Structure, Energetics, and Dynamics of Water Adsorbed on the Muscovite (001) Surface: A Molecular Dynamics Simulation

Molecular dynamics (MD) computer simulations of liquid water adsorbed on the muscovite (001) surface
provide a greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation
in the structural and orientational ordering, hydrogen bond (H-bond) organization, and local density of H2O
molecules at this important model phyllosilicate surface. MD simulations at constant temperature and volume
(statistical NVT ensemble) were performed for a series of model systems consisting of a two-layer muscovite
slab (representing 8 crystallographic surface unit cells of the substrate) and 0 to 319 adsorbed H2O molecules,
probing the atomistic structure and dynamics of surface aqueous films up to 3 nm in thickness. The results
do not demonstrate a completely liquid-like behavior, as otherwise suggested from the interpretation of X-ray
reflectivity measurements and earlier Monte Carlo simulations. Instead, a more structurally and orientationally
restricted behavior of surface H2O molecules is observed, and this structural ordering extends to larger distances
from the surface than previously expected. Even at the largest surface water coverage studied, over 20% of
H2O molecules are associated with specific adsorption sites, and another 50% maintain strongly preferred
orientations relative to the surface. This partially ordered structure is also different from the well-ordered
2-dimensional ice-like structure predicted by ab initio MD simulations for a system with a complete monolayer
water coverage. However, consistent with these ab initio results, our simulations do predict that a full molecular
monolayer surface water coverage represents a relatively stable surface structure in terms of the lowest
diffusional mobility of H2O molecules along the surface. Calculated energies of water adsorption are in good
agreement with available experimental data.