Surfactant-Amino Acid and Surfactant-Surfactant Interactions in Aqueous Medium: a Review.

Appl Biochem Biotechnol DOI 10.1007/s12010-015-1712-1

Surfactant–Amino Acid and Surfactant–Surfactant Interactions in Aqueous Medium: a Review Nisar Ahmad Malik 1

Received: 7 December 2014 / Accepted: 9 June 2015 # Springer Science+Business Media New York 2015

Abstract An overview of surfactant–amino acid interactions mainly in aqueous medium has been discussed. Main emphasis has been on the solution thermodynamics and solute–solvent interactions. Almost all available data on the topic has been presented in a lucid and simple way. Conventional surfactants have been discussed as amphiphiles forming micelles and amino acids as additives and their effect on the various physicochemical properties of these conventional surfactants. Surfactant–surfactant interactions in aqueous medium, various mixed surfactant models, are also highlighted to assess their interactions in aqueous medium. Finally, their applied part has been taken into consideration to interpret their possible uses. Keywords Amino acids . Surfactants . Surfactant–amino acid interactions . Surfactant–surfactant interactions

Introduction Proteins and Amino Acids Proteins are the biomolecules which play a vital role in all the biochemical processes occurring in living organisms. Their behaviour can be governed by their interactions with the surrounding environment. In order to understand the role played by biological molecules in the living organism [1], it is necessary to study the thermodynamic stability of native structure of protein. It has proved quite challenging and still remains a subject of extensive research [2] for the researchers. Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. Generally, the polypeptides with less than 40 amino acids are termed as peptides. Naturally occurring polypeptides with more than 40 amino acids are proteins. The equilibrium conformation adopted by the protein is a sensitive function of residue composition, sequence and solvent environment. There are two major classes of * Nisar Ahmad Malik [email protected] 1

Islamic University of Science and Technology, Awantipora, Pulwama, Srinagar 192122, India

Appl Biochem Biotechnol

conformations, namely native conformation and random coil conformation. In native conformation, chain–chain contacts are thermodynamically favoured over chain–solvent contact while in random coil conformation, chain–solvent conformation is favoured over chain–chain contacts. To be functional and active, there should be energetic balance between native and random coil conformations of proteins. There are an array of vital forces, i.e. hydrophobic interactions, hydrogen bonding, ionic interactions, van der Waals interactions which constitute the main forces responsible for the specific structure and conformation of proteins. Some covalent forces like disulphide linkage also contribute in maintaining the structure of protein. The net free-energy difference which stabilizes the native conformation against transition to other forms are small and around 9–17 kcal mol−1 [3], and this small energy difference can easily be overcome by relatively minor perturbation in the external factors like temperature, pH, pressure, ligand binding, surface charge distribution and hydrophobicity. All these factors are able to disturb different types of forces which are responsible for protein stability. The important contributions to conformational properties and structural stability of proteins come from effect of different additives like electrolytes, surfactants and different organic entities. So, it is essential to have knowledge of interactions responsible for stabilizing the native state of protein in the presence of these additives. Also, the process of denaturation of proteins in aqueous solutions is a fundamental biological process that is not yet completely understood and remains a subject of extensive investigations [2]. To understand the effects of denaturating agents on proteins, it is important to understand the interactions between these additives and proteins in aqueous solutions. But due to structural complexity of proteins, their direct study is somewhat difficult. Therefore, one useful approach is to investigate interactions between extensively employed model compounds of proteins, i.e. amino acids, peptides and their derivatives, in aqueous and additive–aqueous solutions. This approach has received a lot of attention in recent years [4]. Protein hydration is an important factor that is responsible for stabilizing the native structure of globular proteins in aqueous solutions. The specific interactions of water with various functional groups on the proteins as well as other solvent−related effects contribute to the formation of the stable folded structure of proteins in solutions [5]. Direct investigation of important protein–water, protein−ion and protein−alkyl group interactions are difficult because of the complexity of these interactions in a protein macromolecule. Amino acids as structural components of proteins, peptides and certain types of hormones and antibiotics and many other compounds of biological relevance, participate in all the physiological processes in a living cell [6]. Generally, it has been recognized that in the absence of experimental data for proteins, amino acids, peptides and their derivatives can serve as useful models in estimating the properties of proteins [7]. Hydration of dipolar ions is a concept of particular significance in the chemistry and biology. Among the dipolar ions frequently studied, amino acids, peptides, proteins, certain types of hormones and antibiotics are common. In physiological media such as blood, membrane, cellular fluids etc., where water happens to be involved in an important manner, the dipolar nature of biomolecules are very important. In aqueous medium, amino acids exist as dipolar ions (zwitterions) at pH = 7 manifesting a unique hydration behaviour which appears to be subtly linked to the vital biological phenomenon. Because of such subtle linkage, study of amino acids, peptides and proteins are considered important in unfolding the role of dipolar ions in the living organisms. Although in the past [8, 9] studies concerning temperature and pressure effects on dipolar ion hydration have been carried out, yet they are not so extensively studied and are needed for further investigation for a better understanding of the subject. In aqueous medium, amino acids also act as acids or bases depending upon the pH of the solution. Hence, the knowledge of the


acid–base properties of amino acids is extremely important in understanding many properties of proteins [10]. Thermodynamic and viscometric properties of these model compounds in aqueous medium provide information about solute–solvent and solute–solute interactions, which, in turn, may help to understand several biochemical processes such as protein hydration, denaturation and aggregation [11, 12]. Proteins may be composed of as many as 20 different amino acids. In each protein, the precise types and amounts of each amino acid are covalently linked in the linear sequence specified by the base sequence of the DNA-generated mRNA for that protein [10]. The ability of each type of the tens of thousands of different proteins to perform its functions is specified by its unique amino acid sequence. The structures of the 20 amino acids that are commonly found in proteins are shown in Fig. 1. These amino acids are referred to as standard amino acids. Common abbreviations for the standard amino acids are listed in Table 1. Note that 19 of the standard amino acids have the same general structure (Fig. 1.). These molecules contain a central carbon atom (the α-carbon) to which an amino group, a carboxylate group, a hydrogen atom and an R (side chain) group are attached. The exception, proline, differs from the other standard amino acids in that its amino group is secondary, formed by ring closure between the R group and the amino nitrogen. Proline confers rigidity to the peptide chain because rotation about the α-carbon is not possible [11]. Nonstandard amino acids consist of amino acid residues that have been chemically modified after they have been incorporated into a polypeptide or into amino acids that occur in living organisms but are not found in proteins. At a pH of 7, the carboxyl group of an amino acid is in its conjugate base form (−COO−) and the amino group is in its conjugate acid form (−NH3+). Thus, each amino acid can behave as either an acid or a base. The term amphoteric is used to describe this property. Neutral molecules that bear an equal number of positive and negative charges simultaneously are called zwitterions [13].

Surfactant–Amino Acid Interactions From biotechnological and biopharmaceutical formulations’ point of view, surfactant–amino acid interaction is an essential area for research (Tables 1 and 2). To diminish the protein wastage that occurs by colloidal and interfacial mechanisms, e.g. aggregation and adsorption, surfactant–amino acids play a major role [14–19]. In order to achieve this, a fundamental understanding of the mechanisms underlying surfactant effectiveness is necessary. In particular, better understanding of the specific roles of the surfactant, protein and surfactant–protein complex in modulating interfacial behaviour will provide direction for much-needed process improvements in the production and finishing of therapeutic proteins (amino acids). Studies of surfactant–amino acid interactions and surfactant–surfactant interactions in aqueous medium from a thermodynamic point of view are very essential and give an insight into the various types of interactions prevailing in the systems and can help in choosing and designing the systems of interest. Protein–surfactant interactions have been extensively studied in aqueous solutions [20–24]. Both oppositely [25–27] and similarly [28, 29] charged protein–surfactant and protein–nonionic surfactant systems are being studied [30]. The understanding of the interactions is complicated because proteins are complex macromolecules with a distinctive primary structure articulated in terms of their amino acid sequences. A wide variety of interactions with surfactant molecules are shown by these constituents at a molecular level. Proteins and lipids have long been known to interact at interfaces as well as in the bulk solution and can thus affect the solution properties to a significant degree [31–36]. The proteins bind surfactants in the monomer form and also in an aggregated condition, depending on the


Fig. 1 Structures of different amino acids

surfactant concentration [21, 22]. Stabilization or destabilization of the protein structure may occur due to concentration of surfactant and the natural environment of the protein [37, 38]. The anionic surfactants in general interact strongly with proteins and can form protein– surfactant complexes inducing the unfolding of proteins [22], whereas a weaker interaction with proteins is shown by cationic surfactants [22, 39]. The nonionic surfactants bind very


Table 1 Names and abbreviations of standard amino acids

Amino acid

Three-letter abbreviation

One-letter abbreviation

Alanine

Ala

A

Arginine Asparagine

Arg Asn

R N

Aspartic acid

Asp

D

Cysteine

Cys

C

Glutamic acid

Glu

E

Glutamine

Gln

Q

Glycine

Gly

G

Histidine

His

H

Isoleucine Leucine

Ile Leu

I L

Lysine

Lys

K

Methionine

Met

M

Phenylalanine

Phe

F

Proline

Pro

P

Serine

Ser

S

Threonine

Thr

T

Tryptophan Tyrosine

Trp Tyr

W Y

Valine

Val

V

weakly to the proteins in comparison to the anionic surfactants [22]. Due to the absence of the electrostatic interaction between protein and nonionic surfactant and low critical micellar concentration (CMC) of these surfactants, micelle formation in the bulk solution is a more favourable process than binding to proteins [22, 40]. To prevent and/or minimize protein aggregation during purification, fermentation, shipping, freeze drying and/or storage surfactants, particularly nonionic types, are often added [41]. Low-molecular-weight surfactant mixtures and proteins are extensively studied due to their major industrial applications in food processing, pharmacology or cosmetics. The formation and the stabilization of foams and emulsions depend on the interfacial properties of these mixtures, their surface activity, rheological behaviour, and adsorption dynamics, which can be drastically different as compared to those of the single components. The structure and properties of proteins including their solubility, denaturation and dissociation into subunits and the activity of enzymes are largely affected by salt solutions [42, 43]. The behaviour of proteins in solutions is governed by a combination of many specific interactions due to their structural complexity. To reduce the degree of complexity in systems, smaller biomolecules such as amino acids and peptides which require less complex measurement techniques are used to study the interactions. The presence of an electrolyte considerably affects the behaviour of amino acids in solutions, and this information can be used for their purification and separation [44, 45]. Thermodynamic properties of amino acids in aqueous electrolyte solutions provide valuable information about solute–solvent and solute–solute interactions. The effect of electrolytic solutions on amino acids has already been revealed by many works [46–50]. A better insight into the effect of

Appl Biochem Biotechnol Table 2 Types of surfactants commonly used Anionic surfactants Name Sodium dodecyl sulphate

Molecular formula C12H25Na O 4S

Sodium stearate

C18H35Na O2

Sodium myristate

C14H27Na O2

Dodecyl benzene

C18H30

Molecular structure

Cationic surfactants

electrostatic and hydrophobic interactions on the stability of proteins can be obtained from salts such as tetramethylhalides which are known to influence macromolecular conformations by weakening attraction or repulsion of inter and intra charge–charge interactions and by affecting hydrophobic interactions through the side chain of the alkyl groups. The bulkiness of tetraalkylammonium salts, depending on their alkyl chain, is known to orient water molecules

Appl Biochem Biotechnol Table 2 (continued) Dodecyl trimethyl ammonium chloride

C15H34Cl N

Cetyltrimethyl ammonium bromide

C19H42Br N

Cetylpyridinium chloride

C21H38Cl N

Amphoteric (zwitterionic) surfactants

Lauryl amido propyl dimethyl betaine

C12H25CON(C H3)2CH2COOH

Nonionic surfactants

around them [51, 52]. The interfacial properties of protein layers are affected differently by the addition of ionic or nonionic surfactants. In almost all life processes, proteins are very important. They interact with a wide variety of surfactants, drugs and ligands such as bilirubin, fatty acids, hematin and metal ions [53–63]. Due to the applications in biosciences, foods and cosmetics, drug delivery, detergents and biotechnological process interactions of proteins with surfactants have been studied for many years. Blood serum for instance is a mixture of human

Appl Biochem Biotechnol Table 2 (continued) Dodecyl hexaoxyethylene glycol monoether

C12H25(OCH2CH2O)6 OH

tertcetylphenoxypolyethoxyeth anol (Triton X-100)

(a) Polyoxyethylene (20) sorbitanmonolaurate (Tween 20) (b) Polyoxyethylene (20) sorbitanmonopalmitate (Tween 40) (c) Polyoxyethylene (20) sorbitanmonostearate (Tween 60) (d) Polyoxyethylene (20) sorbitan monooleate(Tween 80) (e) Polyoxyethylene (20) sorbitantristearate (Tween 65) (f) Polyoxyethylene (20) sorbitan tri-oleate (Tween 85)

serum albumin with a number of compounds, including low-molecular-surface active molecules. The surface tension of such organic fluids is used as an investigative and therapeutic tool [64]; for example, the complex between sodium dodecyl sulphate (SDS) and bovine serum albumin (BSA) is used in the determination of molecular weights of proteins by polyacrylamide gel electrophoresis [22, 23]. Wool and human hair are proteinacious in nature, which on a regular basis are exposed to surfactants. The single-chain surfactants including cetyltrimethylammonium bromide (CTAB), SDS, and tertcetylphenoxypolyethoxyethanol (Triton X-100) are investigated extensively [65–69] and proteins with double-chain surfactants interactions are studied less. The interactions between nonionic water-soluble polymers and surfactants are of technological importance and have been the subject of intense studies over the last three decades due to their wide range of applications [70]. Protein–surfactant interactions are usually surfactant feature dependent. As cationic surfactants weakly interact with the proteins as compared to anionic surfactants due to smaller relevance of electrostatic interactions at the pH of interest [71], but for both types of surfactants, the binding isotherms are found to be similar [39, 71]. The ionic surfactants at low concentrations adhere to the


oppositely charged sites of proteins, causing them to open out and rendering the more sites for binding. The binding becomes accommodating, and eventually the protein is saturated by the surfactant as the surfactant concentration is increased. The micelle formation in bulk becomes more favourable in nonionic surfactants in comparison to ionics due to the absence of electrostatic interactions [40]. Sodium dodecyl sulphate (SDS) and bovine serum albumin (BSA) have been often used as a representative system for the determination of interactions between ionic surfactants and water-soluble proteins [72]. The surfactant molecules can change the conformation of proteins in the bulk [73] and at the interface [74]. Thus, understanding of interaction between the surfactants and proteins in the bulk and at the interface, formation of protein–surfactant complexes and displacement of protein molecules from the interface by surfactant molecules is important from scientific as well as practical viewpoints. The theoretical and experimental studies on the adsorption behaviour of protein– surfactant mixtures have been covered by a number of papers [72–76], and different mechanisms for the displacement of protein molecules from the interface by the surfactants have been suggested such as orogenic displacement [75] or competitive adsorption [76]. In many investigations [77–81], interaction of cationic gemini surfactants with proteins have exposed that such surfactants interact more competently with proteins as compared to conventional single-chain surfactants because of their distinctive aggregation properties such as lower critical micelle concentration (CMC) and Krafft temperature, special aggregation morphology, strong hydrophobic microdomains and so forth [82–85]. Cationic surfactants, being antimicrobial, have attracted attention with respect to their interaction with deoxyribonucleic acid (DNA) and lipids [86]. The mixed cationic–anionic (decyltriethylammonium bromide + sodium decylsulphonate) surfactants with the BSA due to the strong synergism in mixed micelle formation between the cationic and anionic surfactants in aqueous solutions were reported showing very weak interaction [87]. It has been proposed that hydrophobic and electrostatic interactions are the two main driving forces for the association between surfactants and proteins in aqueous solution. However, the study of protein–surfactant interactions is difficult because of the complexity of interactions in such a large molecule. Several details in the mode of these interactions remain unanswered. Therefore, it is very important to understand the origin and nature of these interactions both qualitatively and quantitatively. To understand the fine details, the interactions of the building blocks of the protein with surfactants must be studied. There have been some investigations on the interaction of surfactants with amino acids [88–91] (Figs. 2 and 3). The widespread application of surfactants in the field of biochemistry has given momentum to fundamental studies of the nature of the interaction between protein and surface active agents in biological phenomena such as biological membranes [92] and protein solubilization [93]. It has also been suggested that surfactant–protein systems can be used as a model for biological membranes. The effect of surfactants, on protein folding and unfolding, depends on the concentrations of surfactant and protein [94], as well as the nature of protein. Anionic surfactants, such as SDS, bind to protein in the monomeric state and in the micellar condition. The interaction between protein and micelle is a complicated situation that plays a role in the folding and unfolding of proteins [95]. Protein refolding and protein reactivation at high concentrations (CMC) using a detergent/phospholipid mixture has been reported. Proteins reactivated by mixed micelles are easy to purify since folding intermediates remain bound to the micelles and are released only after they have folded/reactivated. This is a successful method for preventing misfolding and/or aggregation, as well as promoting correct protein folding. It has also been used in the regeneration of bacterio rhodopsin in mixed micelles [96]


Fig. 2 Schematic representation of SDS micelles with amino acids (water molecules are not shown)

and reactivation of several enzymes by surfactant after treatment with guanidinium chloride [97]. It has also been reported that some aspects of the micelle model of alpha-crystalline can be related to its chaperone activity [98]. Surfactants may also interact with proteins directly by competing for oil–water or air–water interfaces [99] and binding to them, thereby leading to

Fig. 3 Schematic representation of CTAB micelles with amino acids (water molecules are not shown)


substantial changes in protein conformation. Surfactants cause changes in the molecular characteristics of globular proteins, and these changes in turn help protein molecules to bind with other molecules. Mao et al. [100] used the series piezoelectric quartz crystal technique to probe the interaction process between SDS, CTAB and lysozyme. They showed the stepwise opening of four disulphide bridges of lysozyme with increasing concentration of CTAB, due to the hydrophobic interactions of positively charged protein with hydrophobic groups of cationic surfactant. In case of SDS–lysozyme interaction, they found that after injecting a given amount of SDS, anionic ends of some SDS molecules strongly interacted with charged domains of lysozyme to form a complex that leads to the occurrence of charge neutralization. It was stated that such micelle-like structure possessed very strong hydrophilicity. Kelley and co-workers [55] observed that at sufficiently low, free-surfactant concentrations, the entropy of surfactant– mixing dominates the attractive forces between protein and surfactant molecules so that surfactant does not bind to protein. At higher concentration, independent of protein concentration, surfactant molecules bind to strong binding sites on protein surface possibly by electrostatic interaction between the ionic surfactant head group in combination with the hydrophobic interaction between the nonpolar surfactant tail groups and nearby hydrophobic patches on the protein surface. De et al. [54] studied protein–surfactant interaction using florescence probe method. They used albumin with SDS, CTAB and TX-100. Studies indicate that all the three surfactants bind to BSA in a cooperative manner. This cooperative binding affects the binding of the external label to BSA. Kishore and co-workers [90] reported the volumetric properties of some amino acids and two glycine peptides in aqueous SDS and CTAB at 298.15 K. Apparent molar volumes were evaluated to calculate partial molar volume and partial molar volumes of transfer. The linear correlation of partial molar volume with the number of carbon atoms present in the side chain of amino acid was calculated. Hydration number evaluation was also done. The results on partial molar volumes of transfer were discussed in terms of ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The partial molar volumes suggested that ion–ion or ion–hydrophilic group interactions of the amino acids and peptides were stronger with SDS as compared with those of CTAB. Further, they also reported the partial molar volume of transfer of some amino acids and protein (lysozyme) in aqueous TX-100 at 298.15 K [101]. The linear correlation of partial molar volume to the number of carbon atoms was calculated to determine the contribution of charged end groups and other alkyl chains to partial molar volume. The number of water molecules bound to amino acids was also estimated. The results showed the solubilization of amino acids in the palisade layer of Tx-100 where an overall balance between interactions was maintained. The partial specific volume of transfer of protein (hen egg white lysozyme) also showed the balance in hydrophobic and hydrophilic interactions in protein–nonionic surfactant system. Forgacs [102] explored the interaction of nonionic surfactant–nonylphenylhexaethoxylate with Cys, Gln, Glu, Gly, Hyp, Phe and Tyr using charge-transfer reversed-phase thin-layer chromatography. The surfactants investigated were seen to have an insignificant effect on the hydrophobicity of amino acids. However, some amino acids Hyp, Phe, Tyr, Cys, Gln, Glu and Gly showed binding to the surfactant. The strength of interaction was significantly affected by the side chain hydrophobicity of the amino acids. These findings confirm that the nonionic surfactants bind to protein at more than one amino acid residues and a considerable impact on the interactions is caused by the hydrophobic forces. Paz-Andrade et al. [103] determined the enthalpies of interaction of n-CnH2n+2SO4Na (n = 4,6–8,10–12) with poly(L-lysine) hydrobromide, poly(L-arginine) hydrochloride and poly(L-histidine) hydrochloride at 298.15 K. For n > 8, linear relation between the enthalpy of interaction and the C chain length


was observed. The enthalpy of interaction was seen to arise from the surfactant–cationic residue interactions which contributed a major part, and additional contributions from surfactant-apolar amino acid residue interactions were also observed. Bertolotti et al. [104] studied the effect of SDS, CTAB and CTAC on the fluorescence quenching of indolic compounds by aliphatic amino acids. It was seen that the SDS micelles increase the quenching efficiency of glycine, alanine, valine and leucine by less or equal to twofold. On the other hand, CTAB and CTAC micelles reduce the quenching. For methionine and cysteine, both SDS and CTAC reduce the quenching. It was seen that the inhibitory effect is larger for CTAC. From linear Stern–Volmer plots and SDS lifetime measurements, it was concluded that the quenching is of dynamic nature. Cardoso et al. [105] studied the solubilization of amino acids in the presence of cationic reversed micelles. Using different experimental conditions, extraction equilibrium experiments were performed and three amino acids—aspartic acid, phenylalanine and tryptophan—were studied as these differ in their structures. The solute–micelle interfacial interactions and the solute location in the cationic system trioctylmethylammonium chloride (TOMAC)/hexanol/n-heptane were studied by comparing the pH and the initial amino acid concentration in the aqueous phase, and the influence of parameters in the reversed micellar structure, such as surfactant co-surfactant concentration, concentration and ionic strength. Amino acids with the same isoelectric point were seen to be selectively separated by exploring the different interactions which establish with the reversed micellar interface. Singh et al. [90] studied the apparent molar volumes of glycine, alanine, valine and leucine in aqueous solutions of 0.05, 0.5, 1.0 mol kg−1 SDS and 1.0 mol kg−1 CTAB at 298.15 K. The apparent molar volumes of diglycine and triglycine in 1 mol kg−1 SDS and CTAB solutions were also examined. The infinite dilution apparent molar volumes and the standard partial molar volumes of transfer for aqueous SDS and CTAB in the presence of amino acids and peptides were calculated. The ion–ion, ion–polar and hydrophobic–hydrophobic group interactions in terms of partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted. As observed from volume of transfer, ion–ion or ion–hydrophilic group interactions of the amino acids and peptides were found to be stronger with SDS than with CTAB. Sjoegren et al. [106] examined the mutual interaction between peptides and nonionic surfactants by considering the events of surfactants on amphiphilic character and studied the influence of molecular characteristics. The studied amino acids were hydrophobic with phenylalanine or tyrosine residues. The interactions between peptides and surfactants were studied by using binding isotherms, CD spectroscopy, and NMR. Below the CMC, cooperative process of binding of surfactant to peptides was observed, and it was concluded that it is necessary that interaction with nonionic surfactant hydrophobicity of peptide plays an essential role as well. Singh et al. [107] studied the density and molar volumes of glycine, valine and leucine from 0.03 to 0.07 mol kg−1, cetylpyridinium chloride (CPC), and cetylpyridinium bromide (CPB) in aqueous surfactant solution systems at different temperatures. Regression analysis of the data against molality was done to find the solute–solvent interactions and molecular interactions and influence of compounds. It was observed that amino acids with a shorter alkyl chain, such as glycine, had weaker affinity to interact with cationic surfactants, in comparison with the longer alkyl chain amino acids, such as leucine. The greater affinity for molecular interaction with amino acids was shown by CPB. Roy et al. [108] probed the aggregation behaviour of 3 N-acyl amino acid surfactants, Na N-(11-


acrylamidoundecanoyl)- L -serinate (SAUS), Na N-(11-acrylamidoundecanoyl)- L asparaginate (SAUAS) and Na N-(11-acrylamidoundecanoyl)-L-glutaminate (SAUGL) in aqueous solution using surface tension, fluorescence, dynamic light scattering and transmission electron microscopic techniques. The amphiphiles initially formed flexible bilayer structures, which upon increasing the surfactant concentration transformed into closed spherical vesicles. The transmission electron micrographs of the aqueous solutions of the surfactants confirmed the existence of spherical vesicles. Dynamic light scattering measurements were performed to obtain hydrodynamic radii of the vesicles. CD spectra of the amphiphiles indicated formation of chiral helical aggregates in the case of SAUS. Ali et al. [109] studied a number of thermodynamic parameters viz. apparent molar volumes, ϕv , partial molar volumes, ϕ0v , transfer volumes, ϕ0v;trans , Falkenhagen coefficients, A, Jones–Dole coefficients, B, free energies per mole of solute, Δμ0≠ 1 , and per mole of solvent, 0 , molar refraction, R , and limiting molar conductivity, Λ , Δμ0≠ D m by using the experimental 2 measured densities, ρ, viscosities, η, refractive indexes, nD, and specific conductivities, k, data of glycine (0.02–0.10 m) in 0.01 m aqueous SDS, CTAB and Tx-100 solutions at 298.15, 303.15, 308.15 and 313.15 K. The data from these parameters together with fluorescence were interpreted in terms of interactions prevailing in the system. Qiu et al. [110] studied the quaternary ammonium surfactants, [CnH2n+1(CH3)2 N CH2CH2OH]Br (n = 12, 14, 16), at 298.15 K with a micro-calorimeter. Enthalpies involved in transferring amino acids from water to aqueous surfactant solutions were investigated. It has been observed that, at relatively low concentrations of the surfactant solutions, amino acids are still in the water phase and mainly interact with the hydrophilic head groups of the surfactant molecules. With an increase in the concentration of the surfactant solutions, the microenvironment of amino acids changes and the molecules might insert into the micelles and interact with the hydrophobic tail groups of the surfactant molecules. The results were explained in terms of a delicate balance between hydrophobic and hydrophilic groups and the differences in molecular structure of amino acids. Jadav et al. [111] in their study examined different types of amino acid-based cationic surfactants with DNA. Their main focus was to study influence of surfactant head-group geometry on modifying the interaction behaviour of DNA with surfactants. Gel retardation assay, fluorescence spectroscopy, CD and isothermal titration calorimetry at different surfactant/DNA molar ratios were used to assess the cell viability. It was found that the surfactants with bulkier hydrophobic head groups interact more strongly with DNA. Lower cytotoxicity was observed in the presence of more hydrophobic groups adjoining the positive amino charge. The electrostatic and steric effects play an important role in shaping the micelles. Qiu et al. [112] studied the enthalpies of solution and transfer of glycine, L-alanine, L-valine, L-serine and L-threonine from water to aqueous SDS or dodecyltrimethylammonium bromide solutions have been determined at 298.15 K by isothermal calorimetry. Dehydration and ion-ion interactions are important at low concentrations of the surfactant. Hydrophobic– hydrophobic interactions become primary when the amino acid molecules insert into the hydrophobic tail groups of the micelles. Kandpa et al. [113] studied the solution properties of linear alkyl benzene sulphonate (LABS) in water in the presence of amino acids by conductometric approach. The critical aggregation concentration (cac) and critical saturation concentration (csc) were determined. It was observed that at low concentration of surfactant, amino acids interact with hydrophilic head groups of surfactant molecules and form molecular aggregate/micelle-like structure. Nandni et al. [114] studied the effect of several varieties of biomolecules such as amino acids, amino alcohols, dipeptides,


sugars, hydroxy acids and dicarboxylic acids to investigate the surfactant–biomolecule interactions. Nonionic surfactants including triblock polymers (L64, P84) and tritons (Tx-100, Tx-114) were investigated by studying their cloud point. The cloud point of the triblock polymers and tritons showed depression particularly at higher concentrations of additives, as the presence of additives with nonionic surfactant change their clouding behaviour. Both hydrophobicity and molecular geometry played an important role in cloud point variations. Misra et al. [115] considered the apparent molar adiabatic compressibility and apparent molar volume values of glycine, L-alanine, Lvaline, L-leucine, DL-a-amino-n-butyric acid (ABA), diglycine and triglycine in the aqueous surfactant solutions by using sound and density measurements. Calorimetric method was used to determine the heat evolved or absorbed. Polar interactions were observed to be dominant by determining the transfer values of partial molar volume, partial molar adiabatic compressibility and limiting heat of dilution of amino acids and peptides. The zwitterionic centres of the amino acids and peptide bonds of the diglycine and triglycine showed primarily polar interactions. Li et al. [116] studied the hydrophobic or hydrophilic partitioning of various amino acids in aqueous two-phase systems (ATPS). Polyethylene glycol (PEG) and nonionic surfactant (Tx-100 and Tween 80) were used for the study. The effect of hydrophobicity, chain length and their structure influence were taken into consideration to explain various types of interactions. It was seen that the hydrophobic amino acid preferred to the bottom surfactant-rich phase and that the hydrophilic amino acid preferred to partition in the top PEG-rich phase. It was observed that in both the partitioning of amino acids and the partitioning of the small model protein in PEG/nonionic surfactant ATPS, the hydrophobic interactions played a significant part. Ali et al. [117] studied HTAB in aqueous solutions of glycine (Gly) and glycyl–glycine (Gly-Gly) using conductivities, densities and ultrasonic speed measurements. The CMC values of HTAB in the presence of Gly were found to be higher than those in the presence of Gly-Gly. As the zwitterionic nature of Gly and Gly-Gly in aqueous solution form hydrogen bonds with two terminal polar groups to water molecules resulting in the loss of water molecules from the hydrophilic head of HTAB, favouring micellization. Apparent molar volumes, apparent molar volumes at infinite dilution, apparent molar compressibilities of HTAB in the pre- and post-micellar regions and volume change on micellization were also calculated. Less compact micellar structure of HTAB in Gly and Gly-Gly with increase in temperature was ascribed to the increase in volume change on micellization and increase in apparent molar adiabatic compressibility which also suggested their solubilization in the palisade layer of the micelle. Chauhan et al. [118] inspected the densities and sound velocities of an ethoxylated alkyl phenol surfactant in the presence of glycine and leucine. The isentropic compressibilities, apparent molar volumes and apparent molar adiabatic compressions were calculated in order to explain amino acid–surfactant interactions. Micellization behaviour of the surfactant in the presence of glycine and leucine were the main focus to interpret the interactions with these additives. Sharma et al. [119] considered the SDBS and DTAB in water and in aqueous solutions of L-glutamine, L-histidine and L-methionine by measuring their density, viscosity and ultrasonic velocity. Hydrophobic hydration in the presence of additives of these surfactants and their apparent molar adiabatic compression values were determined. Hydration of DTAB-amino acid complex was seen to be observed at low concentration of DTAB as compared to SDBS. Nonionic


surfactant hexadecylpoly[oxyethylene(25)] alcohol, its interaction with amino acids (DL-glycine, DL-alanine, DL-serine, DL-aspartic acid, DL-lysine and DL-leucine) were visualized through volumetric properties using a vibrating tube digital densimeter. Apparent molar volumes, partial molar volumes and volumes of transfer were calculated. The values of volumes of transfer in neutral amino acids (glycine and alanine) and acidic amino acid (aspartic acid) are positive in pre-micellar region and negative in post-micellar region, indicating the hydrophilic–hydrophilic and ion–hydrophilic interactions are prevailing in pre-micellar region while hydrophobic–hydrophobic interactions are leading in post-micellar region [120]. Density, sound velocity and titration calorimetry were used to study the interaction of cationic surfactants: dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) with amino acids (glycine, L-alanine, DL-a-amino-n-butyric acid, Lvaline, L-leucine and peptides glycyl–glycine, glycyl–glycyl–glycine and glycyl–leucine). Ionic–ionic (between Br− of DTAB/TTAB and NH+3 group of amino acids or N+ CH3 group of DTAB/TTAB and COO− group of amino acids), ionic–hydrophobic (between nonpolar parts of amino acids and polar head group of DTAB/TTAB) and hydrophobic–hydrophobic group (between alkyl chain of surfactants and nonpolar parts of amino acids) interactions (Fig. 4) were used to describe the interactions of these amino acids and peptides with DTAB and TTAB using the standard partial molar quantities of transfer from water to aqueous surfactant solutions [121]. Gly, Ala, Phe and Gly-Gly were used as solutes in aqueous TX-100 solution to study the solute–solute and solute–solvent interactions using viscometric and volumetric approaches. Structure-breaking nature of these additives in aqueous TX-100 solution was observed. Stronger solute–solvent and weak solute–solute interactions were attributed to the large positive values of B-coefficients of viscosity than A-coefficients of viscosity for these additives. Gly and Ala showed negative partial molar volumes of transfer as compared to Phe and Gly-Gly which exhibited positive partial molar volumes of transfer. Solute–solute interactions disappeared at infinite dilution, signifying the importance of interactions between solute and solvent [122]. Increasing the concentration of amino acids (glutamine/histidine/methionine) in aqueous solutions of sodium dodecyl benzene sulphonate (SDBS) and

Fig. 4 Schematic representation of alkyl ammonium bromide micelle with amino acids (water molecules are not shown), showing ion–ion/hydrophilic–hydrophilic interactions between Br- of surfactant and NH+3 group of amino acids or N+CH3 group of surfactant and COO− group of amino acids


dodecyltrimethylammonium bromide (DTAB), the CMC was found to decrease in both the surfactants. Nature, concentration, and temperature played an important role in affecting the micellar properties of SDBS and DTAB. Partial destruction of the hydration shell around the alkyl chain of the surfactant monomer due to an increase in amino acid concentration and the ionic heads of surfactant on the addition of amino acid molecules resulted in decreased thickness of the solvation layer [123]. Effect of temperature and organic additives like D-(−)fructose, asparagine and amino acids (glutamic acid and L-arginine) in aqueous solution were investigated on the micellization of cationic surfactant cetyltrimethylammonium chloride (CTAC). CMC values first increased with increasing temperature (up to 298 K) and then showed a reverse trend above 298 K and with organic additives as well [124]. Ali and coworkers [125] used density and ultrasonic speed measurements in binary aqueous solutions of CTAB with amino acids (glycine, L-alanine and L-valine). Apparent molar volume and apparent molar isentropic compression were calculated from these two techniques. Cosphere overlap model was used to interpret the solute–solvent interactions using transfer volumes at infinite dilution from aqueous to aqueous CTAB solutions. Hydrophilic–hydrophilic interactions between Br−/N+−CH3 of CTAB and NH3+/COO− of amino acids favour positive transfer values at infinite dilution due to a decrease in the electrostatic interaction. Hydrophilic–hydrophobic and hydrophobic–hydrophobic interactions between amino acids and CTAB would lead to negative transfer values at infinite dilution due to less hydration. The observed negative values of these three amino acids suggest that hydrophobic–hydrophilic and hydrophobic–hydrophobic interactions are dominant over hydrophilic–hydrophilic. The values of transfer volumes at infinite dilution in 0.0004 m aqueous CTAB solution are less negative than in 0.0012 m aqueous CTAB solution suggesting weaker ion–ion or ion–hydrophilic interactions when concentration of surfactant is above CMC (Fig. 5).

Effect of Amino Acid Ordering on the Assembly of Surfactants Amino acids mostly exist as zwitterions in aqueous solution. These vary in the characteristics of the hydrophobic and hydrophilic groups attached to the α-carbon (Fig. 6). Increase in hydrophobicity in amino acids increases the size of water clathrate surrounding hydrophobic groups, thus increasing molecular associations between amino acids and ionic surfactants [126]. As in the case of SDS with Gly, Ala, Val and Leu, it was observed that the increasing hydrophobicity from Gly to Leu, the water clathrate-like structure increased with increasing hydrophobicity of these amino acids (Leu > Val > Ala > Gly) resulting into a decrease in CMC of SDS with increasing hydrophobicity of these amino acids [127]. With cationic surfactant, HTAB, similar trends were observed, a decrease in CMC in the sequence Gly-Gly-Gly < Gly-Gly < Gly was seen. As the number of carbon atoms increases from Gly to Gly-Gly-Gly, the hydrophobic interaction between the nonpolar groups of the surfactant increases. Lower values of the CMCs of HTAB in pure water than in aqueous Gly/Gly-Gly/ Gly-Gly-Gly were observed which clearly suggest that factors such as the nature of the solvent, its tendency to solvate surfactant ions, favouring the formation of the micelles dominate the effect of the dielectric constant. [128]. Effects of different concentrations of Val and Leu on the micellar behaviour of SDS were studied at different temperatures. It was observed that in 0.01, 0.02, 0.03 Val and 0.01 m Leu, the CMC values of SDS were lower than those in pure water. The presence of Val and Leu in the outer portion of the micelle at low concentrations resulted into a decrease in CMC due to hydrogen bonding with water molecules. Val was found to be more effective in helping micelle formation of SDS than Leu in


Surfactant 1

Surfactant 2

Mixed Surfactant Fig. 5 Schematic representation of mixed micelle formation from two different surfactants

aqueous medium. Breaking up of water structure surrounding the polar head groups were induced, (DS− of SDS which, in turn, interact strongly with DS− through their terminal −NH+ 3 groups) hindering micellization of SDS with increased concentration of Val and Leu [129].

Fig. 6 Structure of amino acid where R is the varying group


SDS micellization in the presence of aqueous Ser and Thr showed a more pronounced decrease in CMC of SDS in the presence of Thr than that of Ser. Larger hydrophilicity of threonine was responsible for the large decrease in CMC of SDS. The determined ΔG0m values were increasingly negative in the order water > Ser >Thr indicating micellization is more favourable in the presence of amino acids [130]. In the presence of amino acids (His, Gln, Met) with SDBS and DTAB, it was observed that the CMC decreases in the order His >Gln > Met in case of SDBS while in case of DTAB, the order is Gln > Met > His. Due to the formation of ion pair between oppositely charged head group of surfactant and amino acids, solubilization takes place more easily. The delayed micellization was more pronounced in case of positively charged histidine with negatively charged head group of SDBS, while micellization with histidine was more favourable in case of positively charged DTAB [123].

Surfactant–Surfactant Interactions Surfactant mixture properties are of immense applied significance particularly in cosmetics and pharmaceuticals, detergents, flotation, coatings, chemical industry and all other fields of industry where surfactants are used. Use of anionic and cationic surfactant mixtures has been evaluated in the following areas: simultaneous washing and softening, analytical chemistry, enhanced oil recovery and pharmaceutical applications [131–133], waste water treatment [134], textile wetting and detergency [135]. Further, the adsorption of surfactants at the solid/liquid interface is fundamentally important in many applications, including detergency, froth flotation, paper manufacturing and pharmaceutical production. Any method that minimizes surfactant adsorption will be a boon so as to reduce surfactant losses and cost. Since anionic–cationic surfactant mixtures greatly reduce the monomer concentration necessary to form mixed micelles [136], adsorption in these systems is of great interest. We know that many natural or synthetic-based self-assemblies and interfaces consist of surfactant mixtures. In biological fluids, many self-assemblies composed of mixed surfactants are present; mixed surfactants find an important role in industrial and pharmaceutical preparations in solubilization, suspension, dispersion etc. [137–139]. Mixed surfactants are preferred over single surfactants in most practical applications. Many detergent preparations nowadays are made up of two or more types of surfactants; attention is paid to add a major component which is usually a conventional pH-insensitive surfactant, sometimes small amounts of pH-sensitive surfactants are added for boosting the performance. There exists a synergism that is mixed system in most cases results in enhanced properties like lowering of critical micelle concentration (CMC) and higher surface activity relative to the individual surfactants. Many studies of different combinations of mixed surfactant systems have been carried out viz. cationic/ cationic [140], nonionic/nonionic [140, 141], anionic/nonionic [142, 143] etc. The solubilization behaviour of different compounds in the mixed micellar solutions has been observed to be better than individual surfactant micelles [144–146]. Micellar stability is directly correlated to Columbic repulsions, and it is believed that detergency is related to micellar stability and that the addition of surfactant of opposite charge is one of the factors enhancing micellar stability. The oppositely charged surfactant addition diminishes the surface charge density of the mixed micelles and hence minimizes the charge repulsion between micelles. The mixed surfactant systems containing anionic and cationic surfactants have attracted much attention because they exhibit spontaneous formation of stable unilamellar vesicles in dilute solutions [147, 148]. The


first to report this in a study on three different cationic–anionic surfactant systems were Kaler et al. [147]. Mixed surfactant system properties are relatively different from than those of single individual surfactant system [149–155]. The two different surfactants in a mixed micelle and their interaction may have synergistic or antagonistic effects on the micellization. The presence of two kinds of surfactants in a solution will make mixed micelles if there are no great electric repulsions or steric obstruction between two surfactants in a micellar state [156, 157]. Many experimental studies have been carried out using a variety of techniques on the micellization of mixed surfactant systems. The ideal solution approach has been used to interpret the properties of mixed micellar system and the adaptation of regular solution theory, where the departure from the ideal is calculated by a single interaction parameter (β), thus has put the foundation of the theoretical study of mixed surfactant solutions [158–161]. The surface or interfacial properties of a surfactant can be improved by adding another surfactant with it which can interact to produce the properties of the mixture that are better than the individual components; this is called synergy and can be predicted from the molecular interactions between the two surfactants to that of the individual surfactants [162, 163]. One of the ways to calculate the interaction in binary mixture and their influence of nature on the interactions can be calculated by the values of their interaction parameter, β [162]. Equations 1 and 2 [162, 163, 168] are used to calculate the interaction parameter for mixed monolayer formation at the aqueous solution/air interface, βσ. X 21 ln α1 C 12 =X 1 C 01 ð1Þ ¼1 ð1−X 1 Þ2 ln ð1−α1 ÞC 12 =ð1−X 1 ÞC 02

σ

β ¼

ln α1 C 12 =X 1 C 01

ð2Þ

ð1−X 1 Þ2

where X1 is the mole fraction of surfactant 1 in the total mixed monolayer; C01, C02 and C12 are the molar concentrations in the solution phases of surfactant 1, surfactant 2 and their mixture, respectively, at the mole fraction α1 of surfactant 1 required to produce a given γ value. Equation 1 is solved numerically for X1, which is then substituted into Eq. 2 to calculate βσ. The value of βM, that is the mixed micelle interaction parameter in an aqueous medium, is calculated from Eqs. 3 and 4 as [162, 166, 169] 2 M M XM ln α1 C M 12 =X 1 C 1 1 M ¼ 1 2 M ln ð1−α1 ÞC M 1−X M 12 = 1−X 1 C 2 1

βM ¼

M M ln α1 C M 12 =X 1 C 1 2 1−X M 1

ð3Þ

ð4Þ

where X1M is the mole fraction of surfactant 1 in the mixed micelle and C1M, C2M and C12M are the CMCs of first and second surfactant and their mixture, respectively, at the mole fraction α1, since free energy of mixing of the system is proportional to the β parameter. Thus, a negative value of β means that the interaction between the two different surfactants is attractive in


nature which is stronger than the attraction between individual ones or in other words the repulsive interactions between different surfactants is weaker than the individual surfactant self-repulsions. A positive value of β means that the attractive interaction of the two different surfactants with each other is weaker than the attractive interaction of the two individual surfactants with themselves or that the self-repulsions between individual surfactants are weaker than between their mixtures. Repulsive interactions are found only in mixtures of hydrocarbon chain and fluorocarbon chain surfactants of the same sign. When the value of the β parameter is close to zero, it indicates almost ideal mixing of the two surfactants. Usually, the interactions, and consequently the values of β parameters, are dominated by electrostatic interaction between the hydrophilic head groups of the two different surfactants [166]. The strength of the interaction between two surfactants in a mixed monolayer at the surface depends on the nature of the surface and the molecular environment, for example, temperature and ionic strength of the solution phase. From the β parameter values in comparison to the individual ones, the existence of synergism can be predicted [162, 163]. The value of βσ(the interaction parameter for mixed monolayer formation at the air/solution interface) is generally not the same as the value of βM (the interaction in mixed micelle formation in the solution phase) for the same two surfactants under the same conditions. Generally, interaction in the mixed monolayer is stronger than that in the mixed micelle, for the same two surfactants under the same conditions. But there are some situations in which there are relatively stronger interactions in the mixed micelle than in the mixed monolayer. The condition for synergism in surface tension reduction effectiveness is that the value of βσ must be more negative than the value of βM (βσ − βM is negative). Since synergism in surface tension reduction effectiveness is the type of synergism that is most sought for practical applications, it is important to know how to maximize that difference. The mixtures of an anionic and a cationic surfactant in aqueous medium have been found to exhibit fundamentally different properties than the solutions of pure ionic surfactant or mixtures of an ionic and a nonionic surfactant [164–172].The prominent property is the large decrease in CMC when in an aqueous solvent two oppositely charged surfactants are mixed [173, 174]. In mixtures of an ionic–nonionic surfactants and nonionic–nonionic surfactants, normally a deviation from ideal behaviour and less synergistic effect is observed. The increasing length of the surfactant tail increases the synergistic effects. Sharma et al. [175] reported the interaction between the alkanediyl-α,ω-type cationic gemini surfactant, [(C16H33N+(CH3)2(CH2)4N+(CH3)2C16H33)2Br−], 16–4–16, and the conventional nonionic surfactant [CH3(CH2)10CH2(OCH2CH2)6OH], C12E6, in aqueous medium. Using a du Nouy tensiometer, the surface tension in aqueous solution at different temperatures, maximum surface excess (Gmax) and minimum area per molecule (Amin) were evaluated from a surface tension vs. log10C plot. Synergism in 16–4–16/C12E6 system at all concentration ratios was seen. Steady-state fluorescence quenching methods at a total surfactant concentration ∼2 mM were done to find the aggregation number and effect on the microenvironment of micelle formation. Kabir-udi-Din et al. [176] studied the mixed micellization and surface properties of cationic gemini surfactant butanediyl-α,ω-bis(dimethylcetylammonium bromide) (G4,16-4-16) with conventional surfactants CPC, Na bis(2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene 10 cetyl ether (Brij56) using conductometric and tensiometric methods. Rubingh, Rosen, Clint and Maeda models were used to obtain the various interaction parameters. Negm et al. [177] studied the surface properties and micellization of cationic and nonionic surfactants dimethyl-, decyl-, tetradecyl- and hexadecylphosphineoxide mixtures using conductance and surface tension measurements. Rubingh, Rosen and Clint models were used to obtain the interaction parameter, minimum area per molecule, mixed micelle


composition, free energies of mixing and activity coefficients. Synergism was found on mixing cationic and nonionic surfactants. Azem et al. [178] studied the interactions between alkanediyl-α,ω-type cationic dimeric surfactant and Tx-100 in aqueous medium. Regular solution theory and Clint’s equations were used to observe the extent of interaction between gemini and conventional surfactant. CMC, aggregation number, micro-polarity, dielectric constant and binding constants were determined using fluorometric technique. Fatma et al. [179] probed and synthesized two cationic ester-bonded cleavable gemini surfactants of d i ff e r e n t h y d r o p h o b i c c h a i n l e n g t h e t h a n e - 1 , 2 - d i y l bi s ( N , N - d i m e t h y l - N alkylammoniumacetoxy)dichloride, CnH2n+1(CH3)2N+(CH2COOCH2)2N+ (CH3)2CnH2n+1.2Cl−(n-E2-n, n = 12,16), having ester linkage in the spacer. Conductivity measurements were employed to calculate the physicochemical properties of the single and binary gemini-conventional mixed micelles of different mole fractions. The conventional surfactants used were DTAC (dodecyltrimethylammonium chloride), CTAC (hexadecyltrimethylammonium chloride), CPC, SDS, SDBS (sodium dodecylbenzene sulphonate), Tx-100 and Brij 58 (polyoxyethylene (20) cetyl ether). Synergistic interactions between the surfactants were analysed by using different models. Akabas et al. [180] studied the interaction between alkanediyl-α-ω-bis (alkyldimethylammonium) dibromide with their conventional monomeric surfactant CTAB using the conductivity and steady-state fluorescence quenching techniques at a certain micellar concentration range and at 303 K. CMC were determined by conductivity and fluorescence techniques. Rubingh’s regular solution theory was used to find the extent of interaction between different mole fraction mixtures. Ideal mixing was observed between these mixtures. Conventional surfactants were seen to play a major role in determining synergistic behaviour. Nagg, micro-polarity, dielectric constant and binding constant were calculated from the fluorescence measurements. The effect of degree of ethoxylation on water solubilization enhancement of pyrene by anionic–nonionic mixed micelles, using sodium lauryl ether sulphate (SLES), and polyoxyethylene (23) lauryl ether (Brij35) mixed micelles were investigated. With increased degree of ethoxylation in SLES, pyrene molar solubilization ratio increased, but no effect was observed with SLES and Brij-35 mixed micelles. It was observed that the experimental CMCs show negative deviation from ideal ones and CMC values in single and mixed micelles were also not affected by the degree of ethoxylation as a result of ion–dipole attractive interactions between the sulphate ion (−SO4−) and the O→CH2 dipole in single (SLES) and mixed (SLES–Brij35) micelles [181]. Surfactants (dodecyltrimethylammoniumchloride, DoTAC; SDS; and zwitterionic d i m e t hy l d od e c y l a m i ne -N - o x i d e , D D A O ) a n d t h e c h e l a t i n g s u r f a c t a n t 2 dodecyldiethylenetriaminepentaacetic acid (4-C12-DTPA) having the same carbon (hydrophobic) chain length were studied using surface tension, pH measurement, and NMR diffusometry. The interaction parameter β for mixed micelle formation were found out to be negative, thus confirming synergism in mixed micelle formation. The negative β in the mixture was found to follow the order DoTAC > DDAO > SDS [182].

Computational Study Quantitative models and simulations are an important tool to understand and give an insight into the various types of interactions and self-assembly processes or to compute quantitatively the energy of adsorption of solute molecules on a given surface. As the quantification of binding properties of ions, surfactants, biopolymers and other macromolecules, their structure and energetics of the self-assembly to nanometre-scale surfaces are often difficult


experimentally and a recurring challenge in molecular simulations [183–185].Computational tools have been used to design and to screen amphiphilic surfactants to stabilize proteins and amphiphilic–amphiphilic systems for their better performance in industrial applications. Computational methods cut the time for screening the suitable and efficient amphiphilic surfactant systems to stabilize the protein [186, 187]. A 3D Lattice Monte Carlo simulation method was used to investigate the micellization of pure gemini surfactants and a mixture of gemini and conventional surfactants. Effects of tail length on CMC and aggregation number for the pure gemini surfactant system were studied, and the simulation results were found to be in excellent agreement with the experimental results. For a mixture of gemini and conventional surfactants, variations in the mixed CMC, interaction parameter and excess Gibbs free energy with composition revealed synergism in micelle formation. Simulation results were compared to estimations made using regular solution theory to determine the applicability of this theory for nonideal mixed surfactant systems. The results indicated that as the chain length of gemini surfactants in mixture is increased, the size parameter remains constant while the interaction and packing parameters increase [188]. Mixed cationic (dodecylamine)/anionic (oleate) surfactants with muscovite (KAl2(Si3Al)O10(OH)2) belong to the group of 2:1 layer silicates and consist of an [AlO6] octahedral sheet sandwiched between two [(Si, Al)O4] tetrahedral sheets were investigated with molecular dynamic simulations [189]. Using hydrophobic theory with Debye–Huckel approximation, surfactant micellization properties such as the CMC and concentration effects were calculated. Experimental data of various ionic surfactant (SDS and cationic dodecyltrimethylammonium chloride, DTAC) types in the presence of salt were also compared with the theoretical ones suggesting good approximation [190].

Conclusions Low-molecular-weight surfactant mixtures and proteins are extensively studied due to their major industrial applications in food processing, pharmacology or cosmetics. The formation and the stabilization of foams and emulsions depend on the interfacial properties of these mixtures, their surface activity, rheological behaviour and adsorption dynamics, which can be drastically different as compared to those of the single components. The structure and properties of proteins including their solubility, denaturation and dissociation into subunits and the activity of enzymes are largely affected by salt solutions. These types of systems can be used in pharmaceutical industry to stabilize proteins and to delay protein aggregation. Surfactant–biomolecule combinations can be used in the design and selection of systems for practical purposes.

References 1. Millero, F. J., Surdo, A. L., & Shin, C. (1978). The apparent molal volumes and adiabatic compressibilities of aqueous amino acids at 25.degree.C. The Journal of Physical Chemistry, 82, 784–792. 2. Timasheff, S. N., & Fasman, G. D. (Eds.) (1969). Structure and stability of biological macromolecules (pp. 65–213). New York:Marcel Dekker. 3. Dill, K. A. (1990). Dominant forces in protein folding. Biochemistry, 29, 7133–7155. 4. Likhodi, O., & Chalikian, T. V. (1999). Partial molar volumes and adiabatic compressibilities of a series of aliphatic amino acids and oligoglycines in D2O. Journal of the American Chemical Society, 121, 1156– 1163.

Appl Biochem Biotechnol 5. Hvidt, A., & Westh, P. (1998). Different views on the stability of protein conformations and hydrophobic effects. Journal of Solution Chemistry, 27, 395–402. 6. Roharkar, P. G., & Aswar, A. S. (2002). Apparent molar volume and apparent molar compressibility of glycine in aqueous vanadyl sulphate solutions at 298.15, 303.15 and 308.15 K. Indian Journal of Chemistry, 41A, 312–315. 7. Tsurko, E. N., Neueder, R., & Kunz, W. (2007). Water activity and osmotic coefficients in solutions of glycine, glutamic acid, histidine and their salts at 298.15 K and 310.15 K. Journal of Solution Chemistry, 36, 651–672. 8. Cohn, E. J., & Edsall, T. J. (Eds.) (1965). Proteins amino acids and peptides (pp. 75–115). New York: Hafner. 9. Zhao, C., Ma, P., & Li, J. (2005). Partial molar volumes and viscosity B-coefficients of arginine in aqueous glucose, sucrose and l-ascorbic acid solutions at T = 298.15 K. The Journal of Chemical Thermodynamics, 37, 37–42. 10. Lehninger, A. L. (2005). Principles of biochemistry 4th edn. (pp. 75–106), New York: W.H. Freeman. 11. Lilley, T. H. (1988). In M. N. Jones (Ed.), Biochemical thermodynamics. Amsterdam: Elsevier. 12. Hedwig, G. R., & Hoiland, H. (1994). Thermodynamic properties of peptide solutions. Part 11. Partial molar isentropic pressure coefficients in aqueous solutions of some tripeptides that model protein sidechains. Biophysical Chemistry, 49, 175–181. 13. Berg, J. M., Tymoczko, J. L., Stryer, L. (2007). Biochemistry, 6th edn., New York: W. H. Freeman and Company. 14. Chi, E. Y., Weickmann, J., Carpenter, J. F., Manning, M. C., & Randolph, T. W. (2005). Heterogeneous nucleation-controlled particulate formation of recombinant human platelet-activating factor acetylhydrolase in pharmaceutical formulation. Journal of Pharmaceutical Sciences, 94, 256–274. 15. Jones, L. S., Kaufmann, A., & Middaugh, C. R. (2005). Silicone oil induced aggregation of proteins. Journal of Pharmaceutical Sciences, 94, 918–927. 16. Liu, J. F., Yang, J., Yang, S. Z., Ye, R. Q., & Mu, B. Z. (2012). Effects of different amino acids in culture media on surfactin variants produced by bacillus subtilis TD7. Applied Biochemistry and Biotechnology, 166, 2091–2100. 17. Li, W., Fedosov, S., Tan, T., Xu, X., & Guo, Z. (2014). Naturally occurring alkaline amino acids function as efficient catalysts on knoevenagel condensation at physiological pH: a mechanistic elucidation. Applied Biochemistry and Biotechnology. doi:10.1007/s12010-014-0840-3. 18. Zhou, M. F., Yuan, X. Z., Zhong, H., Liu, Z. F., Li, H., Jiang, L. L., & Zeng, G. M. (2011). Effect of biosurfactants on laccase production and phenol biodegradation in solid-state fermentation. Applied Biochemistry and Biotechnology, 164, 103–114. 19. Srila, W., & Yamabhai, M. (2013). Identification of amino acid residues responsible for the binding to antiFLAG™ M2 antibody using a phage display combinatorial peptide library. Applied Biochemistry and Biotechnology. doi:10.1007/s12010-013-0326-8. 20. Lindman, B., Thalberg, K., Goddard, E. D., & Ananthapadmanabhan, K. P. (Eds.) (1993). Interactions of surfactants with polymers and proteins. Boca Raton FL:CRC. 21. Cserhati, T. (1995). Alkyl ethoxylated and alkylphenol ethoxylated nonionic surfactants: interaction with bioactive compounds and biological effects. Environmental Health Perspectives, 103, 358–364. 22. Ananthapadmanbhan, K. P. (1993). In E. D. Goddard, & K. P. Ananthapadmanbhan (Eds.), Protein– surfactant interactions, chap. 8, interactions of surfactants with polymers and proteins. London: CRC Press. 23. Zhao, X., Chen, J., Lu, Z., Ling, X., Deng, P., Zhu, Q., & Du, F. (2011). Analysis of the amino acids of soy globulins by AOT reverse micelles and aqueous buffer. Applied Biochemistry and Biotechnology, 165, 802–813. 24. Jones, M. N., & Brass, A. (1991). In E. Dickenson (Ed.), Food polymers, gels and colloids. Cambridge: Cambridge University Press. 25. Jones, M. N., & Manley, P. (1980). Interaction between lysozyme and n-alkyl sulphates in aqueous solution. Journal of the Chemical Society, Faraday Transactions, 1(76), 654–664. 26. Fukushima, K., Murata, Y., Nishikido, N., Sugihara, G., & Tanaka, M. (1981). The binding of sodium dodecyl sulfate to lysozyme in aqueous solutions. Bulletin of the Chemical Society of Japan, 54, 3122–3127. 27. Fukushima, K., Murata, Y., Sugihara, G., & Tanaka, M. (1982). The binding of sodium dodecyl sulfate to lysozyme in aqueous solutions. II. The effect of added NaCl. Bulletin of the Chemical Society of Japan, 55, 1376–1378. 28. Volynskaya, A. V., Murasheva, S. A., Skripkin, A. Y., Shishikova, A. V., & Goldanskii, V. I. (1989). Use of tritium labeling for studying conformational-changes of proteins in solution. Molecular Biology, 23, 265–272. 29. Subramanian, M., Sheshadri, B. S., & Venkatappa, M. P. (1986). Interaction of proteins with detergents: binding of cationic detergents with lysozyme. Journal of Biosciences, 10, 359–371. 30. Nishiyama, H., & Maeda, H. (1992). Reduced lysozyme in solution and its interaction with non-ionic surfactants. Biophysical Chemistry, 44, 199–208. 31. Bakshi, M. S., Kaur, G., Thakur, P., Banipal, T. S., Possmayer, F., & Petersen, N. O. (2007). Surfactant selective synthesis of gold nanowires by using a DPPC-surfactant mixture as a capping agent at ambient conditions. Journal of Physical Chemistry C, 111, 5932–5940.

Appl Biochem Biotechnol 32. Bakshi, M. S., Singh, K., & Singh, J. (2006). Characterization of mixed micelles of cationic twin tail surfactants with phospholipids using fluorescence spectroscopy. Journal of Colloid and Interface Science, 297, 284–291. 33. Bakshi, M. S., Singh, J., & Kaur, G. (2005). Mixed micelles of monomeric and dimeric cationic surfactants with phospholipids: effect of hydrophobic interactions. Chemistry and Physics of Lipids, 138, 81–92. 34. Ali, A., & Ansari, N. A. (2010). Studies on the effect of amino acids/peptide on the micellization of SDS at different temperature. Journal of Surfactants and Detergents, 13, 441–449. 35. Wustneck, R., Wetzel, R., Buder, E., & Hermel, H. (1998). The modification of the triple helical structure of gelatin in aqueous solution I. The influence of anionic surfactants, pH-value, and temperature. Colloid and Polymer Science, 266, 1061–1067. 36. Chen, J., & Dickinson, E. (1995). Protein/surfactant interfacial interactions part 1. Flocculation of emulsions containing mixed protein + surfactant. Colloids and Surfaces, A: Physicochemical and Engineering Aspects, 100, 255–265. 37. Gumpen, S., Hegg, P. O., & Martens, H. (1979). Thermal stability of fatty acid-serum albumin complexes studied by differential scanning calorimetry. Biochimica et Biophysica Acta, 574, 189–196. 38. Waninge, R., Paulsson, M., Nylander, T., Ninham, B., & Sellers, P. (1998). Binding of sodium dodecyl sulfate and dodecyltrimethyl ammonium chloride to β-lactoglobulin: a calorimetric study. International Dairy Journal, 8, 141–148. 39. Nozaki, Y., Reynolds, J. A., & Tanford, C. (1974). The interaction of a cationic detergent with bovine serum albumin and other proteins. The Journal of Biological Chemistry, 249, 4452–4459. 40. Moore, P. N., Puvvada, S., & Blankschtein, D. (2003). Role of the surfactant polar head structure in protein −surfactant complexation: zein protein solubilization by SDS and by SDS/C12En surfactant solutions. Langmuir, 19, 1009–1016. 41. Ruiz-Peña, M., Oropesa-Nuñez, R., Pons, T., Louro, S. R. W., & Pérez-Gramatges, A. (2010). Physicochemical studies of molecular interactions between non-ionic surfactants and bovine serum albumin. Colloids and Surfaces. B, Biointerfaces, 75, 282–289. 42. Von Hippel, P. H., & Schleich, T. (1969). Ion effects on the solution structure of biological macromolecules. Accounts of Chemical Research, 2, 257–265. 43. Jencks, W. P. (1969). Catalysis in chemistry and enzymology. New York:McGraw- Hill. 44. Khoshkbarchi, M. K., & Vera, J. H. (1996). A simplified perturbed hard-sphere model for the activity coefficients of amino acids and peptides in aqueous solutions. Industrial and Engineering Chemistry Research, 35(1996), 4319–4327. 45. Natarajan, M., Wadi, R. K., & Gaur, H. C. (1990). Apparent molar volumes and viscosities of some.alpha.and.alpha.,.omega.-amino acids in aqueous ammonium chloride solutions at 298.15 K. Journal of Chemical & Engineering Data, 35, 87–93. 46. Yan, Z. N., Wang, J. J., & Lu, J. S. (2002). Viscosity behavior of some alpha-amino acids and their groups in water-sodium acetate mixtures. Biophysical Chemistry, 99, 199–207. 47. Wadi, R. K., & Ramasami, P. (1997). Partial molal volumes and adiabatic compressibilities of transfer of glycine and DL-alanine from water to aqueous sodium sulfate at 288.15, 298.15 and 308.15 K. Journal of the Chemical Society, Faraday Transactions, 93, 243–247. 48. Banipal, T. S., Kaur, D., & Banipal, P. K. (2004). Apparent molar volumes and viscosities of some amino acids in aqueous sodium acetate solutions at 298.15 K. Journal of Chemical & Engineering Data, 49, 1236–1246. 49. Wang, X., Xu, L., Lin, R. S., & Sun, D. Z. (2004). Dilution enthalpies of glycine in aqueous potassium chloride solution. Acta Chimica Sinica, 62, 1405–1408. 50. Belibagli, K. B., & Ayranci, E. (1990). Viscosities and apparent molar volumes of some amino acids in water and in 6 M guanidine hydrochloride at 25 °C. Journal of Solution Chemistry, 19, 867–882. 51. Roy, M. N., Sinha, B., Dakua, V. K., & Sinha, A. (2006). Electrical conductances of some ammonium and tetraalkylammonium halides in aqueous binary mixtures of 1,4-dioxane at 298.15 K. Pakistan Journal of Scientific and Industrial Research, 49, 153–159. 52. Blanco, L. H., & Vargas, E. F. (2006). Apparent molar volumes of symmetric and asymmetric tetraalkylammonium salts in dilute aqueous solutions. Journal of Solution Chemistry, 35, 21–28. 53. Sułkowska, A., Bojko, B., Ro’wnicka, J., Pentak, D., & Sułkowski, W. (2003). Effect of urea on serum albumin complex with antithyroid drugs: fluorescence study. Journal of Molecular Structure, 651, 237–243. 54. De, S., Girigoswami, A., & Das, S. (2005). Fluorescence probing of albumin-surfactant interaction. Journal of Colloid and Interface Science, 285, 562–573. 55. Kelley, D., & McClements, D. J. (2003). Interactions of bovine serum albumin with ionic surfactants in aqueous solutions. Food Hydrocolloids, 17, 73–85. 56. Vasilescu, M., Angelescu, D., Almgren, M., & Valstar, A. (1999). Interactions of globular proteins with surfactants studied with fluorescence probe methods. Langmuir, 15, 2635–2643.

Appl Biochem Biotechnol 57. .Kamat, B. P., & Seetharamappa, J. (2004). In vitro study on the interaction of mechanism of tricyclic compounds with bovine serum albumin. Journal of Pharmaceutical and Biomedical Analysis, 35, 655–664. 58. Bai, H. Y., Liu, X. Q., Zhang, Z. L., & Dong, S. J. (2004). In situ circular dichroic electrochemical study of bilirubin and bovine serum albumin complex. Spectrochimica Acta A, 60, 155–160. 59. Farruggia, B., Nerli, B., Nuci, H. D., Rigatusso, R., & Pico, G. (1999). Thermal features of the bovine serum albumin unfolding by polyethylene glycols. International Journal of Biological Macromolecules, 26, 23–33. 60. Watanabe, S., & Sato, T. (1996). Effects of free fatty acids on the binding of bovine and human serum albumin with steroid hormones. Biochimica et Biophysica Acta, 1289(1996), 385–396. 61. Peters Jr., T. (1985). Serum albumin. Advances in Protein Chemistry, 37, 161–245. 62. He, X. M., & Carter, D. C. (1992). Atomic structure and chemistry of human serum albumin. Nature, 358, 209–215. 63. Sulkowska, A. (2002). Interaction of drugs with bovine and human serum albumin. Journal of Molecular Structure, 614, 227–232. 64. Miller, R., Fainerman, V. B., Makievski, A. V., Krägel, J., Grigoriev, D. O., Kazakov, V. N., & Sinyachenko, O. V. (2000). Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface. Advances in Colloid and Interface Science, 86, 39–82. 65. Griffiths, P. C., Cheung, A. Y. F., Jenkins, R. L., Howe, A. M., Pitt, A. R., Heenan, R. K., & King, S. M. (2004). Interaction between a partially fluorinated alkyl sulfate and gelatin in aqueous solution. Langmuir, 20, 1161–1167. 66. Sun, C. X., Yang, J. H., Wu, X., Huang, X. R., Wang, F., & Liu, S. F. (2005). Unfolding and refolding of bovine serum albumin induced by cetylpyridinium bromide. Biophysical Journal, 88, 3518–3524. 67. Deep, S., & Ahluwalia, J. C. (2001). Interaction of bovine serum albumin with anionic surfactants. Physical Chemistry Chemical Physics, 3, 4583–4591. 68. Tribout, M., Paredes, S., Gonza’lez-Manãs, J. M., & Goñi, F. M. (1991). Binding of triton X-100 to bovine serum albumin as studied by surface tension measurements. Journal of Biochemical and Biophysical Methods, 22, 129–133. 69. Sabin, J., Prieto, G., Gonzalez-Perez, A., Ruso, J. M., & Sarmiento, F. (2006). Effects of fluorinated and hydrogenated surfactants on human serum albumin at different pHs. Biomacromolecules, 7, 176–182. 70. Berglund, K. D., Przybycien, T. M., & Tilton, R. D. (2003). Coadsorption of sodium dodecyl sulfate with hydrophobically modified nonionic cellulose polymers. 1. Role of polymer hydrophobic modification. Langmuir, 19, 2705–2713. 71. Few, A. V., Ottewill, R. H., & Parreira, H. C. (1995). The interaction between bovine plasma albumin and dodecyltrimethylammonium bromide. Biochimica et Biophysica Acta, 18, 136–137. 72. Takeda, K., Moriyama, Y., Hachiya, K. (2006). Protein interactions with ionic surfactants part I: binding and induced conformational changes^. In Encyclopedia of surface and colloid science, 2nd ed.; Somasundaran, P., & Hubbard, A. (Eds.); London: Taylor and Francis. 73. Ding, Y., Shu, Y., Ge, L., & Guo, R. (2007). The effect of sodium dodecyl sulfate on the conformation of bovine serum albumin. Colloids and Surfaces, A: Physicochemical and Engineering Aspects, 298, 163–169. 74. Lad, M. D., Ledger, V. M., Briggs, B., Frazier, R. A., & Green, R. J. (2003). Analysis of the SDS−lysozyme binding isotherm. Langmuir, 19, 5098–5103. 75. Mackie, A. R., Gunning, A. P., Ridout, M. J., Wilde, P. J., & Morris, V. J. (2001). Orogenic displacement in mixed β-lactoglobulin/β-casein films at the air/water interface. Langmuir, 17, 6593–6598. 76. Miller, R., Fainerman, V. B., Leser, M. E., & Michel, M. (2004). Surface tension of mixed non-ionic surfactant/protein solutions: comparison of a simple theoretical model with experiments. Colloids and Surfaces, A: Physicochemical and Engineering Aspects, 233, 39–42. 77. Li, Y., Wang, X., & Wang, Y. (2006). Comparative studies on interactions of bovine serum albumin with cationic gemini and single-chain surfactants. The Journal of Physical Chemistry. B, 110, 8499–8505. 78. Pi, Y., Shang, Y., Peng, C., Liu, H., & Hu, Y. (2006). Interactions between bovine serum albumin and gemini surfactant alkanediyl-alpha, omega-bis(dimethyldodecyl-ammonium bromide). Biopolymers, 83, 243–249. 79. Wu, D., Xu, G., Feng, Y., & Li, Y. (2007). Aggregation behaviors of gelatin with cationic gemini surfactant at air/water interface. International Journal of Biological Macromolecules, 40, 345–350. 80. Wu, D., Xu, G., Sun, Y., Zhang, H., Mao, H., & Feng, Y. (2007). Interaction between proteins and cationic gemini surfactant. Biomacromolecules, 8, 708–712. 81. Gull, N., Sen, P., Khan, R. H., & Kabir-ud-Din, (2009). Spectroscopic studies on the comparative interaction of cationic single-chain and gemini surfactants with human serum albumin. Journal of Biochemistry, 145, 67-77. 82. Zana, R., & Xia, J. (Eds.) (2003). Gemini surfactants. New York:Marcel Dekker.

Appl Biochem Biotechnol 83. Zana, R. (2002). Dimeric and oligomeric surfactants. Behavior at interfaces and in aqueous solution: a review. Advances in Colloid and Interface Science, 97, 205–253. 84. Siddiqui, U. S., Ghosh, G., & Kabir-ud-Din (2006). Dynamic light scattering studies of additive effects on the microstructure of aqueous gemini micelles. Langmuir, 22(2006), 9874–9878. 85. Wettig, S. D., Verrall, R. E., & Foldvari, M. (2008). Gemini surfactants: a new family of building blocks for non-viral gene delivery systems. Current Gene Therapy, 8, 9–23. 86. Moulik, S., Dutta, P., Chattoraj, D. K., & Moulik, S. P. (1998). Biopolymer–surfactant interactions: 5: Equilibrium studies on the binding of cetyltrimethyl ammonium bromide and sodium dodecyl sulfate with bovine serum albumin, β-lactoglobulin, hemoglobin, gelatin, lysozyme and deoxyribonucleic acid. Colloids and Surfaces, B: Biointerfaces, 11, 1-8. 87. Lu, R. C., Cao, A. N., Lai, L. H., Zhu, B. Y., Zhao, G. X., & Xiao, J. X. (2005). Interaction between bovine serum albumin and equimolarly mixed cationic-anionic surfactants decyltriethylammonium bromidesodium decyl sulfonate. Colloids and Surfaces B: Biointerfaces, 41, 139–143. 88. Ali, A., Tariq, M., Patel, R., & Ittoo, F. A. (2008). Interaction of glycine with cationic, anionic, and nonionic surfactants at different temperatures: a volumetric, viscometric, refractive index, conductometric, and fluorescence probe study. Colloid and Polymer Science, 286, 183–190. 89. Arutyunyan, N. G., Arutyunyan, L. R., Grigoryan, V. V., & Arutyunyan, R. S. (2008). Effect of aminoacids on the critical micellization concentration of different surfactants. Colloid Journal, 70, 666–668. 90. Singh, S. K., Kundu, A., & Kishore, N. (2004). Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T=298.15 K: a volumetric approach. The Journal of Chemical Thermodynamics, 36, 7–16. 91. Ali, A., Sabir, S., Shahjahan, & Hyder, S. (2007). Volumetric and refractive index behaviour of α-amino acids in aqueous CTAB at different temperatures. Acta Physico-Chimica Sinica, 23, 1007–1012. 92. Jones, M. N. (1975). Biological interfaces. Amsterdam:Elsevier. 93. Helenius, A., & Simons, K. (1975). Solubilization of membranes by detergents. Biochimica et Biophysica Acta, 415, 29–79. 94. Jing, P., Kaneta, T., & Imasaka, T. (2005). On-line concentration of a protein using denaturation by sodium dodecyl sulfate. Analytical Sciences, 21, 37–42. 95. Zardeneta, G., & Horowitz, P. M. (1994). Protein refolding at high concentrations using detergent/ phospholipid mixtures. Analytical Biochemistry, 218, 392–398. 96. Renthal, R., Hannapel, C., Nguyen, A. S., & Haas, P. (1990). Regeneration of bacteriorhodopsin in mixed micelles. Biochimica et Biophysica Acta, 1030, 176–181. 97. Tandon, S., & Horowitz, P. M. (1988). The effects of lauryl maltoside on the reactivation of several enzymes after treatment with guanidinium chloride. Biochimica et Biophysica Acta, 955, 19–25. 98. Aerts, T., Clauwaert, J., Haezebrouck, P., & Paeters, E. (1997). Interaction of detergents with bovine lens alpha-crystallin: evidence for an oligomeric structure based on amphiphilic interactions. European Biophysics Journal, 25, 445–454. 99. Dickinson, E., & Hong, S. K. (1994). Surface coverage of beta-lactoglobulin at the oil-water interface: influence of protein heat treatment and various emulsifiers. Journal of Agricultural and Food Chemistry, 42, 1602–1606. 100. Mao, Y., Wei, W., Zhang, J., & Zhang, S. (2002). Interaction process between ionic surfactant and protein probed by series piezoelectric quartz crystal technique. Journal of Biochemical and Biophysical Methods, 52, 19–29. 101. Singh, S. K., & Kishore, N. (2004). Volumetric properties of amino acids and hen-egg white lysozyme in aqueous triton X-100 at 298.15 K. Journal of Solution Chemistry, 33, 1411–1427. 102. Fargacs, E. (1993). Interaction of amino acids with the nonionic surfactant nonylphenyl hexaethoxylate. Biochemistry and Molecular Biology International, 30, 1–11. 103. Paz-Andrade, M. I., Jones, M. N., & Skinner, H. A. (1978). Enthalpy of interaction between some cationic polypeptides and n-alkyl sulphates in aqueous solution. Journal of the Chemical Society, Faraday Transactions, 1(74), 2923–2929. 104. Bertolotti, S. G., Bohorquez, M., Cosa, J. J., Garcia, N. A., & Previtali, C. M. (1987). Micellar effect on the fluorescence quenching of indolic compounds by amino acids. Photochemistry and Photobiology, 46, 331–335. 105. Cardoso, M. M., Barradas, M. J., Kroner, K. H., & Crespo, J. G. (1999). Amino acid solubilization in cationic reversed micelles: factors affecting amino acid and water transfer. Journal of Chemical Technology and Biotechnology, 74, 801–811. 106. Sjoegren, H., Ericsson, C. A., Evenaes, J., & Ulvenlund, S. (2005). Interactions between charged polypeptides and nonionic surfactants. Biophysical Journal, 89, 4219–4233. 107. Singh, M. (2005). Studies of molecular interactions of α-amino acids in aqueous and cationic surfactant systems investigated from their densities and apparent molal volumes at 283.15, 288.15 and 293.15 K. Pakistan Journal of Scientific and Industrial Research, 48, 303–311.

Appl Biochem Biotechnol 108. Roy, S., & Dey, J. (2007). Effect of hydrogen-bonding interactions on the self-assembly formation of sodium N-(11-acrylamidoundecanoyl)-L-serinate, L-asparaginate, and L-glutaminate in aqueous solution. Journal of Colloid and Interface Science, 307, 229–234. 109. Ali, A., Khan, S., Hyder, S., & Tariq, M. (2007). Interactions of some α-amino acids with tetra–n– alkylammonium bromides at different temperatures. The Journal of Chemical Thermodynamics, 39, 613– 620. 110. Qiu, X., Fang, W., Lei, Q., & Lin, R. (2008). Enthalpies of transfer of amino acids from water to aqueous cationic surfactants solutions at 298.15 K. Journal of Chemical & Engineering Data, 53, 942–945. 111. Jadhav, V., Maiti, S., Dasgupta, A., Das, P. K., Dias, R. S., Miguel, M. G., & Lindman, B. (2008). Effect of the head-group geometry of amino acid-based cationic surfactants on interaction with plasmid DNA. Biomacromolecules, 9, 1852–1859. 112. Qiu, X., Lei, Q., Fang, W., & Lin, R. (2008). A calorimetric study on interactions of amino acids with sodium dodecylsulfate and dodecyltrimethylammonium bromide in aqueous solutions at 298.15 K. Thermochimica Acta, 478, 54–56. 113. Kandpal, N. D., Joshi, S. K., Singh, R., & Pandey, K. (2010). Thermodynamic parameters of micellization and transfer of amino acids from water to aqueous linear alkyl benzene sulphonate. Journal of the Indian Chemical Society, 87, 487–493. 114. Nandni, D., Vohra, K. K., Mahajan, R. K., & Kumar, R. (2012). Phase separation of triblock polymers and tritons in the presence of biomolecules. Journal of Solution Chemistry, 41, 702–714. 115. Misra, P. P., & Kishore, N. (2012). Volumetric and calorimetric investigations of molecular interactions in some amino acids and peptides in the combined presence of surfactants and glycine betaine. The Journal of Chemical Thermodynamics, 54, 453–463. 116. Liu, Y., Wu, Z., Zhang, Y., & Yuan, H. (2012). Partitioning of biomolecules in aqueous twophase systems of polyethylene glycol and nonionic surfactant. Biochemical Engineering Journal, 69, 93–99. 117. Ali, A., Tasneem, S., Bidhuri, P., Bhushan, V., & Malik, N. A. (2012). Critical micelle concentration and self-aggregation of hexadecyltrimethylammonium bromide in aqueous glycine and glycylglycine solutions at different temperatures. Russian Journal of Physical Chemistry A, 86, 1923–1929. 118. Chauhan, S., Sharma, K., Kumar, K., & Kumar, G. (2014). A comparative study of micellization behavior of an ethoxylated alkylphenol in aqueous solutions of glycine and leucine. Journal of Surfactants and Detergents, 17, 161–168. 119. Sharma, K., & Chauhan, S. (2014). Apparent molar volume, compressibility and viscometric studies of sodium dodecyl benzene sulfonate (SDBS) and dodecyltrimethylammonium bromide (DTAB) in aqueous amino acid solutions: a thermo-acoustic approach. Thermochimica Acta, 578, 15–27. 120. Harutyunyana, N. G., Harutyunyana, L. R., & Harutyunyan, R. S. (2010). Volumetric properties of amino acids in aqueous solution of nonionic surfactant. Thermochimica Acta, 498, 124–127. 121. Talele, P., & Kishore, N. (2014). Thermodynamics of the interactions of some amino acids and peptides with dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide. The Journal of Chemical Thermodynamics, 70, 182–189. 122. Ali, A., Shahjahan, Malik, N. A., Uzair, S., & Bhushan, V. (2015). Physico-chemical studies of glycine, lalanine, l-phenylalanine and glycylglycine in aqueous Triton X-100 at different temperatures. Tenside, Surfactants, Detergents, 52, 1–8. 123. Chauhan, S., & Sharma, K. (2014). Effect of temperature and additives on the critical micelle concentration and thermodynamics of micelle formation of sodium dodecyl benzene sulfonate and dodecyltrimethylammonium bromide in aqueous solution: a conductometric study. The Journal of Chemical Thermodynamics, 71, 205–211. 124. Alam, M. S., Siddiq, A. M., Mythili, V., Priyadharshini, M., Kamely, N., & Mandal, A. B. (2014). Effect of organic additives and temperature on the micellization of cationic surfactant cetyltrimethylammonium chloride: evaluation of thermodynamics. Journal of Molecular Liquids, 199, 511–517. 125. Ali, A., Bhushan, V., & Bidhuri, P. (2013). Volumetric study of α-amino acids and their group contributions in aqueous solutions of cetyltrimethylammonium bromide at different temperatures. Journal of Molecular Liquids, 177, 209–214. 126. Wen, W. Y., & Saito, S. (1964). Apparent and partial molal volumes of five symmetrical tetraalkylammonium bromides in aqueous solutions. The Journal of Physical Chemistry, 68, 2639–2644. 127. Chauhan, S., Rana, D. S., Akash, Rana, K., Chauhan, M. S., & Umar, A. (2012). Temperature-dependant volumetric and compressibility studies of drug-surfactant interactions in dimethylsulfoxide (DMSO) solutions. Advanced Science Letters, 5, 1–4. 128. Ali, A., Malik, N. A., Uzair, S., Ali, M., & Ahmad, M. F. (2014). Hexadecyltrimethylammonium bromide micellization in glycine, diglycine, and triglycine aqueous solutions as a function of surfactant concentration and temperatures. Russian Journal of Physical Chemistry A, 88, 1053–1061.

Appl Biochem Biotechnol 129. Ali, A., Malik, N. A., Uzair, S., & Ali, M. (2014). Conductometric and fluorometric studies of sodium dodecyl sulphate in aqueous solution and in the presence of amino acids. Molecular Physics, 112, 2681–2693. 130. Ali, A., Bhushan, V., Malik, N. A., & Behera, K. (2013). Study of mixed micellar aqueous solutions of sodium dodecyl sulfate and amino acids. Colloid Journal, 75, 357–365. 131. Stellner, K. L., Amante, J. C., Scamehorn, J. F., & Harwell, J. H. (1987). Precipitation phenomena in mixtures of anionic and cationic surfactants in aqueous solutions. Journal of Colloid and Interface Science, 123, 186. 132. Holland, P. M., & Rubingh, D. M. (1992). Mixed surfactant systems: an overview In: P.M. Holland, D. N. Rubingh, (Eds.), Mixed surfactant systems, ACS Symposium Series No. 501, (pp. 2). Washington. 133. Attwood, D., & Florence, A. T. (1983). Surfactant systems, their chemistry, pharmacy and biology. New York:Chapman & Hall. 134. USSR Patent 1,028,605 (1983). Chem. Abstr. 100, 12144w. 135. Schwuger, M. J. (1971). Properties of sub-stoichiometric mixtures of anionic and cationic surfactants in water. Kolloid Zeitschrift & Zeitschrift fur Polymere, 243, 129–135. 136. Scamehorn, J. F. (Ed.) (1986). An overview of phenomena involving surfactant mixtures. In: Phenomena in mixed surfactant systems (pp. 20). ACS Symposium Series. 137. Moulik, S. P., Haque, M. E., Jana, P. K., & Das, R. (1996). Micellar properties of cationic surfactants in pure and mixed states. The Journal of Physical Chemistry, 100, 701–708. 138. Sharma, K. S., Patil, S. R., & Rakshit, A. K. (2004). Self-aggregation of a cationic−nonionic surfactant mixture in aqueous media: tensiometric, conductometric, density, light scattering, potentiometric, and fluorometric studies. The Journal of Physical Chemistry. B, 108, 12804–12812. 139. Jana, P. K., & Moulik, S. P. (1991). Interaction of bile salts with hexadecyltrimethylammonium bromide and sodium dodecyl sulfate. The Journal of Physical Chemistry, 95, 9525–9532. 140. Haque, M. E., Das, A. R., Rakshit, A. K., & Moulik, S. P. (1996). Properties of mixed micelles of binary surfactant combinations. Langmuir, 12, 4084–4089. 141. Sulthana, S. B., Rao, P. V. C., Bhat, S. G. T., Nakano, T. Y., Sugihara, G., & Rakshit, A. K. (2000). Solution properties of nonionic surfactants and their mixtures: polyoxyethylene (10) alkyl ether [CnE10] and MEGA-10. Langmuir, 16, 980–987. 142. Castaldi, M., Costantino, L., Ortona, O., Paduano, L., & Vitagliano, V. (1998). Mutual diffusion measurements in a ternary system: ionic surfactant−nonionic surfactant−water at 25 °C. Langmuir, 14, 5994–5998. 143. Sulthana, S. B., Rao, P. V. C., Bhat, S. G. T., & Rakshit, A. K. (1998). Interfacial and thermodynamic properties of SDBS−C12E10 mixed micelles in aqueous media: effect of additives. The Journal of Physical Chemistry. B, 102, 9653–9660. 144. Bury, R., Treiner, C., Chevalet, J., & Makayssi, A. (1991). Peculiar solubilization thermodynamics of pentan-1-ol in mixed surfactant solutions of benzyldimethyltetradecylammonium chloride and trimethyltetradecylammonium chloride: a calorimetric investigation. Analytica Chimica Acta, 251, 69–77. 145. Makayssi, A., Bury, R., & Treiner, C. (1994). Thermodynamics of micellar solubilization for 1-pentanol in weakly interacting binary cationic surfactant mixtures of 25 .degree.C. Langmuir, 10, 1359–1365. 146. Jacobson, A. M., & Crars, F. (1991). Multicomponent solubilization in aqueous micelles of dodecyl- and tetradecyltrimethylammonium bromide: solubilization equilibria. Journal of Colloid and Interface Science, 142, 480–488. 147. Kaler, E. W., Murthy, A. K., Rodriguez, B. E., & Zasadzinski, J. A. (1989). Spontaneous vesicle formation in aqueous mixtures of single-tailed surfactants. Science, 245, 1371–1374. 148. Shioi, A., & Hatton, T. A. (2002). Model for formation and growth of vesicles in mixed anionic/cationic (SOS/CTAB) surfactant systems. Langmuir, 18, 7341–7348. 149. Efrat, R., Abramov, Z., Aserin, A., & Garti, N. (2010). Nonionic−anionic mixed surfactants cubic mesophases. Part I: structural chaotropic and kosmotropic effect. The Journal of Physical Chemistry. B, 114, 10709–10716. 150. Cui, X., Jiang, Y., Yang, C., Lu, X., Chen, H., Mao, S., Liu, M., Yuan, H., Luo, P., & Du, Y. (2010). Mechanism of the mixed surfactant micelle formation. The Journal of Physical Chemistry. B, 114, 7808– 7816. 151. Kabir-ud-Din, Rub, M. A., & Naqvi, A. Z. (2010). Mixed micelle formation between amphiphilic drug amitriptyline hydrochloride and surfactants (conventional and gemini) at 293.15-308.15 K. The Journal of Physical Chemistry. B, 114, 6354–6364. 152. Rodriquez, A., Graciani, M., Moreno-Vargas, A. J., & Moya, M. L. (2008). Mixtures of monomeric and dimeric surfactants: hydrophobic chain length and spacer group length effects on non ideality. The Journal of Physical Chemistry. B, 112, 11942. 153. Singh, J., Unlu, Z., & Ranganathan, R. (2008). Aggregate properties of sodium deoxycholate and dimyristoylphosphatidylcholine mixed micelles. The Journal of Physical Chemistry. B, 112, 3997–4008. 154. Prado, M. C., & Neves, B. R. A. (2010). Mixed self-assembled layers of phosphonic acids. Langmuir, 26, 648–654.

Appl Biochem Biotechnol 155. Tucker, I., Penfold, J., Thomas, R. K., Grillo, I., Mildner, D. F., & Barker, J. G. (2008). Self-assembly in complex mixed surfactant solutions: the impact of dodecyl triethylene glycol on dihexadecyl dimethyl ammonium bromide. Langmuir, 24, 10089–10098. 156. Katarzyna, S., & Bronislaw, J. (2009). Thermodynamics of micellization of aqueous solutions of binary mixtures of two anionic surfactants. Langmuir, 25, 4377–4383. 157. Denkova, P. S., Lokeren, L. V., & Willem, R. (2009). Mixed micelles of triton X-100, sodium dodecyl dioxyethylene sulfate, and synperonic l61 investigated by NOESY and diffusion ordered NMR spectroscopy. The Journal of Physical Chemistry. B, 113, 6703–6709. 158. Tsamaloukas, A. D., Beck, A., & Heerklotz, H. (2009). Modeling the micellization behavior of mixed and pure n-alkyl-maltosides. Langmuir, 25, 4393–4401. 159. Khatua, D., Ghosh, S., Dey, J., Ghosh, G., & Aswal, V. K. (2008). Physicochemical properties and microstructure formation of the surfactant mixtures of sodium N-(2-(n-dodecylamino)ethanoyl)-L-alaninate and SDS in aqueous solutions. The Journal of Physical Chemistry. B, 112, 5374–5380. 160. Rodriguez-Pulido, A., Casado, A., Munoz-Ubeda, M., Junquera, E., & Aicart, E. (2010). Experimental and theoretical approach to the sodium decanoate-dodecanoate mixed surfactant system in aqueous solution. Langmuir, 26(2010), 9378–9385. 161. Poorgholami-Bejarpasi, N., Hashemianzadeh, M., Mousavi-khoshdel, S. M., & Sohrabi, B. (2010). Role of interaction energies in the behavior of mixed surfactant systems: a lattice Monte Carlo simulation. Langmuir, 26, 13786–13796. 162. Rosen, M. J. (2004). Surfactants and interfacial phenomena, 3rd edn. Hoboken: Wiley-Interscience. 163. Tadros, T. F. (2005). Applied surfactants principles and applications. Weinheim:Wiley-VCH Verlag GmbH & Co. KGaA. 164. Rosen, M. J., & Zhao, F. (1983). Binary mixtures of surfactants. The effect of structural and microenvironmental factors on molecular interaction at the aqueous solution/air interface. Journal of Colloid and Interface Science, 95, 443–452. 165. Gu, B., & Rosen, M. J. (1989). Surface concentrations and molecule interactions in cationic-anionic mixed monolayers at various interfaces. Journal of Colloid and Interface Science, 129, 537–553. 166. Rosen, M. J. (1991). Synergism in mixtures containing zwitterionic surfactants. Langmuir, 7, 885–888. 167. Liu, L., & Rosen, M. J. (1996). The interaction of some novel diquaternary gemini surfactants with anionic surfactants. Journal of Colloid and Interface Science, 179, 454–459. 168. Rosen, M. J., & Hua, H. Y. (1982). Surface concentrations and molecular interactions in binary mixtures of surfactants. Journal of Colloid and Interface Science, 86, 164–172. 169. Rubingh, D. N. (1979). Solution chemistry of surfactants, Mittal, K. L., (Ed.). (pp. 337-354) New York: Plenum. 170. Kwan, C., & Rosen, M. J. (1980). Relationship of structure to properties in surfactants. 9. Syntheses and properties of 1,2- and 1,3-alkanediols. The Journal of Physical Chemistry, 84, 547–551. 171. Rosen, M. J. (1981). Purification of surfactants for studies of their fundamental surface properties. Journal of Colloid and Interface Science, 79, 587–588. 172. Reid, V. W., Longman, G. F., & Heinerth, E. (1967). Determination of anionic-active detergents by two phase titration. Tenside, 4, 292–304. 173. Kaler, E. W., Herington, K. L., Murthy, A. K., & Zasadzinski, J. A. N. (1992). Phase behavior and structures of mixtures of anionic and cationic surfactants. The Journal of Physical Chemistry, 96, 6698–6707. 174. Rosen, M. J., & Zhu, B. Y. (1984). Synergism in binary mixtures of surfactants: III. Betaine-containing systems. Journal of Colloid and Interface Science, 99, 427–434. 175. Sharma, K. S., Rodgers, C., Palepu, R. M., & Rakshit, A. K. (2003). Studies of mixed surfactant solutions of cationic dimeric (gemini) surfactant with nonionic surfactant C12E6 in aqueous medium. Journal of Colloid and Interface Science, 268, 482–488. 176. Kabir-ud-Din, Sheikh, M. S., & Dar, A. A. (2009). Interaction of a cationic gemini surfactant with conventional surfactants in the mixed micelle and monolayer formation in aqueous medium. Journal of Colloid and Interface Science, 333, 605–612. 177. Negm, N. A., & El Sabagh, A. M. (2011). Interaction between cationic and conventional nonionic surfactants in the mixed micelle and monolayer formed in aqueous medium. Quimica Nova, 34, 1007–1013. 178. Azum, N., Asiri, A. M., Rub, M. A., Khan, A. S. P., Khan, A., Rahman, M. M., Kumar, D., & Al-Youbi, A. O. (2013). Mixed micellization of gemini surfactant with nonionic surfactant in aqueous media: a fluorometric study. Colloid Journal, 75, 235–240. 179. Fatma, N., Ansari, W. H., Panda, M., & Kabir-ud-Din (2013). Mixed micellization behavior of gemini (cationic ester-bonded) surfactants with conventional (cationic, anionic and nonionic) surfactants in aqueous medium. Zeitschrift für Physikalische Chemie, 27, 133–149. 180. Akbas, H., Boz, M., & Elemenli, A. (2014). Interaction between cationic gemini surfactant and related single-chain surfactant in aqueous solutions. Fluid Phase Equilibria, 370, 95–100.

Appl Biochem Biotechnol 181. Al-Hadabi, B. A., & Aoudia, M. (2014). Surfactant–surfactant and surfactant–solute interactions in SLES– Brij35 mixed micelles: effect of the degree of ethoxylation on pyrene solubilization enhancement in water. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 459, 82–89. 182. Svanedal, I., Persson, G., Norgren, M., & Edlund, H. (2014). Interactions in mixed micellar systems of an amphoteric chelating surfactant and ionic surfactants. Langmuir, 30, 1250–1256.S. 183. Jha, K. C., Liu, H., Bockstaller, M. R., & Heinz, H. (2013). Facet recognition and molecular ordering of ionic liquids on metal surfaces. Journal of Physical Chemistry C, 117, 25969–25981. 184. Heinz, H., & Suter, U. W. (2004). Surface structure of organoclays. Angewandte Chemie, International Edition, 43, 2239–2243. 185. Heinz, H. J. (2010). Computational screening of biomolecular adsorption and self-assembly on nanoscale surfaces. Computers & Chemistry, 31, 1564–1568. 186. Kumari, M., Maurya, J. K., Singh, U. K., Khan, A. B., Ali, M., Singh, P., & Patel, R. (2014). Spectroscopic and docking studies on the interaction between pyrrolidinium based ionic liquid and bovine serum albumin. Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy, 124, 349. 187. Vishvakarma, V. K., Kumari, K., Patel, R., Dixit, V. S., Singh, P., Mehrotra, G. K., Chandra, R., & Chakrawarty, A. K. (2015). Theoretical model to investigate the alkyl chain and anion dependent interactions of gemini surfactant with bovine serum albumin. Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy, 143, 319–323. 188. Gharibi, H., Khodadadi, Z., Mousavi-Khoshdel, S. M., Hashemianzadeh, S. M., & Javadian, S. (2014). Mixed micellization of gemini and conventional surfactant in aqueous solution: A lattice Monte Carlo simulation. Journal of Molecular Graphics & Modelling, 53, 221-227. 189. Wanga, L., Hu, Y., Sun, W., & Sun, Y. (2015). Molecular dynamics simulation study of the interaction of mixed cationic/anionic surfactants with muscovite. Applied Surface Science, 327, 364–370. 190. Jusufi, A., LeBard, D. N., Levine, B. G., & Klein, M. L. (2012). Surfactant concentration effects on micellar properties. The Journal of Physical Chemistry. B, 116, 987–991.

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