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

The lamellar phase produced by surfactants with water exhibits several subphases, such as hydrated crystal (L_c), gel (L_β), tilted gel (L_β'), and liquid crystal (L_α) 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 L_c, L_β', and L_α phases. In the L_α phase, the lateral diffusion of LAS is as fast as that found for phospholipid bilayers in the L_α phase. The diffusion coefficient was undetectably small for the L_c and L_β' phases. The conformation of LAS in the L_α phase relaxes very rapidly, whereas those in the L_c and L_β' 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 L_c, L_β', and L_α 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 L_α phase.