Molecular Insight into Fatty Acid Adsorption on Bare and Hydrated (111) Fluorite Surface

The adsorption of fatty acids with various chain structures on the (111) fluorite surface is investigated using density functional theory, including a correction for dispersive interactions. In the case of the acidic form, we observe that the molecular form is preferred over the dissociated one and the molecule adsorbs on a surface calcium atom with an energy of -78.2 kJ mol^-1. Also, we show that the carboxylate anion adsorbs on the surface under two possible configurations, a bidentate binuclear one or a monodentate one, the bidentate binuclear being favored. At both 0 and 300 K, the chain length does not affect the geometry of the carboxyl group but it strongly impacts the global geometry of the molecule adsorption on the fluorite surface: the "flat" adsorption mode, i.e., when the molecule is parallel to the surface, is favored when the number of carbon atoms is equal to or higher than 6, due to dispersion forces. However, when the molecule is in hydrated condition, the chain folds up by itself to reduce the interactions with water while the carboxylate group adsorbs in monodentate configuration. In aqueous conditions, the chain length does not impact anymore the adsorption energies, the vertical adsorption mode being always favored.