Molecular Behavior of Linear Alkylbenzene Sulfonate in Hydrated Crystal, Tilted Gel, and Liquid Crystal Phases Studied by Molecular Dynamics Simulation.

The lamellar phase produced by surfactants with water exhibits several subphases, such as hydrated crystal (Lc), gel (Lβ), tilted gel (Lβ'), and liquid crystal (Lα) phases, depending on temperature, pressure, and hydration. The dynamics of the surfactant molecules in these phases are still unclear. In the present study, we investigate the translational and conformational dynamics of sodium linear alkylbenzene sulfonate (LAS) molecules in the Lc, Lβ', and Lα phases. In the Lα phase, the lateral diffusion of LAS is as fast as that found for phospholipid bilayers in the Lα phase. The diffusion coefficient was undetectably small for the Lc and Lβ' phases. The conformation of LAS in the Lα phase relaxes very rapidly, whereas those in the Lc and Lβ' phases relax very slowly. The time scale of the relaxations greatly depends on the segment of the LAS molecule for the latter two phases. The relaxation time for the SO3- head group and benzene ring of LAS was much longer than that for alkyl chains. Conformational pattern analyses of LAS alkyl chains revealed that the high fraction of the gauche conformation for the odd-numbered C-C bonds aligns the chain parallel to the bilayer normal and is the main origin of the different relaxation times for different segments in the chain. In the Lc, Lβ', and Lα phases, the orientations of the SO3- group and the benzene ring are locked by the salt bridge among SO3- groups and sodium ions. As a result, the orientational order found for the C-C bonds in the LAS alkyl chains is kept even in the Lα phase.