A computational study of the surface structure and reactivity of calcium fluoride

Abstract

Electronic structure calculations based on the density functional theory (DFT) are employed to investigate the electronic structure of fluorite (CaF₂) and the mode and energies of adsorption of water at the main {111} cleavage plane. Electron density plots show the crystal to be strongly ionic with negligible ionic relaxation of the unhydrated surface. We find associative adsorption of water at the surface with hydration energies between 41 and 53 kJ mol⁻¹, depending on coverage. We next employ atomistic simulation techniques to investigate the competitive adsorption of water and methanoic acid at the planar and stepped {111}, {011} and {310} surfaces. The hydration energies and geometries of adsorbed water molecules on the planar {111} surface agree well with those found by the DFT calculations, validating the interatomic potential parameters. Methanoic acid adsorbs in completely different configurations on the three surfaces, but always by one or both oxygen atoms to one or more surface calcium atoms. Molecular Dynamics simulations at 300 K show that the effect of temperature is to increase the difference in adsorption energy between methanoic acid and water at the planar {111} surface. The methanoic acid remains bound to the surface whereas the water molecules prefer to form a droplet of water between the two surface planes. We show in a series of calculations of the co-adsorption of water and methanoic acid that the presence of solvent makes a significant contribution to the final adsorption energies and that the explicit inclusion of solvent in the calculations is necessary to correctly predict relative reactivities of different surface sites, a finding which is important in the modelling of mineral separation processes such as flotation.