Effects of cage occupancy in hydrate by first-principles calculation and modification of the van der Waals–Platteeuw hypothesis

The changes of methane hydrate lattice with the decrease of cage occupancy were calculated by first-principles methods. The calculation results show that the decrease of the cages occupancy in sII and sH hydrate does not lead to large deformation in the lattice. Even if all the methane molecules are removed so that the hydrates have become new types of ice, the sII and sH lattices remain stable. The same conclusion is also true when the occupancy of the small cages in sI hydrate is reduced. However, the sI hydrate lattice will deform and almost collapse as the large cage occupancy decreases. These calculation results suggest that sI hydrate cannot exist with empty cages. Since the van der Waals–Platteeuw theory is based on the assumption that the stability of host lattice is independent of the occupancy of guest molecule, it would be applicable to sII and sH lattices, but not to sI hydrates. We propose a modification to the van der Waals–Platteeuw hypothesis so that the theory seems more reasonable. (paper)