additives and relative size and charge of the headgroup [32-33]. The formation of micelles and its dependence on different factors such as temperature, additives, dielectric constant of the medium, the extent of counter-ion binding (for ionic surfactants), solubilization etc. are important 10

27 Introduction physicochemical aspects that need detailed and intensive attention for both fundamental understanding and industrial applications. The dominance of the favorable interaction between alkyl chains of the surfactant favors micellization and lead CMC to lower values by stabilizing micelles while the opposing repulsive interaction between the polar/charged head groups disfavor micellization and leads CMC to higher values [32]. To differentiate among these different kinds of intractions, the surfactant solution properties, such as critical micellar concentration (CMC), micelle shape and size, solubility and Krafft temperature have been considerably important [34]. Micelles are known to have an anisotropic water distribution within their structure. In other words, the water concentration decreases from the bulk towards the interior of the micelle, with a completely hydrophobic-like interior. Thus, micellar solution consists of special medium in which hydrophobic organic compounds can be solubilized in aqueous surfactant solution, which are otherwise insoluble in water [35-36]. At low concentration in water, surfactants exist mostly as monomers [37]. At higher concentrations, the surfactants molecules grouped together in a manner that their hydrophobic tails (usally an n-alkyl hydrocarbon chain containing 8 to 18 methylene groups) tend to coaggregate to form more or less spherical micelles with hydrocarbon chains forming a core and the polar hydrophilic heads on the surface providing protection. A major source of stability of micelle is the existence of an electric charge on their surface. On account of this charge, ions of opposite charge tend to cluster nearby, and an ionic atmosphere is formed Critical Micelle Concentration The change in surface properties as the concentration of an aqueous solution of a surfactant rises is characteristic of most surface active molecules. During earlier studies of the solution properties of surfactants, it was recognized that the bulk solution properties of these materials were unusual and could change abruptly over a very small concentration range, indicating the presence of colloidal particles in the solution [39]. Equivalent conductance of any ionic surfactant, plotted against the square root of its concentration gives a curve instead of smooth curve characteristic of ionic electrolyte [Figure 1.4]. This sharp break in the conductivity of the solution indicates a sharp increase in the mass per unit charge of material in solution. That is interpreted as 11

28 Introduction evidence of the formation of micelles from the monomeric surfactant molecules with part of the charge of the micelle neutralized by associated counter ions. The threshold concentration at which micellization begins is known as the critical concentration. Similar behavior in almost all measurable physical properties is observed by all types of surface active materials (anionic, cationic, nonionic, zwitterionic) which depend on size or number of particles in solution [Figure 1.4]. Phillips [38] had used that CMC is the concentration at which the properties of the surfactant solution changes in the most abrupt manner, i.e d 3 φ dc 3 = 0 where φ is any additive property which varies linearly with the concentration of micellized end of unassociated surfactant. The discovery of this discontinuity in physical properties and reasons for it were first described by McBain [39] in 1920s and there has been a considerable volume of work on the subject since then. Figure 1.4: Changes in the concentration dependence of a wide range of physico-chemical changes around the critical micelle concentration (CMC) 12

29 Introduction Factors affecting CMC in aqueous solution (i) The Hydrophobic Group The length of the hydrocarbon chain is a major factor determining the CMC. For a homologous series of linear single-chain surfactants the CMC decreases logarithmically with carbon number. Interestingly, for straight-chain dialkyl sulfosuccinates the value is double than that for the single chain compounds. Alkyl chain branching and double bonds, aromatic groups or some other polar character in the hydrophobic part produce noticeable changes in the CMC. In hydrocarbon surfactants, chain branching gives a higher CMC than a comparable straight chain surfactant [15], and introduction of a benzene ring in the chain is equivalent to about 3.5 carbon atoms [5]. (ii) The Hydrophilic Group For surfactants with the same hydrocarbon chain, varying the hydrophile nature (i.e., from ionic to non-ionic) has an important effect on the CMC values. Ionic surfactants have much higher CMC than nonionic surfactants containing equivalent hydrophobic groups. For instance, for a C12 hydrocarbon the CMC with an ionic headgroup lies in the range of mol dm -3, while a C12 non-ionic material exhibits a CMC in the range of mol dm -3. (iii) Temperature The effect of temperature on the CMC of surfactants in aqueous medium is complex. Rosen [15] pointed out that the value appearing first to decrease with the temperature to some minimum and then to increase with further increase in temperature. The increase of the temperature causes decrease of the hydration of the hydrophilic group, which favors the micellization. However temperature increase also causes disruption of the structured water surrounding of 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 in the literature, the minimum in the CMC temperature curve appears to be around 25 o C for ionic surfactants [40] and around 50 o C for nonionic [41]. 13

30 Introduction (iv) Salts Addition of neutral salts to an aqueous solution of surfactant usually decreases the CMC of ionic surfactants. This effect is less pronounced when the surfactants is nonionic. Salts tend to screen electrostatic repulsions between headgroups and make the surfactant effectively more hydrophobic. This increases hydrophobic interactions among the surfactants cause them to aggregate at a lower concentration, thereby the CMC decreases [42] Cooperative association process in Surfactants When surfactants associate into micelles, they form a liquid like aggregate. As there is no specific mechanism related to specific aggregation number, the association of monomers into micelles is described as stepwise addition of a monomer, S to the aggregate, S n-1 as in S + S n-1 S n (1) By neglecting additional interactions between aggregates and between monomers, the equilibrium would be K n = [S n ] S [S n 1 ] (2) This equation gives description of any stepwise association process in dilute solution. In the case of aggregation, n of order 100, there would be a number of intractable equilibrium constants K n. However, because it is almost impossible to specify all the K n equilibrium steps, approximate model of micellization are being used. (i) Isodesmic model: In this model it is assumed that K n is independent of n where regardless of either the total concentration or of K, [S] K < 1. The aggregation distribution function 14

31 Introduction f(n) = [S n ] n =1[S n ] (3) decays exponentially with [S 1 ] > [S 2 ] > [Sn]. In this model, aggregation is a continuous process that does not show the abrupt onset in a narrow concentration range, which typifies micelle formation. Isodesmic model describe the association of dyes in aqueous solution quite well but it is less successful as a description of the formation of micelles because the model does not predict a CMC. Its basic shortcoming lies in making K n independent of n and thus depriving the process of cooperativity. (ii) Phase separation model In this model aggregation is approximated as a phase separation process in which the activity of the monomer remains constant above the CMC. Micelle formation having several features in common with the formation of a separate liquid phase provides basis for this model in which micelles formally constitute a separate phase. In terms of association described in equation (1), the phase separation model assumes that aggregates with large n, dominate all others except the monomer. This assumption implies strong cooperativity because, once aggregation has started, and it becomes more and more favorable to add another monomer until a large aggregation number is reached. In the pseudo separate phase, the surfactant possesses a certain chemical potential µ (micelle) in the aggregates when monomers and aggregates coexist in equilibrium µ (micelle) = µ θ (solvent) + RT ln[s] (4) [S] is the CMC (neglecting dimers and oligomers). The standard free energy of micelle formation G mic represents the standard free energy difference between a monomer in the micelle and the standard chemical potential in dilute solutions and G mic = µ (micelle) - µ θ (solvent) = RT lncmc (5) This equation provides a useful approximation for obtaining G mic. This phase separation model captures several but not all essential features of micelle formation. It 15

32 Introduction describes the start mechanism of the self-assembly process but does not describe the stop mechanism [43]. (iii) Closed association model This model describes both start and stop features of micellization. It is assumed that aggregation number dominates with only monomers and N-aggregates NS S N (6) K N = [S N ] [S] N (7) The total surfactant concentration in terms of model of monomers is [S] T = N[S N ] + [S] = NK N [S] N + [S] (8) This K N can be related to other equilibrium constant in equation (2) as K N = N K n 2 (9) Fraction of added surfactants enters into an aggregate is given by derivative {N S N } [S] T (10) Figure 1.5 shows three curves with varying values of N, the larger the N value, the more abruptly the derivative N S N / [S] T changes from a low concentration value of zero to the high concentration value of unity. When N, discontinuity in the derivative at CMC is regained. As the aggregation number, N increases, the fraction of added surfactant that goes to the micelle varies more and more steeply with total concentration [S] T. In the limiting case in which the aggregation number becomes infinite the transition becomes a step function that unambiguously defines the CMC while small aggregation numbers to less defined values of CMC. In the case of ionic surfactant an equilibrium between surfactant monomers, S, counterions, C + and micelles, S N is written as (N P)C + + NS P S N (11) 16

33 Introduction for which K N = [S N P ] [S ] N [C + ] N P (12) Thermodynamics of micellization To evaluate the thermodynamic parameters of micellization two approaches are generally used: the phase separation model [44] and mass action model or the equilibrium model [45]. If, however the aggregation number of the micelle is small, the mass action model is used, while if the aggregation number is large, the phase separation model is applied. According to the mass action model, the micelles and monomeric species are considered to be in a kind of chemical equilibrium, while in phase separation model, the micelles are considered to constitute a new phase formed in the system at and above the critical micelle concentration. In each case classical thermodynamic approaches are used to describe the overall process of micellization. Analysis of both approaches produces the same general results in terms of the energetic of micelle formation Figure 1.5: Effect of "N" on fraction of added surfactant that goes to micelle 17

34 Introduction In the case of ionic surfactants the equilibrium model is preferable because it is possible to take into consideration, in an explicit way, the effect of the counterion dissociation. The equilibrium model considers that the micellization process can be described by equilibrium between monomers, counterions, and monodisperse micelles. In the case of a cationic surfactant this equilibrium can be represented by ns + + (n p)c M p+ (13) The corresponding equilibrium constant can be written as K = [M p + ] [S + ] n [C ] n p or, lnk = ln[m p+ ] nln S + (n p)ln[c ] or, RT lnk = RT (ln[m p+ ] nln S + (n p)ln[c ]) where S + represents the surfactant cations, C the corresponding counterions, and M p+ the micelle formed by n monomers with an effective charge of p. The standard free energy of micellization per mole of surfactant, G mic, is given by or, G mic = RT 1 n ln a MP+ + ln as p n lnac [ G = RT lnk] (14) where a is the activity of the respective species. For large n values the first term of the parenthesis is negligible and both as + and ac can be replaced by the activity at the CMC. Moreover, since the micellar formation occurs in dilute solutions, the activity can be replaced by the surfactant concentration (expressed in mole fraction) at the CMC. Considering these approximations, Eq. (14) can be expressed as [46] G mic = (2 β) RT ln X CMC (15) Where β = P is the degree of dissociation of the counterion binding. For a completely n ionized micelle, β = 1 and for neutral β = 0. 18

35 Introduction Counterions drawn into the regions of charged head groups reduce the repulsive electrostatic interactions between them, and this is the heuristic physical basis for the model of counterion binding. In the case of ionic surfactants the relative contribution of enthalpy and entropy determines the temperature dependence of the CMC. Since the thermodynamic parameters are related by the Gibbs-Helmholtz equation, G mic can be separated into its enthalpic and entropic components G mic = H mic T S mic (16) For the cases when the aggregation number and the degree of ionization are temperature dependent. In classical thermodynamics, H mic is also given by the relation H mic = RT 2 2 β ln X CMC T. p ln X CMC β T. p (17) If the change in β with temperature is small, Eq. (17) yields H mic = 2 β RT 2 ln X CMC T. p (18) In this way, the enthalpy of micellization can be evaluated from the slope of a tangent to a plot of ln X CMC versus T at a particular temperature. Once G mic and H mic have been obtained, the entropy of micellization can be estimated from Eq. (16). T S mic = H mic G mic The micellization process is governed primarily by the entropy gain associated with it and the driving force for the process is the tendency of the lyophobic group of the surfactant to transfer from the solvent environment to the interior of the micelle [47]. The increased freedom of the hydrophobic chain in the nonpolar interior of the micelle compared to the aqueous environment plays an important role in entropy of micellization. Any structuranl or environmental factors that may affect solvent-lyophobic group interactions or interactions between the lyophobic groups in the interior of the micelle, therefore, affect G mic and consequently the value of the CMC. 19

36 Introduction Micellar Solubilization An important property of micelles is their ability to increase the solubility of sparingly soluble or insoluble substances in water. Solubilization, as defined by McBain and Hutchinson [48, 49], is a particular mode of bringing into solution of substances that are otherwise insoluble in a given medium, involving the previous presence of colloidal solution whose particles take up and incorporate within or upon themselves the otherwise insoluble material. Solubilization by micelles is of importance in many industrial processes such as detergency, micellar catalysis and extraction, emulsion, polymerization, oil recovery, etc. [50] and in a variety of fundamental research oriented studies like micellar modeling of biological membrane [11]. Below the CMC surfactant molecules exist as monomers and have only little or no influence on the solubility of water-insoluble compounds but above this concentration solubility increases sharply with surfactant concentration. If the solubility of a normally solvent-insoluble materials is plotted against the concentration of the surfactant solution, the solubility is very limited at concentrations below the CMC of the surfactant but rise abruptly, once the CMC has been reached as shown in Figure 1.6. This indicates that solubilization is a micellar phenomenon. In solubilization, the solubilized material is in the same phase as the solubilizing solution, and the system is consequently thermodynamically stable. The extent of solubilization depends on many factors such as the structure of the surfactant, aggregation number, micellar geometry, and temperature, ionic strength of the medium and the nature of the solubilizate. The locus of solubilization of poorly water-soluble compounds in micellar systems depends on the polarity of solubilizate. Non-polar molecules are solubilized in the micelle core and substances with intermediate polarity are distributed along surfactant molecules in certain intermediate position [50]. An increase in surfactant concentration in solution increases the extent of solubilization of hydrophobic solutes because of an increase in the number of micelles in the bulk. The solubilizing capacity of a surfactant is usually expressed quantitatively by molar solubilization ratio (MSR). The MSR can be expressed as the number of moles of the substance solubilized per mole of the surfactant in solution [51]. 20

37 Introduction Figure 1.6: Relation between the solubilized material and concentration of surfactant Solubilization Theory The formation of additive-surfactant aggregates in the micellar solution can also be explained based on solubilization theory [52]. The stepwise association between an additive (D) molecule and the micelle (M) gives rise to the following equilibria K 1 M+D MD 1 K 2 MD 1 + D MD 2 K 3 MD m-1 +D MDm Where MD 1 is the micelle associated with 1(one) molecules of the dye and K 1 is the stepwise association constant between MD 1 and D. Assuming that the additive molecules that solubilize within micelles obey a position distribution, the first stepwise association constant, K 1, can be obtained from the relation- D 1 [D] [D] = K 1 [M 1 ] 21

38 Introduction Here [M 1 ] is total micelle concentration, [D 1 ] is the total equivalent concentration of the dye and [D] is the average number of additive incorporated into a single micelle [D] = D 1 [D] [M 1 ] Figure 1.7: The chemical and physical solubilization (incorporation) of drugs within micelle Factors affecting solubilization (i) Effect of structure of solubilizer There are a number of factors regarding the structure of solubilizer such as chain length, substitutions in the chain and position of hydrophilic group, which effect the solubilization. The amount of material solubilized generally increases with increasing the size of the micelles. The factors that cause an increase in either the diameter of the micelle or its aggregation number can be expected to produce increased solubilization. 22

39 Introduction An increase in the chain length of the hydrophobic portion of the surfactant generally results in an increased solubilization capacity for hydrocarbons in the interior of the micelle in aqueous media. Bivalent counterions show greater solubilizing power than the corresponding univalent [53]. Nonionic surfactants, because of low CMC, are better solubilizing agents than ionic surfactants in dilute solutions. In general, the solubilizing power for hydrocarbons and polar compounds having same hydrophobic chain length follows the order: [54] nonionics > cationics > anionics. (ii) Effect of structure of the solubilizate For polar solubilizates, the structure of the solubilizate shows variation in the depth of penetration into the palisade layer of the micelle. In the case of more or less spherical micelle, the polar compounds are solubilized close to the micelle-water interface, to a greater extent than nonpolar solubilizates that are located in the inner core. Usually the molecules having longer alkyl chain length and less polarity in nature show the smaller degree of solubilization [55]. For condensed aromatic hydrocarbons the extent of solubilization appears to decrease with an increase in the molecular size [56]. (iii) Effect of electrolytes Neutral electrolytes in ionic surfactant solution decrease the repulsion between the charged ionic surfactant headgroups, thereby decrease the CMC and increase the aggregation number and volume of micelles. The increase in aggregation number of the micelles presumably results in an increase in hydrocarbon solubilization in the inner core of the micelle. (iv) Effect of organic additives The presence of solubilized hydrocarbons in the surfactant micelles generally increases the solubility of polar compounds in these micelles. The solubilized hydrocarbon causes the micelle to swell, and this may make it possible for the micelle to incorporate more polar material in the palisade layer. The long chain polar compound which are less capable of forming hydrogen bond, show the greater power to increase the solubilization of hydrocarbons. 23

40 Introduction (v) Effect of temperature For ionic surfactants an increase in temperature generally results in an increase in the extent of solubilization for both polar and nonpolar solubilizates, possibly because increased thermal agitation increases the space available for solubilization in the micelle [57]. For nonionic surfactants, the effect of temperature increase depends on the nature of the solubilizate. Nonpolar materials, which are solubilized in the inner core of the micelle, appear to show increased solubility as the temperature is raised. Increase in temperature above 10 C, causes the increase in thermal agitation of the surfactant molecules in the micelles which results in increased in solubilization. Further an increase in temperature decreases the amount of material solubilized due to increased dehydration and tighter coiling of the chains, decreasing the space available in the palisade layer Reasons for self-aggregation of surfactant molecules (i) Hydrophobic Interaction One of the important features that make water unique as a solvent is its response to a- polar solutes. The tendency for a-polar molecules or molecular fragments to avoid contact with water is said to be due to the hydrophobic interaction, which thus gives rise to a thermodynamic force rather than a mechanical force. The hydrophobic interaction and the mechanism of surfactant self-assembly has been studied extensively [58]. From a thermodynamic point of view, surfactant self-assembly is entropy driven process [59]. When temperature is increased, entropy of water is increased due to the destruction of structured water around the hydrophobic tail and entropy of surfactant is decreased a little compared to the water. Even though it is an endothermic process, the free energy of the whole process is negative which suggests micelle formation is a spontaneous process. Generally, the water molecules are arranged in an ordered way around the monomeric units of surfactants, which can be defined as iceberg. During micellization, due to the destruction of the iceberg a positive entropy change occurs. Despite this micellizationfavoring phenomenon, a negative entropy change can occur if the ordering of the randomly oriented amphiphile molecules from the solvated form into a micelle structure 24

41 Introduction is more pronounced than disordering effect due to the destruction of icebergs around the alkyl chains. At the same time, the motion of the water molecules bound to the hydrophilic heads become more restricted, contributing to the decrease in entropy [60]. (ii) Hydration Due to its highly structured nature, water as a solvent displays a very complex behavior. Thus in addition to direct ion-molecule interactions, the effect of a solute on the hydrogen bonded network is of great importance. It is important to note that non-polar solutes have particularly profound influences on water structure. Thus the alkyl groups markedly reduce both the rotational and the translational mobility of the water molecules [61]. This entropically unfavorable solution of nonpolar molecules or group in water is termed hydrophobic hydration. X-ray diffraction studies have established their structure to be of the clathrate type, with the solute surrounded by a layer of hydrogen-bonded water molecules forming, for example, pentagonal dodecahedra. Thus even if the detailed structure is not presently established, it is assumed that alkyl chain of an amphipile monomer in water is surrounded by a hydrogen-bonded organized water layer. The polar heads of the monomer interact with water in away similar to simple polar solutes and electrolytes through hydrogen-bond, dipole-dipole and ion-dipole interactions. But when the amphiphiles are in micelles these hydration features get affected. The nature and the extent of this effect are interesting for both fundamental understanding and applied aspects. Very few studies have been done on the hydration of non-ionic surfactants because of the sensitive effects of temperature and concentration on their micellar size and shape. There are also various spectroscopic methods for the study of amphiphile hydration. Deuteron quadruple splitting studies may provide information on the number of water molecules influenced in their orientation by the amphiphile aggregates in liquid crystals [62]. For the lamellar phase of the systems alkali octanoate-decanol-water, for example, at most about 5 water molecules per octanate are appreciably oriented [63]. 25

42 Introduction (iii) Counter-ion Binding A counter ion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium cation is the counter ion for the chlorine anion and vice versa. In a charged transition metal complex, a (i.e. non-coordnated) ion accompanying the complex is termed the counterion. Counterions have a large influence on the aggregation of the surfactant molecules in solution mainly through changes in the ionic strength of the solution [64]. In addition, the valency of the counterion also influences the CMC to a larger extent. The degree of the counterion binding is due to the balance between the electrostatic forces which pull the counterion towards the oppositely charged head group of micelles and the hydration forces which tends to inhibit the binding [65]. The CMC value normally decreases as counterion binding increases. Counterions or ions with opposite charge to that of the surface active moiety of the surfactant are known to have an additional specific effect. For example, sodium bromide was found to induce the growth of micelles of the cationic surfactant cetylpyridinium bromide whereas sodium chloride did not [66]. Aromatic counterions like benzoate, tosylate, salicylate, because of their strong binding at the micellar surface lower the CMC while increasing the counterion binding [67]. Salicylate in particular is effective in inducing micellar growth. The counterion binding also increases with increasing counterion hydrophobicity enhancing the micelle formation [68]. Hydrophobic counterions are interesting as charge carrier or quencher in biomembranes and membrane photochemistry [69]. Addition of cationic surfactant to SDS is a special case of hydrophobic counterion interaction. The CMC of a mixture of anionic and cationic surfactant in aqueous solution is considerably lower than that of the individual surfactants due to the synergistic interaction between the surfactant molecules and they exhibit properties superior to their constituent single surfactant in many surfactant applications [70]. 26

43 Introduction 1.5 SURFACTANT SOLUBILITY In aqueous solution, when all available interfaces are saturated, the overall energy reduction may continue through other mechanisms. Depending on the system composition, a surfactant molecule can play different roles in terms of aggregation (formation of micelles, liquid crystal phases, bilayers or vesicles, etc). The physical manifestation of one such mechanism is crystallisation or precipitation of surfactant from solution that is, bulkphase separation. While most common surfactants have a substantial solubility in water, this can change significantly with variations in hydrophobic tail length, head group nature, counterion valence, solution environment, and most importantly, temperature The Krafft temperature As for most solutes in water, increasing temperature produces an increase in solubility. However, for ionic surfactants, which are initially insoluble, there is often a temperature at which the solubility suddenly increases very dramatically. This is known as the Krafft point or Krafft temperature, T K, which varies for each surfactant and is defined as the intersection of the solubility and the CMC curves, i.e., it is the temperature at which the solubility of the monomeric surfactant is equivalent to its CMC at the same temperature. This is illustrated in Figure 1.8. At the T K an equilibrium exists between solid hydrated surfactant, micelles and monomers. Below T K, surfactant monomers only exist in equilibrium with the hydrated crystalline phase, and above T K, micelles are formed providing much greater surfactant solubility. Above the T K maximum reduction in surface or interfacial tension occurs at the CMC because the CMC then determines the surfactant monomer concentration. The T K of ionic surfactants is found to vary with counterion [71], alkyl chain length and chain structure. The knowledge of the T K is crucial in many applications since below the T K the surfactant does not perform efficiently; hence typical characteristics such as maximum surface tension lowering and micelle formation cannot be achieved. The development of surfactants with a lower T K but still being very efficient at lowering surface tension (i.e., long chain compounds) is usually achieved by 27

44 Introduction introducing chain branching, multiple bonds in the alkyl chain or bulkier hydrophilic groups thereby reducing intermolecular interactions that would tend to promote crystallisation. Figure 1.8: The Krafft temperature T K is the point at which surfactant solubility equals the critical micelle concentration. Above T K, surfactant molecules form a dispersed phase; below T K, hydrated crystals are formed The Cloud point Nonionic surfactants do not exhibit krafft points. Instead, the solubility of nonionic surfactants decreases with increasing temperature, and these surfactants may begin to lose their surface active properties above a transition temperature referred to as the cloud point. Above the cloud point, the system consists of an almost micelle-free dilute solution at a concentration equal to its CMC at that temperature, and a surfactant-rich micellar phase. This separation is caused by a sharp increase in aggregation number and a decrease in intermicellar repulsions [72] that produces a difference in density of the micelle-rich and 28

45 Introduction micelle-poor phases. Since much larger particles are formed, the solution becomes visibly turbid with large micelles efficiently scattering light. As with T K, the cloud point depends on chemical structure. For polyoxyethylene (PEO) non-ionics, the cloud point increases with increasing EO content for a given hydrophobic group, and at constant EO content it may be lowered by decreasing the hydrophobe size, broadening the PEO chain-length distribution, and branching in the hydrophobic group [73]. 1.6 APPLICATIONS OF SURFACTANTS In all processes that take place at interfaces, surfactants can become effective. By application of surfactants, work processes may be simplified, accelerated, or economized. Also, the quality, as well as the efficiency of much differing products, may be optimized. An overview of the manifold application areas is given below: Consumer Products An important field of application for surfactants is consumer products. These products are detergents, dishwashing agents, cleaning agents and personal products Detergents and dishwashing: The primary traditional application for surfactants is their use as soaps and detergents for a wide variety of cleaning processes. Soap has been used in personal hygiene for well over 2000 years with little change in the basic chemistry of their production and use. New products with pleasant colors, odors, and deodorant and antiperspirant activity have crept in to the market since the early twentieth century. The soaps and detergents are used mainly in washing our clothes, dishes, houses, and so on to remove unwanted dirt, oils, and other pollutants from the substrate Cosmetics and Personal Care Products: Cosmetics and personal care products make up a vast multi-billion-dollar market worldwide, continues to grow as a result of improved overall living standard. Such products are formulated mainly from surfactants and other amphiphilic materials. It is probably safe to say that few, if any, cosmetic 29

46 Introduction products known to women (or men, for that matter) are formulated without at least a small amount of a surfactant or surface-active component. That includes not only the more or less obvious creams and emulsions but also such decorative products as lipstick; rouge; mascara; and hair dyes, tints, and rinses. An important aspect of such products is that it may produce an adverse reaction in some cases. Unfortunately, our understanding of the chemical reactions or interactions among surfactants, biological membranes, and other components and structures is not sufficiently advanced to allow the formulator to say with sufficient certainty what reaction an individual will have when in contact with a surfactant. The possible adverse effects of surfactants in cosmetics and personal care products, of course, be studied in depth for obvious safety reasons Industrial Applications Food products: The food industry utilizes surfactants as cleaners and emulsifiers [74]. Through application of natural or synthetic emulsifiers, O/W emulsions (milk substitutes, ice cream, mayonnaise, sauces, etc.) and W/O emulsions (e.g., margarine) can be improved in their consistency Pharmaceutical industry: The primary application of surfactants in the pharmaceutical industry is as emulsifiers for creams and salves, but they are also used as dispersing agents in tablets or as synergists for active ingredients. The most important criterion for a specific application is the pharmacological or toxicological product safety Insecticides and herbicides: Active substances for the protection of growing plants [75] are offered as powder or liquid concentrates, which are diluted to so-called spray liquors for application. Surfactants are used here as aids for preparing satisfactorily dispersed spray liquors for adequate wetting of the target, as well as for promoting penetration of active substances into the plant Agriculture: In agriculture, surface active polymeric carboxylic acids or short chain alkane sulfonates effect hydrophilizing of heavy soils. To prevent caking of fertilizers in mixers and to achieve uniform distribution of fertilizers in the soil, dilute 30

47 Introduction solutions of fatty alcohol polyglycol ethers, alkyl benzene sulfonates or cationic surfactants are advantageous Textiles and fibers: In the manufacture and further processing of textiles, surfactants have a role as auxiliaries in a number of process steps. In pretreating of textile material, natural fibers are freed of accompanying substances (waxes, fats, pectines, seed hulls and other impurities). The detergents and wetting agents needed for this are primarily mixtures of different surfactant types. In the manufacture of textiles, surfactants are applied to optimize individual processing steps (drawing, spinning, twisting, texturizing, coning, weaving, knitting, etc.) Chemical industry: The wetting and dispersing power of surfactants is being utilized in chemical processes to aid processing. In systems containing immiscible components, the reaction speed may be increased by the emulsification effect of surfactants, e.g., in splitting of fats by the Twitchell process, in hydrolytic splitting of wool wax and in hydrolysis of polyvinyl acetate. Also worth mentioning is phenol manufacture by the cumene process, the preparation of ethylene carbamates, as well as chlorination reactions. Surfactants may also be applied to increase the yield in extraction processes Plastics industry: The application for surfactants in the plastics industry is in the preparation of plastics dispersions (emulsion polymerization), pearl polymerizates, polyurethane foams, mold release agents and in micro encapsulation processes etc Paints and laquers: Surfactants are also of great importance in the manufacture of coating materials, paints, varnishes, lacquers, dyestuff pigments, binding materials, and binders. Paints and lacquers are, for the most part, dispersed systems of dyestuff pigments, binding materials and solvents. Therefore, surface active substances can speed up the preparation of dispersions, and improve the dispersion degree and stability Cellulose and paper: Surfactants are employed in the pulp and paper industry for the following purposes: rosin removal in pulp and paper manufacture, foam inhibition and pigment dispersion in the manufacture of paper, emulsifying in paper sizing and finishing processes, cleaning machinery, and regeneration of waste paper. In the 31

48 Introduction regeneration of waste paper (deinking flotation process), wetting agents are used to improve removal of substances adhering to the paper Leather and furs: The broad spectrum of the raw goods occurring in the leather and fur industry [76] necessitates various wet treatment processes in which surfactants and emulsifiers play a big role. In the finishing surface treatment (trimming) of the dry leather, polymer films are applied to the leather surface, whereby the quality is improved. The coatings can consist of polyacrylate-polyurethane-or polybutadiene dispersions Photographic industry: Surfactants are utilized in the photographic industry as wetting agents in casting solutions and lubricants, as aids in the preparation of dye emulsions and as additives in processing baths. In the application of antihalation layers, filter layers, or other supercoats to photographic films various surfactants have proven useful Metal processing: Surfactants do find broad application in the various processes employed in the metal processing industry. In addition to the specific cleaning processes, application in cooling lubricants, tempering oils, hydraulic emulsions, anticorrosion agents, polishing pastes, mold separating agents, and metal drying agents is especially noteworthy Electroplating: Surface active substances are applied in electrochemical processes for removal of soil and grease from substrate surfaces prior to the actual electrolytic process Adhesives: Surfactants are added to adhesives to effect a fast spreading on the respective surfaces by lowering of interfacial tension between the substrate surfaces and the adhesives. As a rule, surfactants find application only in aqueous adhesive formulations, since organic solvents have inherently low interfacial tensions Road construction and building materials: Surfactants are applied in road construction, in construction and building materials, in the preparation of bitumen emulsions, as dispersants in the cement industry, in the utilization of polymer dispersions, as additives to plasters and cement coatings and in wood impregnation. 32

49 Introduction Firefighting: In fighting fires where water cannot penetrate toward the inside of the fire source such as in fires of cotton, paper bales, wood flour, forest floors, etc., water with wetting agent containing however may be fought more effectively. For fires of storage tanks, in mines, on ships, in warehouses with combustible solid or liquid materials, on airport runways, etc., heavy foams are better suited Mining and flotation: For prevention of coal dust explosions and as dust binding agents for mineral dust in mining operations, calcium chloride pastes which are brushed onto the rock surface, are being used with surfactants to improve the wettability of the pastes. In the separation of raw material minerals, differences in surface properties of individual mineral species are being utilized. Following suspension of finely milled ore in water, air is sparged into the suspension. Minerals of value should float upwards by attachment to the air bubbles and thus be separated from the accompanying burden. The surface of the valuable mineral particles has to be hydrophobic to affect their attachment to the air bubbles and surfactant works here actively Oilfield chemicals: Surfactants find manifold applications in crude oil extraction activities [77]. In drilling operations, the properties of drilling fluids can be improved. The application of drilling fluids, i.e. the continuous flushing of the bore hole, has as its purpose to lubricate and cool the drilling tool, to flush the drilled out rock particles upwards, to support the wall of the bore hole, and to prevent the sudden eruption of oil or gas after penetration of the deposit. The basis of most drilling mud is bentonite. Additionally used are heavy spar, protective colloids and thickeners Cleaning agents: The cleanliness of homes, work places, and public facilities, is of great importance for reasons of hygiene, esthetics and value maintenance. Although highly developed machines are available for the cleaning of both textiles and tableware, the mechanical cleaning of fixed hard surfaces is only feasible on the large surface areas found in the commercial sector. Hence, to a large extent hard surfaces have to be cleaned by manual procedures. To simplify this work, cleaning agents are extensively utilized Other: Surfactants in biological systems: The understanding of the pulmonary surfactant system, although discovered in 1929, has only been applied clinically since 33

50 Introduction about 1990 for the treatment of respiratory distress syndrome. Surfactant replacement therapy may also be used in treating other forms of lung disease, such as meconium aspiration syndrome, neonatal pneumonia and congenital diaphragmatic hernia. 1.7 THE SCOPE AND OBJECTIVES OF THE PRESENT STUDY Due to their immense application potential, surfactant-based systems are a topic of major research interest in both academia and industry. They are one of the most important groups of organic chemicals, and are used in vast amounts in domestic and industrial applications. They are designed to remove dirt, sweat, sebum, and oils from the skin and other surfaces. The main characteristic of these compounds is to decrease the surface tension of solvent, and resulting many properties such as contact angle, foam properties etc.and forming colloidal sized clusters known as micelle in solution [78]. These clusters or aggregates of different morphologies endow the surfactant solutions with useful properties. Such unique properties encouraged their applications in various field of study, such as microbiology, pharmaceuticals industry, food industry, personal care, cosmetics, catalytic reaction, oil recovery and polymerization, etc. [79]. However to initiate aggregates or micelle the solution must attain a certain concentration level known as the critical micelle concentration (CMC). Below the CMC surfactant molecule exist as monomers and cannot show its activity. Many of its properties changes upon the formation of micelles. Micellar solutions have the special characteristics of solubilizing the hydrophobic organic compounds [80]. An increase in surfactant concentration in solution increases the extent of solubilization of hydrophobic solutes because of an increase in the number of micelles in the bulk. Studies of the solubilization of poorly water-soluble compounds in non-aqueous and aqueous system have revealed a lot of application in the practical fields such as drug formulations and drug carrier, drug solubilization, separation, toxic waste removal etc. [81-82]. Therefore, it is a matter of great research interest is to reduce the CMC to a lower value for wider application of surfactants. In this study such an attempt has been taken to tune the CMC to a lower value. A major area of concern nowadays is the micelle formation in the presence of additives, among which surfactant- inorganic salts interactions are of great interest. Net 34

51 Introduction charge, either on the molecules of one component or on both [81], determines the nature of surfactant-salt interactions. When a salt is present in any aqueous surfactant system it decreases the electrostatic repulsion between the charged head groups which causes a decrease in the (CMC). For example, the CMC of cetyltrimethylammonium bromide (CTAB) decreases from 0.92 to 0.56 mm when the NaCl concentration is 0.01 M [83]. Other major factors, which are playing an important role, are the length of the surfactant hydrophobic tail, and temperature. The CMC values increase with temperature. The temperature effect on the process of micellization of surfactants in water has usually been analyzed in terms of two opposing factors. With an increase in temperature, the degree of hydration of hydrophilic group decreases and this process is in favour of micellization. On the other hand, it also breaks the water structure surrounding the hydrophobic groups and is unfavourable for micellization [84] of the surfactant. The predominated one thus determines CMC formation in aqueous surfactant solution. Increasing the number of carbon atoms in the hydrophobic alkyl chain, decreased the (CMC). Longer chain length of HTAB than that of TTAB increases the surface area of the micelle and, thus, reduces the electrostatic repulsions [85]. The opposing repulsive interaction between the polar/charged head groups disfavor micellization and leads CMC to higher values. So it is a delicate balance between the interaction between hydrophobic alkyl chain and between opposing repulsive head groups. Another interesting characteristic feature shown by the ionic surfactant is their limited in solubility below a certain critical temperature but above this temperature they are fully soluble. This temperature is known as Krafft Temperature (T K ). Below this temperature the surfactant molecules remain as crystalline hydrated solids. At this state surfactant solution loses many of its activities. The T K can also be termed as the melting temperature of the hydrated solid surfactant [86]. The monomer solubility is essential for the formation of micelle. At T K the surfactant monomers become soluble enough for the formation of micellar aggregates and the solubility of an ionic surfactant becomes equal to the CMC and there is an establishment of equilibrium state between crystalline hydrated solid and micelle formation [15]. Above this temperature equilibrium state shifted towards micelle formation and the solubility of the surfactant monomer increases and micellar formation become thermodynamically favored [5]. For surfactants being 35

52 Introduction used below T K, show lower effectiveness in reducing surface tension than similar materials that are being used above their T K. The maximum reduction in surface tension is determined by the concentration of surfactant at solution saturation [15]. The T K increases with increase in the number of carbon atoms in the hydrophobic group and decreases with branching or unsaturation in that group in a homologous series of ionic surfactants [87]. Oxyethylenation of alkyl sulfates decreases their T K ; oxypropylenation decreases them even further. Alkane sulfonates have higher T K than their corresponding alkyl sulfates. The substitution of triethyl for trimethyl in the head groups of cationic alkyl trimethylammonium bromides leads to significant reduction in their T K values [88]. This probably explains why traditional surfactants bear a hydrocarbon chain usually shorter than C18 [15]. On the other hand, increase in the number of head group in the surfactant molecule increases the solubility of the surfactant in water and increases its surface activity. When the surfactant contains two hydrophilic groups, however, its solubility in water increases compared to conventional surfactants and shows much lower Krafft points and at this stage the molecule can accommodate more carbon atoms in the hydrophobic groups without becoming water-insoluble. The solubility of surfactant also increases with the increasing the size of the head group. The concept of T K is very important as below the T K surfactant cannot show their detergency, dispersing and emulsifying properties as well as their characteristic properties of maximum lowering of surface tension, formation of micelle thus solubilization of water insoluble organic compounds. Therefore, it is essential to lower the T K of surfactants below room temperature for their wider industrial applications. In many commercial formulations, the solution contains a certain amount of dissolved salt, in addition to the surfactant ions and their counterions [89, 90]. Usually added salts lower the critical micelle concentration (CMC), increase the viscosity and surface activity of surfactants, which is favorable for their industrial applications. Unfortunately, added salts elevate the T K of surfactants which limits their industrial applications. The T K values of a number of ionic surfactants have been measured in the presence of added electrolytes [91, 92]. These studies have revealed that the T K increases with increasing the concentration of the added electrolyte. At present, it has been the subject of many research to use surfactant with lower CMC and depressed T K in comparison with pure surfactant. 36

53 Introduction In the present work, we attempted to study the effect of some electrolytes on the T K and micellar behavior of Octadecyltrimethylammonium Bromide (OTAB) and Sodium Dodecyl Sulfate (SDS) in aqueous solution. Here we will show an important point that was clothed for a long time of specific ion effect on T K of ionic surfactant. It is engrossing to note here that the T K can increase or decrease depending on the nature of electrolytes and the CMC can be depressed stunningly upon addition of electrolytes to the surfactant solution. It is important to note here that except for Br (common ion), SCN and I, the rest of the anions used in this study are effective in lowering the T K and all the anions are effectual to lower the CMC of the OTAB. Only Li + is found to be effective in lowering the T K while all cations used in this study are effective in lowering the CMC of SDS. Moreover, a water insoluble dye, Sudan Red B (SRB) was solubilized in aqueous micellar solution of OTAB and SDS in pure water and in aqueous salt solution. Since many of the applications of surfactants lie in their capacity to form micelles, it can be expected that the depression of the T K and lowering of the CMC in the presence of the added electrolytes will favor wider industrial applications of OTAB and SDS. REFERENCES [1] Lomax, E. G.; Amphoteric surfactants. Marcel Dekker, New York, 1996 [2] Eichhorn, P.; Knepper, T. P.; J Mass Spectrom. 36, 6, 677, 2001 [3] Tharwat, F.; Tadros; Applied Surfactants, WILEY-VCH Verlag GmbH & Co., KGaA, Weinheim, United Kingdom, 2005 [4] Salager J. L.; J. Colloid and Interf. Sci.105, 21, 1985 [5] Mayers, D.; Surfactant Science and Technology, 3rd edn, John Wiley & Sons, Interscience, New Jersey, [6] Islam, M.N.; Kato, T.; Langmuir, 19, 7201, 2003 [7] Porter, M. R.; Handbook of surfactants. London, Blackie Academic and Professional, 1994 [8] Shchukin, E. D.; Colloid Chemistry: Textbook for Universities and High School, Vysshaya, Shkola, Moscow,

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55 Introduction [28] Evan, D. F.; Mitchell, D. J.; Ninham, B. W.; J. Phys. Chem. 88, [29] Kresheek, C.; Wate, a comprehensive treatise, Edt. By F. Franks, Plenum Press, New York, 1975 [30] Evan, D. F.; Ninham, B. W.; J. Phys. Chem. 87, 5025, 1983 [31] Del Rio, J. M.; Prieto, G.; Sarmiento, F.; Mosquera, v.; Langmuir. 11, 1511, 1995 [32] Roy, J. C.; Islam, M. N.; Aktaruzzaman, G.; J Surfact Deterg. 17, 231, 2014 [33] Crook, E. H.; Trebbi, G. F.; Fordice, D. B.; J. Phys. Chem. 68, 3592, 1964, [34] Bakshi, M. S.; Bull. Chem. Soc. Jpn, 69, 2723, 1996 [35] Liu, C.; Desai, K. G. H.; Liu, C.; J. Chem. Eng. Data.49, 1847, 2004 [36] Metha, S. K.; Bhasin, K. K.; Chauhan, R.; Dham, S.; Colloids and Surfaces A. 235, 153, 2005 [37] Drakenberg, T.; Lindman, B.; J. Colloid and Interf Sci.44, 184, 1973 [38] Phillips, J. N.; Trans Faraday Soc. 51, 561, 1958 [39] MacBain, J. W.; Third Colloidal Report of the British Association [40] Flockhart, B. D.; J. Colloid and Interf Sci.16, 484, 1961 [41] Crook, E. H.; Fordyce, D. B.; Trebbi, G. F.; J. Phys. Chem.67, 10, 1987, 1963 [42] Lange, K. R.; Surfactant: A Practic al Handbook. Hanser Gardner Publications, Inc., 6915 V alley Avenue, Cincinnati, Ohio, 1999 [43] Mayers, D.; Surfaces, Interfaces, and Colloids: Principles and Applications. VCH Publishers, Inc., New York, 1999 [44] Shinoda, K.; Bull. Chem. Soc. Jpn. 26, 101, 1953 [45] Philips, J. N.; Trans. Faraday Soc. 51, 561, 1955 [46] Hiemenz, P. C.; and Rajagopalan, R.; Principles of Colloid and Surface Chemistry. Dekker, New York, 1997 [47] Ballerat-Busserolles, K.; Roux-Desgranges, G.; Roux, A. H.; Langmuir. 13, 1946, 1997 [48] McBain, J. W.; Richards, P. H.; Ind. Eng. Chem. 38, 642, 1946 [49] Hutchinson, E.; Mosher, C. S.: J. Colloid and Interf Sci.11, 352, 1956 [50] Choucair, A.; Eisenberg, A.; J. Am. Chem. Soc. 125, 11993, 2003, [51] Paria, S.; Yust, P.K.; Ind. Eng. Chem. Res. 45, 3558, 2006 [52] Moroi, Y.; Adv Colloid Interf Sci.73, 47,

56 Introduction [53] Stake, I.; Matsuura, R.; Bull. Chem. Soc. Japan. 36, 813, 1963 [54] Saito, S.; J. Colloid and Interf Sci. 24, 227, 1967 [55] Nakagawa, T.; Tori, K.; Kolloid-Z. 168, 132, 1960 [56] Schwuger, M. J.; Kolloid-Z. Z. Polym. 240, 872, 1970 [57] Elworthy, P. H.; Florence, A. T.; MacFarlane, C. B.; Solubilization by Surface- Active Agents. Chapman & Hall, London, 68, 1968 [58] Franks, F.; water a comprehensive treatise, ed, F. Franks, Plenum Press, New York, 2, 1, 1973 [59] Marilyn, F. E.; Holtzer, A.; J. Phys. Chem.71, 10, 3320, 1967 [60] Tanford, C.; The hydrophobic effect: Formation of micelles and biological membranes. J. Wiley, New York, 1980 [61] Zeidler, M. D.; in: water: a comprehensive treatise, ed, F. Franks, Plenum Press, New York, 2, 1973 [62] Johansson, A.; Drakenberg, T.; Mol. Cryst. Liquid Cryst. 14, 23, 1971 [63] Persson, N.O.; Lindman, B.; J. Phys. Chem. 79, 1410, 1975 [64] Lindman, B.; Wennerstrom, H.; Spinger, Verlag, New York, 1980 [65] Berr, S.; Jones, R. R. M.; Johnson, J. S. J.; J. Phys. Chem. 96, 5611, 1992 [66] Porte, G.; Appell, J.; Poggl, Y.; J. Phys. Chem. 84, 3105, 1980 [67] Bijma, K.; Engberts, J. B. F. N.; Langmuir. 13, 4843, 1997 [68] Manet, S.; Karpichev, Y.; Bassani, D.; Ahmad, R. K.; Odo, R.; Langmuir. 26, 10645, 2010 [69] Almgren, M.; Swarup, S.; J. Phys. Chem. 87, 876, 1983 [70] Puvvada, S.; Blankschtein, D.; J. Phys. Chem. 96, 5567, 1992 [71] Hato, M.; Tahara, M.; Suda, Y. J. Colloid and Interf Sci. 72, 458, 1979 [72] Staples, E. J.; Tiddy, G. J. T. J.; Chem. Soc., Faraday Trans. 1, 74, 2530, 1978 [73] Schott, H. J.; Pham. Sci. 58, 1443, 1969 [74] Kuchbuch, B.; Fette-Seifen-Anstrichmittel. 77, 407, 1975 [75] MUller, A.; in: Proceedings of the World Surfactants Congress, Miinchen, Vol. IV, Gelnhausen, Kiirle, 1984 [76] Hollstein, M.; in: Proceedings of the World Surfactants Congress, Miinchen, Vol. IV, Gelnhausen, Kiirle,

57 Introduction [77] Friedel, H.; in: Proceedings of the World Surfactants Congress, Miinchen, Vol. I, Gelnhausen, Kiirle, 1984 [78] Staszak, K.; Wieczorek, D.; Michocka, K.; J Surfact Deterg.18, 321, 2015 [79] More, U.; Kumari, P.; Vaid, Z.; Behera, K.; Malek, N. I.; J Surfact Deterg 19, 75, 2016 [80] Paria, S.; Yust, P. K.; Ind. Eng. Chem. Res. 45, 3558, 2006 [81] Metha, S. K.; Bhasin, K. K.; Chauhan, R.; Dham, S.; Colloids and Surfaces A. 255, 153, 2005 [82] Kim, J.H.; Domach, M. M.; Tilton R. D.; Langmuir. 16, 10037, 2000 [83] Roy, J. C.; Islam, M. N.; Akhtaruzzaman, G.; J Surfact Deterg. 17, 231, 2013 [84] Varade, D.; Joshi, T.; Aswal, V. K.; Goyal, P. S.; Hassan, P. A.; Bahadur, P.; Colloid Surf A. 259, 95, 2005 [85] Banipal, T. S.; Kaur, H.; Banipal, P. K.; Sood, A.K.; J Surfact Deterg17, 1181, 2014 [86] Tsuji, K.; Mino, J.; J. Phys. Chem. 82, 1610, 1978 [87] Gu, T.; Zhu, B-Y.; Rupprecht, H.; Prog. Colloid Polym. Sci. 88, 74,1992 [88] Davey, T. M.; Ducker, W. A.; Hayman, A. R.; Simpson, J.; Langmuir.14, 3210, 1998 [89] Diamant H.; Andelman, D.; J. Phys. Chem. 100, 13732, 1996 [90] Vijayan, S.; Ramachandran, C.; Shah, D. O.; J. Am. Oil Chem. Soc. 58, 566, 1981 [91] Carolina, V. G.; Bales, B. L.; J. Phys. Chem. B. 107, 5398, 2003 [92] Bakshi, MS.; Sood, R.; Colloids Surf A. 233, 203,

58 Chapter Two Experimental section

59 Experimental 2.1 MATERIALS Surfactants 1. Octadecyltrimethylammonium Bromide (OTAB) Linear formula: CH 3 (CH 2 ) 17 N(Br)(CH 3 ) 3 Structure: 2. Sodium Dodecyl Sulfate (SDS) Linear formula: CH 3 (CH 2 ) 11 OSO 3 Na Structure: Salts 1. Sodium Fluoride (NaF) 2. Sodium Chloride (NaCl) 3. Sodium Bromide (NaBr) 4. Sodium Iodide (NaI) 5. Sodium Thiocyanate (NaSCN) 6. Sodium Nitrate (NaNO 3 ) 7. Sodium Sulfate (Na 2 SO 4 ) 8. Sodium Benzoate (C 7 H 5 O 2 Na) 42

60 Experimental 9. Sodium Salicylate (C 7 H 5 O 3 Na) 10. Sodium Benzene Sulfonate (C 6 H 5 SO 3 Na) 11. Lithium Chloride (LiCl) 12. Potassium Chloride (KCl) 13. Cesium Chloride (CsCl) Dye 1-({3-methyl-4-[(3-methylphenyl)diazenyl]phenyl}diazenyl)naphthalen-2-ol Linear formula: C 24 H 20 N 4 O Structure: The cationic surfactant Octadecyltrimethylammonium Bromide (OTAB) was supplied by Sigma-Aldrich, with a purity of > 99 % and was used without any further purification. The anionic surfactants Sodium Dodecyl Sulfate was collected from MERCK and was highly pure samples and was used as received. Some salts were obtained from BDH and some from MERCK and Sigma-Aldrich with a purity > 99 % and were used as received. The dye SRB was obtained from MERCK. Triple-distilled water from all-pyrex glass apparatus was used for the preparation of solutions. All the measurements were carried out two or three times until reproducible data was obtained and when the data were found to agree within ±1%, then the results were confirmed. 43

61 Experimental 2.2 METHOD Measurement of Krafft Temperature To determine T K, clear aqueous solutions of surfactant, SDS and OTAB in pure water and in the presence of salt of counter-ion were prepared and placed in a refrigerator at about 2 C for at least 24h, where the precipitation of surfactant hydrated crystals occurred. The system was then taken out of the refrigerator when precipitation of the hydrated surfactant occurred and then the temperature of the precipitated system was raised gradually under constant stirring with a glass rod, and its conductance was measured with the help of a EUTECH CON 510 conductivity meter. Figure 2.1: Hydrated crystal in the beaker (left side) and arrangement for Krafft temperature measurement (right side: EUTECH CON 510 conductivity meter and Froilabo RE 5 refrigerated bath circulator) 44

62 Experimental At each temperature, the conductance reading was checked every 2 min until it reach a steady value. The temperature was measured using a sensor combined with conductivity meter (precision of ±0.01) immersed in the investigated system. The Krafft temperature was taken as the temperature where the conductance versus temperature plots showed an abrupt change in slope. Operationally, T K values were determined from plots of the second derivative of the data. This temperature was the same as that required to completely dissolve the hydrated solid surfactant, judged visually to be the point of complete clarification of the system. The reproducibility of T K measurements on a single sample (typically ± 0.05 C) was superior to the reproducibility in samples presumably prepared identically (averages about ±0.1 C). Details of the experimental procedure are to be found elsewhere [1] Measurement of Critical Micelle Concentration Conductometric method: Conductivity measurements were carried out by using a EUTECH CON 510 conductivity meter. Experiments were started with a dilute solution and the subsequent concentrated solutions were obtained by adding a previously prepared stock solution into a 100-mL beaker. The solution was stirred with glass rod after each addition and the conductance of the solution was measured. The CMC was then taken from the sharp break in the conductance vs concentration plot. The temperature of the solution was kept constant by using a circulating water bath (Froilabo RE 5 refrigerated bath circulator) with a precision of ±0.1 C. To observe the effect of electrolytes on the CMC, surfactant solutions were prepared in various electrolytes solutions of desired concentrations [2]. Surfacetensiometric method: To measure CMC, the surface tensions of the aqueous surfactant solutions of different concentrations were measured by a surface tensiometer (Kruss K9) furnished with a platinum plate. Before each measurement, the plate was thoroughly washed by red heat. The solution was transferred into a vessel that was thermostated by circulating water at the desired temperature. Before measurement, the surface tension of the double distilled deionized water was confirmed to be in the range 45

63 Experimental ±0.3 mn/m at the respective temperature. Two readings were acquired under all experimental conditions and standard deviations were 0.4 mn/m. The surface tension measurements were started with a dilute solution and the subsequent concentrated solutions were made by adding a previously prepared stock solution into the vessel. Care was taken that the platinum plate was properly wetted with the solution. The establishment of equilibrium was checked by repeated measurements at 5-min intervals until the surface tension readings stabilized; this generally required min. Details of the experimental procedure are to be found elsewhere [2]. Figure 2.2: Surface tension measurement: Surface tensiometer (Kruss K9) and refrigerated bath circulator (JSRC-13C) 46

64 Experimental Solubilization Solubilization studies of Sudan Red B (SRB) in OTAB and SDS solution in pure water and in the presence of Na 2 SO 4 and NaCl respectively were conducted under the condition of maximum solubilization at a temperature of 30±1 C for SDS and associated salt, NaCl and 38±1 C for OTAB and with Na 2 SO 4. The temperature for each system was chosen above the Krafft temperature to ensure the micelle formation of the surfactants in aqueous solution. 50-mL reagent bottles were used for this study. Surfactant solutions of different concentrations were poured separately into some reagent bottles, where the surfactant concentrations in the first few were below the CMC and the last few were above the CMC. A fixed amount of the solubilizate (SRB) was added to maintain excess product at least three times its solubility limits for achieving solubilization equilibrium. To equilibrate the solution the bottles were continuously agitated using a shaker (Stuart Orbital shakers, SSL1) at 250 rpm for 24 hours held in a horizontal position. The solutions were then filtered in order to separate the non- solubilized excess of dye from the solution using Whatman 41 Ashless Quantitative Filter Paper 2.5µm and filtrate was then analyzed by using the UV visible spectrophotometer (Jenway Spectrophotometer- 7315). The absorbance of each solution was measured by using a quartz cell of path length 1 cm. The concentration of SRB in surfactant micelles was calculated from a calibration curve obtained from the absorption spectra of known concentrations of SRB in OTAB and SDS against a blank. The strong absorbance at λ max = 517 nm for OTAB and λ max = 524 for SDS gave a satisfactory Beer s law plot. 47

65 Experimental Figure 2.3: Shaking of the surfactant solution with dye (Top: Stuart Orbital shakers, SSL1) and solution after shaking (Below) 48

66 Experimental Figure 2.4: Jenway UV-spectrophotometer, model 7315 (Top) and a spectrophotogram of SRB (Below) 49