DYNAMICS OF MICELLE-VESICLE TRANSITIONS IN AQUEOUS ANIONIC CATIONIC SURFACTANT MIXTURES/

The dynamics of vesicle formation following the rapid mixing of cation ic and anionic surfactant solutions, and of their breakup, have been m onitored using time-resolved Light scattering. Two anionic/cationic su rfactant pairs were used in the se experiments-sodium octyl sulfate (S OS)/cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate ( SDS)/dodecyltrimethylammonium bromide (DTAB). The observed temporal va riations in scattering intensities over a wide range of surfactant com positions are consistent with vesicle formation being a sequence of a fast process (complete within 4 ms, the 'dead' time in our stopped-flo w apparatus), followed by one or more processes which, when analyzed a s a series of first-order events, yield apparent characteristic time c onstants of similar to 10, similar to 100, and similar to 2000 s. Scat tering intensities at 4 ms after mixing are two-to four-fold higher th an those from either of the feeds, indicating that the first process c onsists of the formation of nonequilibrium mixed aggregates. Addition of salt has no impact on this 'initial' scattering intensity, nor on t he three larger time constants. For a fixed SOS/CTAB ratio, the scatte ring intensity at 4 ms is proportional to the overall surfactant conce ntration, indicating a rise in the number concentration of these initi al aggregates only with increasing amounts of surfactant. Vesicle form ation rates depend strongly upon the difference between the final surf actant composition and an optimal one that produces the natural curvat ure for the bilayer. A combination of simultaneous time-resolved dynam ic and static light-scattering measurements suggests that the final pr ocess observed here is the relaxation of nonequilibrium vesicles towar d their ultimate composition and size distribution. In contrast, vesic le breakup to mixed micelles appears to be a rapid single-step process , with system-dependent time constants of <4 ms for the SOS/CTAB vesic les and similar to 10 s for the SDS/DTAB vesicles.