Micellar Compositions in Mixed Surfactant Solutions

Micellization of aqueous mixtures of alkyltriethylammonium bromide and sodium alkylsulfate in the presence of excess sodium bromide has been studied by surface tension measurements. The molecular ratio of the cationic surfactant to the anionic surfactant in the mixed micelles is deduced by applying the Gibbs-Duhem equation to the measured critical micelle concentrations. Approximately equimolar amounts of the surfactant components in the mixed micelles over a wide range of aqueous mixing ratio are found in the systems of components similar in chain lengths. Large deviations of the surfactant molecular ratio deduced by the regular solution approach (Rubingh's model) when compared with that deduced by our approach are discovered, which suggests a limitation in applying the regular solution approach to mixed systems of cationic/anionic surfactants.