Vesicle−Micelle Equilibrium of Anionic and Cationic Surfactant Mixture Studied by Surface Tension

The phase behavior of a very dilute aqueous solution of a sodium decyl sulfate (SDeS)−decyltrimethylammonium bromide (DeTAB) mixture was investigated by the surface tension measurement and partly by the density measurement. The surface tension γ and density ρ were measured as functions of total molality of surfactants m̂ and composition of DeTAB X̂ 2 . Break points were observed on the curves and identified as the critical vesicle concentration and the concentrations of the initial and final points of the vesicle−micelle coexisting region, respectively. These break points were plotted against X̂ 2, analyzed thermodynamically, and then compared with the ones in the diagrams predicted from a simple model. The composition of DeTAB in the vesicles at the critical vesicle concentration (cvc) was obtained from the cvc versus X̂ 2 curve, and it was found out to be 0.5 at almost all X̂ 2 values. From the other break point versus X̂ 2 plots, the composition region of the vesicle−micelle coexistence in the phase diagram was suggested to be from 0.62 to 0.65. Furthermore, vesicle formation was clearly indicated to take place even at X̂ 2 = 0.9999, and therefore, the vesicle−micelle transition on the critical aggregation concentration Ĉ versus X̂ 2 curve was expected to sit between X̂ 2 = 0.9999 and 1.