Water orientation and hydrogen-bond structure at the fluorite/water interface

TL;DR

This study combines experimental and theoretical methods to analyze the molecular-level structure and orientation of water at the fluorite-water interface, revealing how surface charge and ion dissolution influence water ordering.

Contribution

It provides a detailed, quantitative understanding of the fluorite-water interface using advanced spectroscopic and ab initio simulation techniques, highlighting the effects of pH and surface chemistry.

Findings

At low pH, the surface is positively charged, inducing water ordering.

Fluoride ion dissolution primarily causes surface charge, not proton adsorption.

High pH leads to Ca-OH groups with weak water interactions, affecting spectral signatures.

Abstract

Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical-experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important flourite-water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the free OH signature in the SFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of flourite-water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects.