Model of Rod-to-sphere Transition of the Mixed Micelles of Cationic-Anionic Surfactants

It has been known that([1][2]), with the increase of surfactant concentration, the micelles of mixed cationic-anionic surfactants undergo a transition from sphere to rod at dilute concentrations. The sphere-to-rod transition of micelles and the coacervation or flocculation of long rodlike micelles have been used successfully to explain the occurrence of precipitation or turbidity of the aqueous mixtures of cationic-anionic surfactants at dilute concentrations (usually slightly above CMC)([1,2]). It has been found that with the continuing increase of surfactant concentration, the aqueous mixtures of cationic-anionic surfactants with precipitate or turbidity turn into homogeneous transparent solutions at high concentrations ([2,3,4]). This unusual phenomenon can't be explained by the model of sphere-to-rod. transition of mixed micelles. In the present paper, we propose the model of rod-to-sphere transition of micelles. In this model, it is thought that, at relative high concentration, with the continuing increase of surfactant concentration, the micelles of mixed cationic-anionic surfactants undergo a transition from long to short rod, and then to spherical shape.