The DODAB-AOT-water system: vesicle formation and interactions with salts or synthetic polyelectrolytes

The phase behavior of the dioctyldimethylammonium bromide (DODAB)-sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-water system was determined. Studies dealt with dilute concentration regimes, at an overall surfactant concentration lower than 30.0 mmol kg(-1), and in the cationic rich side. The system was investigated by combining visual inspection with optical microscopy, fluorescence microscopy, dynamic light scattering, small-angle X-ray scattering, and electrophoretic mobility. At high [DODAB]/[AOT] mole ratios, stable and unilamellar catanionic vesicles are observed in a narrow region of the phase diagram. Vesicles are characterized by nearly constant hydrodynamic diameters, in the 300-400 nm size range, and positive zeta-potentials (about 40-50 mV). A further increase of AOT content induces the onset of a biphasic region, where vesicles are in equilibrium with a lamellar phase. In the above concentration regimes, addition of sodium bromide destabilizes vesicles and favors the formation of a lamellar phase. Under the same conditions, conversely, a synthetic polyelectrolyte, sodium polyacrylate, does not alter the phase behavior. It induces the growth of particle size and decreases the zeta-potential. The complexes formed by catanionic vesicles and sodium polyacrylate aggregate and form clusters, and readily re-dissolve when more polymer is added.