Thermodynamics of vesicle formation from a mixture of anionic and cationic surfactants

Detailed model calculations on mixed sodium dodecyl sulfate/dodecylammonium chloride vesicles show that the work of bending a planar bilayer into a geometrically closed bilayer vesicle rises steeply as the molar ratio between aggregated anionic and cationic surfactant approaches unity. Likewise, the bending work increases as the mole fraction of either of the surfactants approaches unity, resulting in a bending free energy minimum on each side of the equimolar composition. In the vicinity of that composition the bending work is too large to permit vesicle formation to any appreciable extent, while at compositions where one of the surfactants is in excess, the bending free energy is much lower, thus enabling the formation of small unilamellar vesicles (R < 500 Å). These calculation results are in good qualitative agreement with recent experimental findings on mixed anionic/cationic vesicles. Moreover, the fluctuations in composition, chain packing density, shape, and size contribute to making the vesicle size distribution fairly polydisperse with a relative standard deviation of σR/R_max equal to 0.283.