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

Rémi Khatib,Mischa Bonn,María-José Perez-Haro,Marie-Pierre Gaigeot,Marialore Sulpizi,Ellen H G Backus

doi:10.1038/srep24287

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 CaF2/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 VSFG 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 CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects.

Highlights

At the interface, the properties of a given system may be drastically different from those observed in the bulk
In the Vibrational Sum Frequency Generation Spectroscopy (VSFG) experiment an infrared and visible laser pulse are in space and time overlapped on the CaF2/water interface and the reflected VSFG signal is detected
We present a combination of phase sensitive VSFG and ab initio molecular dynamics modelling which permitted to elucidate the details of the fluorite/water interface

Summary

Introduction

The properties of a given system may be drastically different from those observed in the bulk. Still experiments seem not to be able to distinguish between the two possible scenarios[5] As another surface sensitive technique, Vibrational Sum Frequency Generation Spectroscopy (VSFG) has the ability to selectively address the nanometric interfacial water layer, and has contributed substantially to our understanding of the physical and chemical properties of the CaF2/ water interface[6,7]. From the experimental point of view, we move beyond the current state of the art providing the first phase-resolved VSFG spectra for buried CaF2/water interfaces and the first broadband phase-resolved VSFG spectra at the solid-liquid interface in general In this way we obtain information about the absolute orientation of the interfacial water molecules. Phase-resolved experimental spectra contain a wealth of spectral information which is crucial to characterise these solid/liquid interfaces and reflect e.g. hydrogen bond strength and water dipole orientation, yet the interpretation of the spectroscopic data in terms of microscopic, atomistic water structures at the interfaces remain challenging, and require theoretical spectroscopic modelling, which simulations can provide

Methods

Results

Conclusion