Influence of salt and polymer on the critical vesicle concentration in aqueous mixture of zwitterionic/anionic surfactants

Vesicles can be formed spontaneously in aqueous solution of surfactant mixture of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and lauryl sulfonate betaine (LSB). Different from the catanionic vesicles, the formation is dependent on surfactant concentration, mixing ratio and salinity due to the weak attraction between the two kinds of surfactant molecules, which provides possibility to determine the critical vesicle concentration (CVC), just as the critical micelle concentration (CMC). In this paper, a simple but sensitive conductivity titration was employed to determine the critical concentration since it is concentration-continuous and the structural transition of the ionic surfactant relates directly to their conductivity. Especially, after vesicles are formed, some of the surfactant molecules form the inner layer of the vesicle and do not conduct any more, which will lead to a much evident change in conductivity. As a result, the turning points appear in both the conductivity and molar conductivity titration curve at the transition from monomers to micelles and micelles to vesicles with increasing concentration and/or salinity, with assistant proof from surface tension measurement and TEM. The CVC drops markedly after the addition of salt due to the compression of salt on the double electric layer of the surfactant polar head. But the addition of nonionic polymer PVP (K30) delays the vesicle formation due to the adsorption of surfactant molecules to the polymer. The results may make much contribution to the understanding of the mechanism of vesicle formation.