At the solid/liquid interface water properties may be quite different from the bulk.
Here we use Density Functional Theory -based molecular dynamics simulations to describe
the water structure at mineral/water interfaces. Using a recently developed approach to calculate interface
vibrational spectra we are able to provide a molecular picture
of the interfacial water and a microscopic interpretation of the experiments.
In the case of the fluorite/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 weakly with the surrounding water, and are responsible
for the “free OH” signature in the vibrational spectrum.