PHASE-BEHAVIOR OF AQUEOUS MIXTURES OF CETYLTRIMETHYLAMMONIUM BROMIDE (CTAB) AND SODIUM OCTYL SULFATE (SOS)

The phase behavior and aggregate morphology of mixtures of the opposit ely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodi um octyl sulfate (SOS) are explored with cryotransmission electron mic roscopy, quasielastic light scattering, and surface tensiometry. Diffe rences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and preci pitate phases), and vesicles form spontaneously over a wide range of c ompositions in both CTAB-rich and SOS-rich solutions. Bilayer properti es of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones. We observe two modes of microstru ctural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there i s macroscopic phase separation. This transition occurs in CTAB-rich so lutions and in SOS-rich solutions at higher surfactant concentrations. In the second transition mode, mixtures rich in SOS at low surfactant concentrations exhibit no phase separation. Instead, small micelles a bruptly transform into vesicles over a narrow range of surfactant conc entration. Since the vesicles that form in mixtures of oppositely char ged surfactants are equilibrium microstructures, the microstructural e volution is related solely to the phase transition and is thus under t hermodynamic control. This differs from experiments reported on the di ssolution of metastable vesicles, such as the detergent solubilization of biological phospholipid membranes, which may be controlled by kine tics. Despite these differences, we find that the evolution in microst ructure in our mixtures of oppositely charged surfactants is analogous to that reported for biological membrane solubilization.