Structural transitions in a bicationic amphiphile system studied by light-scattering, conductivity, and surface tension measurements

Vesicles were found to form easily in diluted aqueous mixtures of two catio nic amphiphiles, the double-tailed, bilayer-forming didodecyldimethylammoni um bromide (DDAB) and the single-tailed, micelle-forming dodecyltrimethylam monium chloride (DTAC), and therefore are believed to be spontaneous. A com bination of Equilibrium tensiometry, conductivity, and static light-scatter ing techniques enabled the determination of critical aggregation concentrat ions corresponding to the structural transitions monomers <----> vesicles ( CVC) and vesicles <----> micelles (CMC), at several DDAB/DTAC molar ratios, thus delimiting the composition range of the vesicle stability, at 25 degr ees C. It was found that the monomers <----> vesicles transition, at lower total surfactant concentrations, is driven by the double-chained DDAB, whil e the vesicles <----> micelles transition, at higher total surfactant conce ntrations, is mainly determined by the single-chained DTAC. A region of coe xistence of vesicles and micelles was found, whose limiting lower and upper CMC values (CMCL, CMCU) were defined by static light-scattering measuremen ts.