Effects of Layer-Charge Distribution of 2:1 Clay Minerals on Methane Hydrate Formation: A Molecular Dynamics Simulation Study.

Molecular dynamics simulations were used to investigate the effects of the external surface of a 2:1 clay mineral with different charge amounts and charge locations on CH hydrate formation. The results showed that 5, 56, 56, and 56 were formed away from the clay mineral surface. The surface of the clay mineral with high- and low-charge layers was occupied by Na to form various distributions of outer- and inner-sphere hydration structures, respectively. The adsorbed Na on the high-charge layer surface reduced the HO activity by disturbing the hydrogen bond network, resulting in low tetrahedral arrangement of HO molecules near the layer surface, which inhibited CH hydrate formation. However, more CH molecules were adsorbed onto the vacancy in the Si-O rings of a neutral-charge layer to form semicage structures. Thus, the order parameter of HO molecules near this surface indicated that the arrangement of HO molecules resulted in a more optimal tetrahedral structure for CH hydrate formation than that near the negatively charged layer surface. Different nucleation mechanisms of the CH hydrate on external surfaces of clay mineral models were observed. For clay minerals with negatively charged layers (i.e., high and low charge), the homogeneous nucleation of the CH hydrate occurred away from the surface. For a clay mineral with a neutral-charge layer, the CH hydrate could nucleate either in the bulk-like solution homogeneously or at the clay mineral-HO interface heterogeneously.