Molecular dynamics simulations on the structure and dynamics of H2O, K+, NH4+ on hydrated muscovite surface

The investigation of water-mineral interfaces plays a pivotal role in the study of various environmental and geochemical problems. The prime reason for exploring the ion-water-mineral interactions lies in the fact that it determines several phenomena ranging from transport of elements, swelling, dispersion, and mineral alteration and so on. These interactions would be different for different surface cations and at different degrees of hydration. Recent experimental studies support the notion that the K+ ions on the hydrated muscovite surface can be exchanged for the hydronium ions. Similarly, surface K+ ions can also be exchanged for ammonium in the aqueous solution (e.g., in tobelite). We have investigated the structure and dynamics of these ionic species at the hydrated surface of muscovite by molecular dynamics (MD) computer simulations using CLAYFF force field. In addition to replacing the interlayer cations, a series of MD simulations have been performed with varying amounts of water on the muscovite surface at ambient conditions for each of the exchanged cations. Atomic density profiles as functions of the distance from the muscovite surface were calculated for all atoms types present in the simulation. The comparison of hydration states and the topological details of the H-bonding network around the three ions on the surface help to interpret the structure and dynamics of the ions in the confined geometries. The angular distributions of the H2O, H3O+, and NH4+ molecules with respect to the muscovite surface have been studied. The dynamics of water molecules were further examined by self-diffusion coefficients calculated from the mean square displacement of oxygen (or nitrogen) atoms and by exchange/reorientation time correlation functions of the surface molecules. Results obtained from the simulations are compared with the available experimental data and other previous simulations and provide reliable molecular-scale view of the structure and dynamics of ions and water on the muscovite surface