Thermodynamics of precipitation from mixed micellar solutions

Abstract

The equilibrium of the precipitate with mixed micellar solutions of the following types, (1) two ionic surfactants with the same hydrophobic chain but different counterions and (2) anionic+semipolar surfactants, are considered. Simple thermodynamic models that yield the composition dependence of the dissolution temperature in the ternary system (at fixed overall surfactant content) are discussed. The description is performed in terms of composition variables of aqueous pseudo phase (solubility product approach) or micellar pseudo phase. The latter approach has appeared efficient for prediction of solubility diagrams in concentrated ternary systems with the use of data for aqueous solutions of pure surfactants. Results of calculations are compared with experimentally studied solubility diagrams for mixed aqueous systems of (1) dodecylsulfates of sodium and magnesium and (2) sodium dodecylsulfate+dodecylamine oxide. The mixtures are studied at overall surfactant contents significantly higher than the CMC.