Temporal Evolution of Microstructures in Aqueous CTAB/SOS and CTAB/HDBS Solutions

Vesicles are formed spontaneously when aqueous solutions of cetyl trimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS), as well as CTAB and dodecyl benzene sulfonic acid (HDBS), are mixed in well-defined ratios. Microstructures in the starting solutions are composition-dependent and, in these experiments, include spherical and rodlike micelles as well as monomers. Starting from these initial morphologies, relaxation to the equilibrium vesicle state can take several hours to months in the CTAB/SOS system, but the transition occurs within minutes in the CTAB/HDBS system at the concentrations studied. In this paper, the temporal evolution of aggregate microstructures from a range of initial states was monitored using time-resolved turbidity, dynamic light scattering, and cryogenic transmission electron microscopy (cryo-TEM). For the CTAB/SOS system, the turbidity changes slowly over a period of 2 h. The rate of growth of the aggregates, measured by dynamic light scattering, was found to be independent of the specific morphology of the initial aggregates and of the added NaBr concentration. The morphologies of intermediate-state aggregates were directly identified by cryo-TEM observations of solutions quenched at different times after mixing and confirmed to be wormlike micelles, disks, and vesicles. The model that emerged for the transitions is that the micelles grow to floppy, undulating disks. The competition between the edge and bending energies drives the transition to small vesicles at a critical disk size. These vesicles then grow to the final size distribution. Varying proportions of each of these aggregates exist at all time points. In contrast to the CTAB/SOS results, both turbidity and dynamic light scattering reveal that the transition to the final size is rapid in the CTAB/HDBS system. Within the time resolution of the cryo-TEM measurements, only vesicles, and no disks are observed. These observations indicate that the bilayer bending energy dominates in this system. The solubility difference between SOS and HDBS could also play a role in the observed difference in kinetics.