Transition from micelle to vesicle in aqueous mixtures of anionic/zwitterionic surfactants studied by fluorescence, conductivity, and turbidity methods

Vesicles form spontaneously in a aqueous mixture of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT) and lauryl sulfonate betaine (LSB). Different from catanionic vesicles, the formation or disaggregation of such zwitterionic/anionic vesicles may be easily controlled by adjusting the relative amount of LSB and salinity. The participation of LSB reduces the polydispersity of the vesicles and even results in the formation of monodispersed vesicles at a certain salinity. But as LSB exceeds a certain proportion, vesicles cannot form at any concentration and salinity, making convenient the study of the structural transitions. We applied pyrene as a fluorescence probe and monitored the transition among the monomer, micelle, and vesicle through the variation of I(1)/I(3), accompanied by conductivity and turbidity measurements. In LSB solution and LSB-rich mixture, an abrupt change of the ratio of I(1)/I(3) was found in the transition from monomer to micelle with increasing concentration, as well as in the transition from micelle to vesicle with increasing salinity, which shows that a difference of the polarity of the microenvironment between the micelle and the vesicle bilayer resulted from the composition change. But in AOT solution and AOT-rich mixture, only a gradual change in the transition is observed due to the existence of intermediate structures, which have different microenvironments from micelles and vesicles. So the formation of vesicle experiences a process of monomer to premicelle to micelle to bilayer segment with increasing concentration by combining the conductivity method. The ratio of I(1)/I(3) is independent of the vesicle size once formed.