An ab initio molecular dynamics study of hydronium complexation in Na-montmorillonite

The Car-Parrinello molecular dynamics simulation technique was used to predict the structure and dynamics of hydronium solvation in mono-, bi- and trihydrated Na-montmorillonite. In monohydrated montmorillonite, hydronium ions are located within the hexagonal rings of the basal clay plane. Oxygen sites of hydronium ions point towards the clay surface and hydrogen atoms towards the water layer. In bi- and trihydrated montmorillonite, hydronium ions form water-solvated, outer-sphere complexes. Similar to the solvation mechanism in bulk water, hydronium ions donate three hydrogen bonds to interlayer water molecules. In all studied hydration states, hydronium ions do not form hydrogen bonds with the basal oxygen sites. Similar to bulk water, the free energy barrier for a classical proton transfer between interlayer water molecules is of the order of kT and therefore not the limiting factor for the proton diffusion. The diffusivity of hydrogen in the interlayer is controlled by the structural rearrangements of the solvating water molecules.