TRANSITION FROM MICELLES TO VESICLES IN AQUEOUS MIXTURES OF ANIONIC AND CATIONIC SURFACTANTS

Vesicles form spontaneously in a variety of aqueous mixtures of opposi tely charged surfactants. Here we report the morphological transition from spherical micelles to vesicles observed in mixtures of dodecyltri methylammonium chloride (DTAC) and sodium dodecylbenzenesulfonate (SDB S) as probed by the complementary techniques of quasielastic light sca ttering (QLS), NMR self-diffusion and relaxation measurements, and tim e-resolved fluorescence quenching (TRFQ) experiments. In these mixture s, there is limited growth of the micelles with changes in composition , and vesicles abruptly begin to form at a characteristic mixing ratio of the two surfactants. As the composition moves further into the ves icle region, the quantity of micelles decreases in proportion to the n umber of vesicles that form. Thus, in mixtures of DTAC and SDBS, the t ransition from micelles to vesicles is continuous. This is in contrast to the first-order phase transition exhibited by other aqueous mixtur es of oppositely charged surfactants, in which micelles first grow int o extended threadlike micelles and samples intermediate to the micella r and vesicle phases separate into two macroscopic phases.