Dynamics of micelle-vesicle transitions in aqueous anionic/cationic surfactant mixtures

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The dynamics of vesicle formation following the rapid mixing of cationic and anionic surfactant solutions, and of their breakup, have been monitored using time-resolved light scattering. Two anionic/cationic surfactant pairs were used in these experiments-sodium octyl sulfate (SOS)/cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS)/dodecyltrimethylammonium bromide (DTAB). The observed temporal variations in scattering intensities over a wide range of surfactant compositions are consistent with vesicle formation being a sequence of a fast process (complete within 4 ms, the 'dead' time in our stopped-flow apparatus), followed by one or more processes which, when analyzed as a series of first-order events, yield apparent characteristic time constants of ∼10, ∼100, and ∼2000 s. Scattering intensities at 4 ms after mixing are two- to four-fold higher than those from either of the feeds, indicating that the first process consists of the formation of nonequilibrium mixed aggregates. Addition of salt has no impact on this 'initial' scattering intensity, nor on the three larger time constants. For a fixed SOS/ CTAB ratio, the scattering intensity at 4 ms is proportional to the overall surfactant concentration, indicating a rise in the number concentration of these initial aggregates only with increasing amounts of surfactant. Vesicle formation rates depend strongly upon the difference between the final surfactant composition and an optimal one that produces the natural curvature for the bilayer. A combination of simultaneous time-resolved dynamic and static light-scattering measurements suggests that the final process observed here is the relaxation of nonequilibrium vesicles toward their ultimate composition and size distribution. In contrast, vesicle breakup to mixed micelles appears to be a rapid single-step process, with system-dependent time constants of <4 ms for the SOS/CTAB vesicles and ∼10 s for the SDS/DTAB vesicles.

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