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 m and composition of DeTAB X₂. 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 X₂, 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 X₂ curve, and it was found out to be 0.5 at almost all X₂ values. From the other break point versus X₂ 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 X₂ = 0.9999, and therefore, the vesicle-micelle transition on the critical aggregation concentration C versus X₂ curve was expected to sit between X₂ = 0.9999 and 1.