The phase equilibria of surfactant aqueous mixtures, hexadecyltrimethylammonium bromide and sodium dodecyl sulfate, have been studied by polarizing microscopy, quasielastic light scattering, conductivity, potentiometric, electrophoretic, and surface tension measurements. Adsorption at the air/solution interface, association and precipitation in bulk solution strongly depended on the molar ratio and actual concentration of surfactants. Catanionic vesicles coexisted with crystalline catanionic salt in a broad concentration range. The relative proportions of crystallites and vesicles varied according to the concentration and molar ratio of surfactants. Solid crystalline phase was progressively converted to the catanionic vesicles with surfactant molar ratio increasing. At the highest excess of one of the surfactant the transition from catanionic vesicles to mixed micelles occurred. The formation and stability of different phases are discussed in terms of surfactant molecular packing constraints and electrostatic interactions in the headgroup region. Surfactant tail-length asymmetry and change of electrostatic interactions in headgroup region from attractive to repulsive are governing factors for the transition from planar to curved bilayers.

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