Water ordering around methane during hydrate formation

Abstract

Neutron diffraction with isotopic substitution has been used to investigate the changes in the local water structure around methane during methane hydrate formation. It has been observed that the solvation cage around methane during hydrate formation is subtly more disordered than in the stable solution and significantly smaller than that observed in the hydrate crystal structure. The number of water molecules in the solvation cage is correspondingly smaller in the liquid and the formation stages than in the final hydrate structure. The results of the experiments also show that as the temperature is decreased solvent separation of the methane molecules increases as hydrate formation progresses. The major finding is that water forms clusters around methane even in the liquid phase: it is not something which occurs only during hydrate formation. These hydration shells are only loosely ordered with respect to one another in the liquid phase, but grow in size and become spatially ordered when hydrate forms.