Crystal structures of methyl 6-O-n-alkanoyl-β-d-glucopyranosides

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 (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.

Structure of Hydrated Crystal (Lc), Tilted Gel (Lβ'), and Liquid Crystal (Lα) Phases of Linear Alkylbenzene Sulfonate (LAS) Studied by X-ray Diffraction and Molecular Dynamics Simulation

Precipitation of crystals in aqueous surfactant solutions hampers their use as liquid detergents. Understanding the molecular structure that causes the crystal formation is of key importance in designing a new surfactant. In this study, we first identified the hydrated crystal (L_c) structure of linear alkylbenzene sulfonate (LAS) at ambient temperature by combining X-ray diffraction (XRD) with all-atom molecular dynamics (MD) simulation. It is known that the L_c domain involves only a stoichiometrically small number of water molecules, and the water number of existing interlamellar layers may change when the system shows the thermally induced phase transition. Therefore, we explored possible crystal structures with five different hydration levels of 0, 0.5, 1, 2, and 4 water per LAS using MD simulations with the d-spacing measured by XRD. Among them, only the diffraction pattern calculated for the monohydrate LAS well coincided with the experimental diffraction pattern at 300 K. A structural change of the monohydrate from the L_c phase to the tilted gel (L_β') phase was also observed by heating from 300 to 360 K in the MD simulations, where a cross-sectional structure of the alkyl groups of LAS molecules changed from a face-centered rectangular lattice to a hexagonal one. Further heating to 400 K resulted in a disordered liquid crystal (L_α) phase. Thus, configurational changes of the detergent during thermally induced phase transitions were well explored and characterized by MD simulations with the aid of XRD data. This approach can be broadly applied where we explore the phase behavior of hydrated crystals of any surfactant species.