Molecular dynamics simulation of shape and structure evolution of preassembled cylindrical cetyltrimethylammonium bromide micelles induced by octanol

Abstract

United-atomic molecular dynamics simulation was performed to probe the shape and structure evolution of the preassembled cylindrical cetyltrimethylammonium bromide (CTAB) micelles at constant CTAB content with an addition of octanol (C8OH). Without or at low amount of C8OH, e.g., r=C8OH/CTAB≤1/3, the simulation results indicate that the cylindrical CTAB micelles are unstable and apt to split into smaller spherical-like micelles, which is dominated by CTAB content in terms of the principle of the lowest Gibbs free energy. Specifically, although C8OH and CTAB may form molecule pairs by electrostatic attraction because of the positive charge of CTA+ and the partial negative charge of O in C8OH (the O atom having −0.700 e in our MD simulation) and there are no additional interactions existing between these pairs because the C8OH content is too low. While in the case of r≥2/3, the micelles become stable and is transformed into rod-like and then disc-like micelles with bilayers. The situation has been changed at the high content of C8OH, a quasi-2D lattice structures begin to form by the electrostatic attraction and the hydrogen bonds. According to the simulation results, this phenomenon can be explained by the distribution of CTAB induced by C8OH. In other words, C8OH as cosurfactants can modify the arrangement of CTAB in aggregates, which result in closer distance between CTAB molecules and lead to the smaller average equilibrium head-group area (a) of the surfactant. Meanwhile, the C8OH molecules embed into the hydrophobic chains of CTAB and increase the average volume (v0) of the hydrophobic chains, but do not cause the change of the length (l0) of the hydrophobic chains. Therefore, a larger packing parameter (P=v0/a·l0) can be obtained, which gives rise to the formation of the stable rod-like and disc-like micelles at high C8OH content.