Dynamics of Micelle−Vesicle Transitions in Aqueous Anionic/Cationic Surfactant Mixtures

Abstract

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.

Formation kinetics and stability of surfactant vesicles

“…Earlier preliminary interpretations were made along these lines for a system containing SXS as the hydrotrope (11,12). It is instructive to compare our results with those of O'Connor et al (13). In their case two different micellar structures with opposite charges are mixed, and relaxation times are extremely long compared with the present ones.…”

Reversible Switching of a Micelle‐to‐Vesicle Transition by Compressed CO2

“…Thus, the decay curves of samples containing large aggregates appear in a monoexponential form. If micelles and vesicles coexist in the solution, the fluorescence decay curves obey a linear combination of Equations (1) and (3). If the curves are forced to fit the form of Equation (1), A 4 shows dependence on the concentration of the quencher and the ratio of vesicles to micelles.…”

“…[3] Study of the transformation from micelles to vesicles is very interesting because such a phenomenon offers, in principle, an easy way of encapsulating active agents by dissolving them in the micellar phase prior to vesicle formation, which is very important from both practical [4] and fundamental [5] points of view. The study of the micelleto-vesicle transition (MVT) is also of particular relevance for biological systems in which the extraction of proteins from cell membranes is required.…”

Review on Anionic/Cationic Surfactant Mixtures

“…Andrea J. O'Connor and co-workers pictured a more detailed model for the micelle-to-vesicle transition [33]. Figure 12 illustrates that model.…”