Molecular insights into clathrate hydrate nucleation at an ice-solution interface.

Clathrate hydrates are specific cage-like structures formed by water molecules around a guest molecule. Despite the many studies that have been performed on clathrate hydrates, the actual molecular mechanism of both their homogeneous and heterogeneous nucleation has yet to be fully clarified. Here, by means of molecular simulations, we demonstrate how the interface of hexagonal ice can facilitate the heterogeneous nucleation of methane clathrate hydrate from an aqueous methane solution. Our results indicate an initial accumulation of methane molecules, which promote induction of defective structures, particularly coupled 5-8 ring defects, at the ice surface. Structural fluctuations promoted by these defective motifs assist hydrate cage formation next to the interface. The cage-like structures formed then act as a sink for methane molecules in the solution and enhance the stability and growth of an amorphous nucleus, which can evolve into a hydrate crystal upon annealing. These results are illustrative of how a surface that is structurally incompatible can serve to facilitate heterogeneous nucleation of a new crystalline phase. They should also further our general understanding of the formation of gas hydrates and their critical roles in various industrial and environmental processes, including carbon capture and storage.