Density functional theory calculations of adsorption of water at calcium oxide and calcium fluoride surfaces

Electronic structure calculations using the density functional theory (DFT) within the generalized-gradient approximation and ultra-soft pseudopotentials have been used to investigate the adsorption of water on the main cleavage planes of CaO and CaF2. The calculated structural parameters are found to be in good agreement with experiment. The unhydrated surfaces show negligible ionic relaxation from bulk termination due to the minimal distortion of the electron density in the surface layer. Electron density plots show both crystals to be strongly ionic. We found on both mineral surfaces that associative adsorption of water is energetically preferred. Water molecules which were initially dissociatively adsorbed, recombined to form associatively adsorbed species. The water molecules are adsorbed by their oxygen ion to surface calcium ions, but electron density plots show strong interactions between surface anions and hydrogen atoms. The calculated hydration energies of approximately 70 kJ mol−1 on the CaO {100} surface and 41–53 kJ mol−1 on the CaF2 {111} surface indicate physisorption on both surfaces.