Micelle Formation in Mixed-Surfactant Systems

When one discusses the solution behavior of many, if not most, industrially important surfactants, it is important to remember that experimental results must be interpreted in the context of a surfactant mixture rather than a pure homogeneous material. Studies of such systems are important both academically, assuming that the mixture can be properly analyzed, and practically, since most detergents and soaps contain homologues of higher or lower chain length than that of the primary component. 

Determinations of the cmc of well-defined, binary mixtures of surfactants have shown that the greater the difference in the cmc between the components of the mixture, the greater is the effect of the chain length of the more hydrophobic member. The analysis of results for binary mixtures of an homologous series of surfactants must take into consideration the fact that at the cmc 

Interpretations may also be complicated by such effects as relatively small changes in the mole fraction of the smaller chain component due to preferential aggregation of the more hydrophobic material and the difficulty of inclusion of the longer chain into micelles of the shorter material. In some cases where the difference is very large, the component with the higher cmc may simply act as an added electrolyte, rather than becoming directly involved in the micellization process. When ternary surfactant mixtures are considered, it is usually found that the cmc of the mixture falls somewhere between the highest and lowest value determined for the individual components. 

The presence of an ionic surfactant in mixture with a nonionic usually results in an increase in the cloud point of the nonionic component. In fact, the mixture may not show a cloud point, or the transition may occur over a broad temperature range, indicating the formation of mixed micelles. As a result of that effect, it is possible to formulate mixtures of ionic and nonionic surfactants for use at temperatures and under solvent conditions (electrolyte, etc.) in which neither component alone is effective. 

Many mixtures of surfactants, especially ionic with nonionic, exhibit surface properties significantly better than do those obtained with either component alone. Such synergistic effects greatly improve many technological applications in areas such as emulsion formulations, emulsion polymerization, surface tension reduction, coating operations, personal care and cosmetics products, pharmaceuticals, and petroleum recovery, to name only a few. The use of mixed surfactant systems should always be considered as a method for obtaining optimal performance in any practical surfactant application. 

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A.P. Graeiaa, J. Lachaise and R.S. Schechter, The thermodynamics of mixed micelle formation, in Mixed Surfactant Systems, Eds. K. Ogino and M. Abe Surfactant science series, V. 46, Marcel Dekker, NY 1993, pp. 63-97. 

Sugihara, G. Miyazono, A. Nagadome, S. Oida, T. Hayashi, Y. Ko, J-S. Adsorption and Micelle Formation of Mixed Surfactant Systems in Water. II A Combination of Cationic Gemini-type Surfactant with MEGA-10. /, Oleo Sci. 2003,52,449-461. 

Fig.1 Formation of reverse micelles in a self-assembled mixed surfactant system. The addition of water tends to link these droplets to form a highly viscous bi-continuous microemulsion with aqueous and isooctane nanochannels separated by the surfactants...

The same effect is seen when a non—aromatic cationic surfactant/nonionic surfactant system is used. Since the nonideality of mixed micelle formation in this case is due almost entirely to the electrostatic effects and not to any specific interactions between the dissimilar hydrophilic groups, the geometrical effect just discussed will cause the EO groups to be less compactly structured... 

In this paper, we report the solution properties of sodium dodecyl sulfate (SDS)-alkyl poly(oxyethylene) ether (CjjPOEjj) mixed systems with addition of azo oil dyes (4-NH2, 4-OH). The 4-NH2 dye interacts with anionic surfactants such as SDS (11,12), while 4-OH dye Interacts with nonionic surfactants such as C jPOEn (13). However, 4-NH2 is dependent on the molecular characteristics of the nonionic surfactant in the anlonlc-nonlonic mixed surfactant systems, while in the case of 4-OH, the fading phenomena of the dye is observed in the solubilized solution. This fading rate is dependent on the molecular characteristics of nonionic surfactant as well as mixed micelle formation. We discuss the differences in solution properles of azo oil dyes in the different mixed surfactant systems. 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

In many products or processes, two surfactants are used together to improve the properties of the system. In some cases, the two surfactants interact in such fashion that the CMC of the mixture (Cf2) is always intermediate in value between those of the two components Cf, C2). In other cases they interact in such fashion that Cf2 at some ratio of the two surfactants is less than either Cf1 or C2. When the latter case occurs, the system is said to exhibit synergism in mixed micelle formation. In still other cases, Cf2 at some ratio of the two surfactants may be larger than either Cf or C2. Here the system is said to exhibit antagonism (negative synergism) in mixed micelle formation. 

The two fundamental properties of surfactants are monolayer formation at interfaces and micelle formation in solution for surfactant mixtures, the characteristic phenomena are mixed monolayer formation at interfaces (Chapter 2, Section RIG) and mixed micelle formation in solution (Chapter 3, Section VIII). The molecular interaction parameters for mixed monolayer formation by two different surfactants at an interface can be evaluated using equations 11.1 and 11.2 which are based upon the application of nonideal solution theory to the thermodynamics of the system (Rosen, 1982) ... 

A remarkable contribution in recent years was to have shown for the first time the formation of highly viscoelastic worm-like micelles (Figure 12) in mixed nonionic surfactant systems [110]. This finding allowed to clarify the relation between packing constraints of hydrophobic chains and micellar growth because the complex interactions between counterions (present in ionic surfactant systems) and headgroups had not to be taken into consideration. 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

For a binary system of surfactants A and B, the mixed micelle formation can be modeled by assuming that the thermodynamics of mixing in the micelle obeys ideal solution theory. When monomer and micelles are in equilibrium in the system, this results in ... 

The conditions for synergism in surface tension reduction efficiency, mixed micelle formation, and Surface tension reduction effectiveness in aqueous solution have been derived mathematically together with the properties of the surfactant mixture at the point of maximum synergism. This treatment has been extended to liquid-liquid (aqueous solution/hydrocarbon) systems at low surfactant concentrations.) The effect of chemical structure and molecular environment on the value of B is demonstrated and discussed. 

The nature of surface adsorption and micelle formation of various mixed FC- and HC-surfactants systems can be conveniently and well investigated by the non-ideal solution theory semi-emplrlcally applied in the surface layer and micelles. The weak "mutual phobic" interaction between FC- and HC-chains has been clearly revealed in the anionic-anionic and nonlonic-nonionic systems as Indicated by the positive values. value cannot be obtained... 

Micelles are often present in surfactant systems. In some processes, such as solubilization, they are directly involved. Micelles indirectly affect many other processes because monomer concentrations or activities of the surfactant components are dictated by the monomer— micelle equilibrium at total surfactant concentrations above the CMC. Therefore, interest in mixed micelle formation will continue to grow. 

In the real world, soluble organic materials may be present in the aqueous surfactant system. As with micelles, it is important to know how these affect mixed admicelle formation and how well these organics are solubilized in the admicelle (adsolubi1ized). 

The effects of macromolecules other than surfactants on the rates of organic reactions have been investigated extensively (Morawetz, 1965). In many cases, substrate specificity, bifunctional catalysis, competitive inhibition, and saturation (Michaelis-Menten) kinetics have been observed, and therefore these systems also serve as models for enzyme-catalyzed reactions and, in these and other respects, resemble micellar systems. Indeed, in some macromolecular systems micelle formation is very probable or is known to occur, and in others mixed micellar systems are likely. Recent books and reviews should be consulted for a more detailed description of macromolecular systems and for their applicability as models for enzymatic catalysis and other complex interactions (Morawetz, 1965 Bruice and Benkovic, 1966 Davydova et al., 1968 Winsor, 1968 Jencks, 1969 Overberger and Salamone, 1969). 

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