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.
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