Type 2 Hydrates Research Paper

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TYPE II HYDRATES:

The structure of the Type II hydrates is signifi cantly 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 twelve-sided polyhedron where each face is a regular pentagon, and (2) Hexakaidecahedron, a sixteen-sided polyhedron with twelve 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, the theoretical composition is X • 5 2/3 H 2 O, where
X is the hydrate former. Or, as is more commonly the case, if the guest occupies only the large cages, the theoretical …show more content…
To form this type of hydrate requires a small molecule, such as methane, and a Type H former. The Type H hydrates are constructed from three types of cages: (1) Dodecahedron, a twelve-sided polyhedron where each face is a regular pentagon, (2) An irregular dodecahedron with three square faces, six pentagonal faces, and three hexagonal faces, and (3) An irregular icosahedron, a twenty-sided polyhedron, with twelve pentagonal faces and eight hexagonal faces. The unit crystal is made up of three dodecahedral cages (small), two irregular dodecahedral cages (medium), and one icosahedral cage (large). It is made up of 34 water molecules. Type H hydrates are always double hydrates. Small guest molecules, such as methane, occupy the small and medium cages of the structure while a larger molecule, such as those given below, occupies the large cage. Since two formers are required to form a Type H hydrate, it is diffi cult to give the theoretical formula, However, if we assume that the small molecule, X, only enters the two smaller cages and we know that the large molecule, Y, only enters the large cages, then the theoretical formula is Y • 5 X • 34 H 2
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