Structure, Energetics, and Dynamics of Smectite Clay Interlayer Hydration: Molecular Dynamics and Metadynamics Investigation of Na-Hectorite

This paper presents a classical molecular dynamics (MD) and metadynamics investigation of the relationships between the structure, energetics, and dynamics of Na-hydroxyhectorite and serves to provide additional, molecular-scale insight into the interlayer hydration of this mineral. The computational results support a model for interlayer H2O structure and dynamics based on H-2 NMR spectroscopy and indicate that H2O molecules undergo simultaneous fast librational motions about the H2O C-2 symmetry axis and site hopping with C-3 symmetry with respect to the surface normal. Hydration energy minima occur at one-, one-and-one-half-, and two-water-layer hydrates, which for the composition modeled correspond to 3, 5.5, and 10 H2O/Na+, respectively. Na+ ions are coordinated by basal O atoms (O-MIN) at the lowest hydration levels and by H2O molecules (O-H2O) in the two-layer hydrate, and H2O molecules have an average of three H-bonds at the greatest hydration levels. The metadynamics calculations yield activation energies for site hopping of H2O molecules of similar to 6.0 kJ/mol for the one-layer structure and similar to 3.3 kJ/mol for hopping between layers in the two-layer structure. Computed diffusion coefficients for water and Na+ are substantially less than in bulk liquid water, as expected in a nanoconfined environment, and are in good agreement with previous results.