Structure of the Clathrate/Solution Interface and Mechanism of Cross-Nucleation of Clathrate Hydrates

Clathrate hydrates mostly occur in two cubic crystal structures, sI and sII. Cross-nucleation between these clathrate crystals has been observed in simulations and may be relevant to the transformation between clathrate polymorphs reported in experiments. Nevertheless, the mechanism by which clathrate crystals cross-nucleate and the structure of the interface between the distinct crystals have not yet been fully characterized. In this work, we use extensive molecular dynamics simulations to investigate the structure of the clathrate/solution interface for sI and sII guest-free and methane-filled hydrates at different degrees of supercooling and the mechanism of cross-nucleation between clathrate polymorphs. We find that 5(12)6(3) water cages, which are not native to the sI or sII crystals, occur assiduously in their interfaces with the solution and play a central role in the mechanism of cross-nucleation of clathrate hydrates: cross-nucleation between sI and sII requires the formation of an interfacial layer tiled by 5(12)6(3) cages connected by dodecahedra. We characterize the structure of the interfacial layer, estimate the size of the critical surface nucleus required for its formation, and assess the role of other variables in the reaction coordinate of cross-nucleation of clathrate hydrates. In agreement with previous reports of cross-nucleation of quite different systems, we observe cross-nucleation of clathrate hydrates both from the stable to the metastable crystal and from the metastable to the stable hydrate. The new crystal that forms is, in all cases, the one that has the fastest growth rate.