A note on transformation between clathrate hydrate structures I and II

Clathrate hydrates formed from binary gas mixtures of methane and other small lipophilic molecules change from the sI phase to sII and back depending on the concentration of methane in the mixtures. In contrast, pure methane hydrate under increasing pressure transforms first from sI to sII and then finally to sH. But the mechanism of the transformation from sI to sII or sH has not yet been identified. Here we present a fundamental cluster unit in the transformation from a structural point of view, which consists of 48 cages of 624 waters, called a 624W-cluster. In the solid state, the 48 cages of 14-hedron in the 624W-cluster of sI would be transformed to 24 cages of 16-hedron and 24 cages of 12-hedron in sII. The 624W-clusters in the sI hydrate each would work as a nucleus in the transformation of sI to sII. Hence, the transformation could occur throughout the solid according to the symmetry of the crystalline hydrate.