Coarse-grained molecular dynamics simulation of ammonium surfactant self-assemblies: micelles and vesicles.

Large-scale coarse-grained molecular dynamics simulations have been performed to investigate the self-assemblies of dodecyltrimethylammonium bromide (DTAB) and gemini surfactants 12-S-12 (S = 6, 14, and 20). At the concentrations investigated, the surfactants experience fast aggregation of monomers into oligomers until the cluster numbers reach maxima. For DTAB, larger aggregates grow at the expense of monomers, but for gemini surfactants, the growth of clusters is accomplished via the merging of oligomers. In the final stage, spherical and worm-like micelles coexist in the systems of DTAB and 12-6-12, and in gemini systems with longer spacers, namely, 12-14-12 and 12-20-12, well-defined vesicles are formed through expansion and curving of bilayer-like structures. Through detailed analysis of the vesicle structures, many of the 12-20-12 surfactants are found to have their headgroups at different surfaces, with the spacers bridging the inner and outer surface of the vesicle.