VESICLE FORMATION AND GENERAL PHASE-BEHAVIOR IN THE CATANIONIC MIXTURE SDS-DDAB-WATER - THE ANIONIC-RICH SIDE

Catanionic mixtures are aqueous mixtures of oppositely charged surfact ants which display novel phase behavior and interfacial properties in comparison with those of the individual surfactants. One phase behavio r property is the ability of these systems;to spontaneously form stabl e vesicles at high dilution. The phase behavior of the mixture sodium dodecyl sulfate (SDS) - didodecyldimethylammonium bromide (DDAB) in wa ter has been studied in detail, and two regions of isotropic vesicular phases (anionic-rich and cationic-rich) were identified. Cryo-transmi ssion electron microscopy allowed direct visualization of relatively s mall and polydisperse unilamellar vesicles on the SDS-rich side. Monit oring of the microstructure evolution from mixed micelles to vesicles as the surfactant mixing ratio is varied toward equimolarity was also obtained. Further information was provided by water self-diffusion mea surements by pulsed field gradient spin-echo NMR. Water molecules can be in fast or slow exchange between the inside and outside of the vesi cle with respect to the experimental time scale, depending on membrane permeability and vesicle size. For the SDS-rich vesicles, a slow-diff using component of very low molar fraction observed for the echo decay s was traced down to very large vesicles in solution, Light microscopy confirmed the presence of vesicles of several microns in diameter. Th us, polydispersity seems to be an inherent feature of the system.