Surfactants micelle compositions

The bile salts and their ability to form mixed micelles is discussed in some detail in order to foster a better understanding of their applications. It is highly important for the electrophoretic characterization of the micellar phase, and therefore for the calculation of the distribution coefficients, to have a thorough understanding of the mode of micelle formation and structural changes achieved by alteration of the surfactant concentration and micelle composition as well as to develop strategies for micelle optimization. 

Monomer—Micelle Equilibria. The distribution of surfactant components between micelles and monomeric state in aqueous solutions depends on surfactant structures as well as on overall solution composition. For example, for a binary system of surfactants A and B in solution, the micelle may contain SO mole % A/SO X B while the monomer may be 90 /. A/10 X B. Since either the monomer or the micelle composition may be crucial to behavior of the system, the ability to predict the relative distribution of surfactant components between monomer and micelle, given the overall solution composition, is an important one. 

In order to illustrate the eFFect oF micellar nonidealities oF mixing on total surFactant monomer concentrations and micelle compositions in a system at the CHC, consider a hypothetical binary surFactant pair, A and B. Assume CMCa = 1 mli and CMCb = 2 mil. For a equimolar mixture oF A and B as monomer, the values oF Cn and micelle compositions are tabulated in Table I at various values oF W/RT. 

The mass action model (MAM) for binary ionic or nonionic surfactants and the pseudo-phase separation model (PSM) which were developed earlier (I EC Fundamentals 1983, 22, 230 J. Phys. Chem. 1984, 88, 1642) have been extended. The new models include a micelle aggregation number and counterion binding parameter which depend on the mixed micelle composition. Thus, the models can describe mixtures of ionic/nonionic surfactants more realistically. These models generally predict no azeotropic micellization. For the PSM, calculated mixed erne s and especially monomer concentrations can differ significantly from those of the previous models. The results are used to estimate the Redlich-Kister parameters of monomer mixing in the mixed micelles from data on mixed erne s of Lange and Beck (1973), Funasaki and Hada (1979), and others. 

Azeotrope Micellization. Here the micelle mole fraction of 1 is the same as that of the monomers (10.111. For fixed a = the monomer and micelle compositions are equal to a for all total surfactant concentrations. If an azeotrope exists, this azeotropic condition and Equation 5 imply that... 

This equation contains the counterion concentration Cj + which depends on the total surfactant concentration. It follows that x would depend on c j+ and hence would vary above the cmc. This contradiction implies that azeotrope micellization cannot occur if = J3(x). Of course, if c c, the C + would be constant and azeotropy can again occur. If d fdx = 0, azeotropy can be also possible. For ySj = / 2 = 0.7, Cj = 0, c /cj = 3.0, and w(x) = A + B ( 2x-l), which is the Redlich-Kbter expansion (12), with A = -3 and B = 0, one finds from Equation 21 that 0.8113. No value of Qj can be calculated if = 0.7, / 2 = 0.3, and / (x) = iX + /32(1 x). Figures 1 and 2 illustrate this point showing monomer and micelle concentrations (or inventories) for a = 0.8113. In the ionic/ionic case, the micelle composition x and the ratio Cj/c2 are constant above the cmc. In the ionic/nonionic case (Figure 2) the micelle composition varies with total surfactant concentration. Osborne-Lee and Schechter (22) have found evidence of azeotrope micellization for... 

Figure 1. Inventories and micelle compositions for azeotrope micell-ization of mixed ionic surfactants, c /cj = 3.0, A = - 3.0, B = 0.0, j3i = 2 0-7 AZ 0.8113.

Figure 4. Surfactant 1 monomer (cj), surfactant 2 monomer (C2), counterion (cj +j and micelle (c ,) inventories and micelle compositions (x) for binary surfactants with c /c = 3.0, A = 0, B = 0,...

It is evident that the non-ideal solution theory of surface adsorption and micellization is a convenient and useful tool for obtaining the surface and the micelle compositions and for studing the molecular interaction in the binary surfactant system. 

Bergstroem, M. 1996. Derivation of size distributions of surfactant micelles taking into account shape, composition, and chain packing density uctuatidng.oll. Interface Scil81 208-219. 

An amine oxide surfactant solution can be modeled as a binary mixture of cationic and nonionic surfactants, the composition of which is varied by adjusting the pH. The cationic and nonionic moieties form thermodynamically nonideal mixed micelles, and a model has been developed which quantitatively describes the variation of monomer and micelle compositions and concentrations with pH and... 

It Is Instructive to compare these behaviors In a phase diagram (Figure 7). For all systems of polymers and particles, there Is a line of compositions where the polymers exactly saturate the surfaces of all particles. If the spheres are large. It will take many macromolecules to saturate the surface of each one If they are small, one macromolecule will saturate many spheres and hold them In a necklace. On either side of the stoichiometric line, the behaviors of oxide particles and surfactant micelles diverge ... 

Another possible extension is to consider an excess oil phase which is a mixtnre of two or more species. Provided that mixing within the micelle can still be considered ideal and that activity coefficients for all species in the bulk oil mixture are known, an expression for for each solnte is readily obtained. Micelles formed from surfactant mixtures can be treated provided that micelle composition is known or can be calculated from theories of mixed micelles such as regular solution theory and that solubilization is low enough not to affect micelle shape or composition. Finally, nonideal mixing in the micelles can be included if some model for the nonideality is available as well as data for evaluating the relevant parameters. Perhaps the simplest scheme for incorporating nonideality with nonpolar solutes is to use volume fractions instead of mole fractions in the spirit of Flory-Huggins theory. 

Polymeric micelles form stable pseudostationary phases with a critical micelle concentration of virtually zero (aggregation number of 1), and are tolerant of high organic solvent concentrations in the electrolyte solution. Mass transfer kinetics are slow compared with conventional surfactant micelles, and peak distortion from mass overloading is a problem for some polymer compositions. Preliminary studies indicate that polymeric surfactants are effective pseudostationary phases in micellar electrokinetic chromatography, but only a limited number of practical applications have been demonstrated, and uptake has been slow. 

The use of microemulsions in the dying process [186] is relatively new and little practiced. The major part of patents and other references are dedicated to the solubilisation of various dyes in surfactant micelles, and microemulsion compositions are given, which consist of a classical triad surfactant, co-surfactant, solubilisable substance (dye). In some cases, o/w emulsions were added, in which xylene or hexadecane were used as oil phase. Only few examples are... 

The cooperative self-assembly route is the most commonly used synthesis procedure for surfactant templated materials. It uses aqueous solutions at a much lower initial surfactant concentration than for the true liquid templating route, reducing the required amounts of expensive surfactant template. In these solutions, the surfactants are at a high enough concentration to form micelles, which may be spherical, elliptical, rod-like or vesicular, but do not form the ordered aggregates found in the final templated silica-surfactant composites. The solutions are in thermodynamic equilibrium so are stable at a given temperature until the silica precursors are added. Once added, a series of interactions between the inorganic species and the surfactant micelles occur, which involve simultaneous association of all components, hence the name, cooperative self-assembly. The result of the interactions is formation of the silica-surfactant composite, usually a precipitate, with an ordered nanoscale structure similar to those found in concentrated surfactant solutions. 

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