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The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry. Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of compositions in both CTAB-rich and SOS-rich solutions. Bilayer properties 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 microstructural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there is macroscopic phase separation. This transition occurs in CTAB-rich solutions 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 abruptly transform into vesicles over a narrow range of surfactant concentration. Since the vesicles that form in mixtures of oppositely charged surfactants are equilibrium microstructures, the microstructural evolution is related solely to the phase transition and is thus under thermodynamic control. This differs from experiments reported on the dissolution of metastable vesicles, such as the detergent solubilization of biological phospholipid membranes, which may be controlled by kinetics. Despite these differences, we find that the evolution in microstructure in our mixtures of oppositely charged surfactants is analogous to that reported for biological membrane solubilization.

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Vesicles form spontaneously in a variety of aqueous mixtures of oppositely charged surfactants. Here we report the morphological transition from spherical micelles to vesicles observed in mixtures of dodecyltrimethylammonium chloride (DTAC) and sodium dodecylbenzenesulfonate (SDBS) as probed by the complementary techniques of quasielastic light scattering (QLS), NMR self-diffusion and relaxation measurements, and time-resolved fluorescence quenching (TRFQ) experiments. In these mixtures, there is limited growth of the micelles with changes in composition, and vesicles abruptly begin to form at a characteristic mixing ratio of the two surfactants. As the composition moves further into the vesicle region, the quantity of micelles decreases in proportion to the number of vesicles that form. Thus, in mixtures of DTAC and SDBS, the transition from micelles to vesicles is continuous. This is in contrast to the first-order phase transition exhibited by other aqueous mixtures of oppositely charged surfactants, in which micelles first grow into extended threadlike micelles and samples intermediate to the micellar and vesicle phases separate into two macroscopic phases.