Structure of Hydrated Crystal (L_c), Tilted Gel (L_β′), and Liquid Crystal (L_α) 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.