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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1H and 13C nuclear relaxation times for the ligand positions of Pd(acac)2 and Co(acac)3 (acac = acetyl-acetonate) have been measured as a function of temperature. Nuclear Overhauser enhancement factors are used to demonstrate that except for the methyl carbon nucleus internuclear dipolar interactions dominate the relaxation times. Consequently TR can be extracted from the data. It is found that TR for a 0.05 M solution in CDCl3 varies with temperature (T) as TR = 3.92x10-13exp(1.38x103/T) (Co) TR = 4.42x10-13exp(1.20x103/T) (Pd) Activation energies for rotational reorientation calculated from the data are found to be 9-10 kJ mol-1 for both complexes. There appears to be a slight concentration dependence of ER, ER being about 12 kJ mol-1 for a 0.2 M solution of the palladium complex. Spin internal reorientation affects the T1 value of the CH3 carbon of Pd(acac)2, with T1 getting shorter as the temperature is raised. A simple single-axis model of rotational reorientation in solution is presented whereby it can be shown that ER is the average barrier energy encountered by a molecule during a single angular reorientation.

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