Molecular Dynamics Simulations of Quartz (101)-Water and Corundum (001)-Water Interfaces: Effect of Surface Charge and Ions on Cation Adsorption, Water Orientation, and Surface Charge Reversal

Quartz and corundum surfaces in water are capable of adsorbing and releasing protons, a behavior attributed to the amphoteric character of their silanol and ab initio calculations are used to obtain different charge densities on crystalline (101) quartz and (001) corundum surfaces and the corresponding charge delocalization after deprotonation of the silanol and aluminol groups, respectively. Then, classical molecular dynamics simulations are used to study the interaction of water with the charged quartz and corundum surfaces in the presence of aqueous solutions of monovalent alkali and alkaline earth metal chlorides. Results include density profiles of adsorbed cations, and the effect of cations on the orientation profiles of water molecules close to the mineral surfaces and the distance at which such surfaces become neutral or reverse their charges. In all cases where there are experimental or simulation data, the results here compare very well. The adsorption density of cations on quartz increases with the size of the cations, either monovalent or divalent. The density of adsorbed monovalent cations on corundum decreases for larger cation sizes, while this behavior on quartz is the opposite. In both cases the adsorption of cations is enhanced by the increase of the surface charge. Adsorption on corundum is much more extensive compared to that on quartz for all surface charges and cations. The sequence of simulations of cation adsorption on silica and alumina provide support to the idea that high isoelectric point materials preferentially adsorb well-hydrated cations and low isoelectric point materials preferentially adsorb poorly hydrated cations. The results of this work are expected to contribute to improving current knowledge on the interaction of mineral oxides with macromolecules, such as polyelectrolytes in solid liquid separation processes and biomolecules in lung inflammatory processes.