TABLE OF CONTENTS The structure of the Type II hydrates is significantly more complicated than that of the Type I. The Type II hydrates are also constructed from two types of cages. The unit structures of a Type II hydrate are (1) dodecahedron, a 12-sided polyhedron where each face is a regular pentagon; and (2) hexakaidecahedron, a 16-sided polyhedron with 12 pentagonal faces and four hexagonal faces. The dodecahedral cages are smaller than the hexakaidecahedron cages.
The Type II hydrate consists of 136 molecules of water. If a guest molecule occupies all of the cages, then the theoretical composition is: X 5 2/3 H 2O, where X is the hydrate former. Or, as is more commonly the case, if the guest occupies only the large cages, then the theoretical composition is X 17 H 2O.
As with Type I hydrates, the Type II hydrates are not stoichiometric, so the compositions of the actual hydrates differ from the theoretical values.
Among the common Type II formers in natural gas are nitrogen, propane, and isobutane. It is interesting that nitrogen occupies both the large and small cages of the Type II hydrate. On the other hand, propane and isobutane occupy only the large cages.
