Hydration of Hydroxylated and Fluorinated Synthetic Micas Exchanged with Monovalent Cations: A NAP-XPS and Molecular Dynamics Investigation

Abstract

In order to better understand the wettability of mica minerals, we combined molecular simulations with near-ambient pressure XPS measurements of the early hydration stages of phlogopite mica containing either structural OH atoms or structural F atoms, for various monovalent compensating cations. The evolution of the environment of the atoms constituting the system (structural atoms, counterions, and water molecules) was studied as a function of increasing relative humidity. The evolution with relative humidity of the NAP-XPS component of O assigned to liquid water was considered as representative of a water adsorption isotherm. At low RH (≤0.2), fluorinated samples display significantly lower water adsorption than their hydroxylated counterparts, which agrees with a delay in hydration of surface cation for the fluorinated samples, evidenced by NAP-XPS. In parallel, alchemical transformations at different surface water contents were performed using molecular dynamics. This allowed calculating for two different micas the ratio between relative humidities for a given surface water amount. In the case of mica–OH, simulations showed that the surface with Na+ is always more hydrophilic than that with Cs+. For mica-F, the Cs+ sample appears to be more hydrophilic at low hydration states (≤≈0.5 monolayer of adsorbed water), while the opposite is observed for higher amounts of adsorbed water. It may be that at low surface coverages, the interactions between fluorine and small Na cations are stronger than with Cs+. Despite the uncertainties associated with both the NAP-XPS experiment and the force field used in simulations, the comparison between experiments and simulations yields good agreement at high relative humidity and minor discrepancies at low relative humidity. The hydrophilic/hydrophobic nature of the surface appears to strongly depend on the very first hydration stages, i.e., on the balance between cation/surface and cation/water interactions.