Sensitivity of Interfacial Water to Structural Charge Distribution in Biotite Mica

Abstract

Wettability of a material provides insight on the hydrophilicity of a material, which governs how a material may interact with water. This is of great importance to many fundamental chemical and physical interactions occurring at various interfaces, particularly mineral-water interfaces. The objective of this study is to investigate the effects of fluorine substitutions and exchangeable surface cations on the hydrophilicity of the mineral biotite mica, particularly the region closest to the mineral surface (~12 Å from the surface). We also investigated the energetics of water adsorption at the biotite surface. We planned to examine this mineral-water interface using Molecular Dynamics (MD) simulations, as well as X-ray reflectivity (XRR) experiments, and compare the results from each method*. We conducted 15 MD simulations to examine biotite with different degrees of fluorination and the exchangeable cations Na+, K+, Rb+, and Cs+ with different amounts of bulk water, and examined N = 20 hydration states from full saturation (3 monolayers of water) to dehydration (0 monolayers of water) to determine the change in enthalpy as a function of water coverage. Of the four cations studied, Na+-bearing biotite promoted the most hydrophilic behavior due to its strong interaction with water molecules, which is consistent with findings from previous research that have studied Na+ on other mineral surfaces. Our simulations on the structure of interfacial water revealed that the more fluorinated the biotite was, the less hydrophilic the surface behaved. Our analysis of the energetics of water adsorption also corroborated this finding, as it demonstrated that the less fluorinated the biotite was, the more energetically favorable it was for water molecules to adsorb to the surface at low water coverages. This study is among the first to examine both structural and energetic differences in biotite-water systems with various exchangeable cations and different degrees of fluorination.

*Due to the Covid-19 situation, we were unfortunately not able to carry out the XRR experiments.