Molecular dynamics simulations of the interactions between water and inorganic solids

Molecular dynamics simulations of three solid surfaces, namely, the (00.1) and (01.2) hematite surfaces and the (10.4) calcite surface, in contact with an aqueous solution have been performed and the structure of water near the interface investigated. We initially calculated the hydration and hydroxylation energies of the two hematite surfaces using static calculations to determine the adsorbed state of water on these surfaces before studying hydration using molecular dynamics. The dynamics simulations show that, in each case, the water density exhibits a damped oscillatory behaviour up to a distance of at least 15 Å from the surface. Next, we investigated the adsorption of ions on the (10.4) calcite surface by calculating their free energy profile. These profiles show a strong correlation with the water structure at the interface. This implies that the adsorption of water at the surface of the solid causes the density fluctuations, which in turn control further adsorption. Further analysis revealed that, in each case, the solid surface had a strong effect on the self-diffusion coefficient and the orientation order parameter of water near the interface. Finally, to consider the effect of the crystal size on the solid/water interface, we modelled a calcite nanoparticle in vacuum and immersed in water. We found that the nanoparticle undergoes a phase change in vacuum but that, in the presence of water, the calcite structure was stabilised. Also, the water residence time in the first hydration shell of the surface calcium ions suggested that the dynamics of water in the vicinity of the nanoparticle resemble that around an isolated calcium ion.