Microscopic Insights into the Fluorite/Water Interfaces from Vibrational Sum Frequency Generation Spectroscopy

Abstract

Water/mineral interfaces are central to a wide range of environmental and technological processes. In this report we provide a quantitative, molecular-level understanding of the CaF2/water interface using Density Functional Theory-based molecular dynamics simulations. In particular through the comparison of calculated Vibrational Sum Frequency Generation spectra to the experimental ones, we give a structural characterisation of the interface at different pH. At low pH, the surface is positively charged, causing a substantial degree of water ordering. Our results suggest that the surface charge originates from the dissolution of fluoride ions of the topmost layer, rather than from proton adsorption to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. Such OH groups do not establish hydrogen bonds with the surrounding water, and are therefore responsible for the “free OH” signature which is recorded in the Vibrational Sum Frequency Generation spectrum.