Spontaneous Vesicles and other Solution Structures in Catanionic Mixtures

We have prepared spontaneous, single-walled, equilibrium vesicles of controlled size and surface charge from aqueous mixtures of simple, commercially available, single-tailed cationic and anionic surfactants. We believe vesicle formation results from the production of anion-cation surfactant pairs which then act as double-tailed zwitterionic surfactants. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time. These catanionic vesicles are stable for periods as long as several years and appear to be the equilibrium form of aggregation. Here we review the phase behavior and structural studies of several such mixtures, with particular focus on the effect of surfactant tail lengths on size and location of the vesicle phase in the appropriate phase diagram. The approach to equilibrium is also discussed.