Effect of Cations (Na+, K+, and Ca2+) on Methane Hydrate Formation on the External Surface of Montmorillonite: Insights from Molecular Dynamics Simulation

In this study, molecular dynamics simulations were performed to investigate the effects of montmorillonite with different surface cations (i.e., Na+, K+, and Ca2+) on CH4 hydrate formation. The results showed that CH4 hydrate cages are mainly formed beyond the montmorillonite surface. The inner-sphere adsorption of K+ and the outer-sphere adsorption of Na+ and Ca2+ occurred on the montmorillonite surface, leading to differences in order parameters and hydrogen bond number of H2O molecules. The number of structure I cages increased faster than that of structure II cages in different models and were in agreement with the fact that CH4 molecules can only form sI hydrate crystals. The number of 5(12) cages increased in the order: Na-Mt < Ca-Mt < K-Mt. The aqueous environment dominated by K+ on the external surface of montmorillonite facilitate heterogeneous nucleation of CH4 hydrate rather than that by Ca2+ or Na+. The abovementioned findings suggest that the coordination structure of cations on the external surface of montmorillonite plays an important role in CH4 hydrate formation through altering the occupation of CH4 hydrate.