CO2-induced microstructure transition of surfactant in aqueous solution: insight from molecular dynamics simulation.

A molecular dynamics simulation study is reported to investigate a CO(2)-induced microstructure transition of surfactant AOT4 in aqueous solution. The lamellar bilayer changes into a spherical micelle induced by CO(2) at ambient temperature, while a thermotropic aggregate transition occurs in the absence of CO(2) above 140 degrees C. In the lamellar bilayer, AOT4 shows a bimodal density distribution. The bilayer thickness and the average area per AOT4 are estimated to be 19.2 A and 83.3 A(2). The AOT4 bilayer possesses a sandwich structure and consists of a hydrophobic region in the center and a hydrated layer on both sides. Upon CO(2) dissolving, the lamellar bilayer is swollen and becomes loose and unstable. CO(2) molecules in the lamellar bilayer are initially near the ester groups of AOT4 and then accumulate in the center of the hydrophobic region. With increasing amounts of CO(2), the AOT4 bilayer expands gradually and the density distribution of each leaflet becomes broader. Driven by surface tension, the lamellar bilayer tends to reduce the surface area. The lamellar bilayer changes into a 3D cubic network in a small simulation box, attributed to the influence of neighboring images. In a sufficiently large box, the lamellar bilayer transforms into spherical micelles. CO(2)-active surfactants such as fluorinated surfactants and oxygenated AOT analogues are proposed to substitute CO(2)-inactive AOT and may reduce the critical pressure in microstructure transition.