Mixed Micelle Solutions

Abstract

Aqueous solutions of two surfactants exhibit striking alterations in physical properties such as surface tension and intensity of scattered light which can be associated with the formation of micelles. The micelles in binary surfactant solutions contain both surfactant components in equilibrium with the monomeric species in the aqueous phase. A theory which treats the mixed micelle as an ideal solution of the two surfactants and predicts critical micelle concentrations (CMCs), micelle composition and monomer concentrations is available in the literature (J. Clint J.C.S. Faraday 1, 1975, 71, 1327). This theory provides an adequate description where nearly ideal mixing might be expected (i.e., homologous series with similar head groups) but fails to predict either the CMC or monomeric concentrations for mixtures of surfactants of differing head groups. A theory is presented based on a regular solution theory concept for the mixed micelle which provides a way to deal with enthalpic as well as entropic factors in mixed micelle formation. Comparison of the predictions of the theory with experimental data for the dependence of the CMC on composition for ionic-nonionic and nonionic-nonionic mixtures shows that the non-ideal mixed micelle theory provides a much better description than the ideal mixed micelle theory. In addition experimentally determined monomer concentrations are shown to be more accurately predicted by the non-ideal mixed micelle model.