On the Hydrogen Bonding Structure at the Aqueous Interface of Ammonium-Substituted Mica: A Molecular Dynamics Simulation

Molecular dynamics (MD) computer simulations were performed for an aqueous film of 3 nm thickness adsorbed at the (001) surface of ammonium-substituted muscovite mica. The results provide a detailed picture of the near-surface structure and topological characteristics of the interfacial hydrogen bonding network. The effects of deuterium/hydrogen isotopic substitution in N(H/D)₄⁺ on the dynamics and consequently on the convergence of the structural properties have also been explored. Unlike many earlier simulations, a much larger surface area representing 72 crystallographic unit cells was used, which allowed for a more realistic representation of the substrate surface with a more disordered distribution of aluminium/silicon isomorphic substitutions in muscovite. The results clearly demonstrate that under ambient conditions both interfacial ammonium ions and the very first layer of water molecules are H-bonded only to the basal surface of muscovite, but do not form H-bonds with each other. As the distance from the surface increases, the H-bonds donated to the surface by both N(H/D)₄⁺ and H₂O are gradually replaced by the H-bonds to the neighbouring water molecules, with the ammonia ions experiencing one reorientational transition region, while the H₂O molecules experiencing three such distinct consecutive transitions. The hydrated N(H/D)₄⁺ ions adsorb almost exclusively as inner-sphere surface complexes with the preferential coordination to the basal bridging oxygen atoms surrounding the aluminium/silicon substitutions.