Molecular dynamics simulation of cetyltrimethylammonium bromide and sodium octyl sulfate mixtures: aggregate shape and local surfactant distribution.

To examine the self-assembly of cationic-anionic (catanionic) surfactant mixtures, we performed molecular dynamical (MD) simulations at fixed surfactant numbers but different ratios of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium octyl sulfate (SOS) aqueous mixtures, which were investigated previously (J. Phys. Chem. 1996, 100, 5874-5879). The simulation results show that with an increase of CTAB, there are two different potential aggregation evolving paths. For SOS-rich mixtures, the aggregation transition is sphere-disc-rod, while in CTAB-rich mixtures, it is rod-sphere. Furthermore, a disc micelle model was built to explain the shape of the aggregates with varying compositions of CTAB and SOS. In the model, the surfactant distribution in disc micelles is spontaneously adjusted according to the different curvature of the disc surface. The short-tailed SOS tends to stay in the edge region of high curvature, while in the disc center, where the curvature is very low, equimolar mixing of cationic and anionic surfactants is better for the arrangement of CTAB and SOS. Based on this model, the relation between the shape and composition of CTAB and SOS aggregates is well established by analyzing the local surfactant distribution. These new simulations on the evolving mechanism of aggregate shape are very important for the full understanding of the complex phase behavior in cationic and anionic mixtures and for the self-assembly of other mixed surfactant systems.