Abstract
Clathrate hydrates are crystalline compounds in which an expanded ice lattice forms cages that contain gas molecules. There are two principal gas hydrate structures. Structure I, with a 12 A cubic unit cell, contains 46 water molecules and 8 cages of two types, giving an ideal formula (for CH4) of CH4·5-3/4H2O. The actual formula contains somewhat more water as the cages are not completely filled. Examples of gases that form Structure I hydrates are, ethane, N2, O2, Ar, Xe, CH3CI, H2S. Structure II, with a 17 A cubic unit cell, contains 136 water molecules, and 8 large cages and 16 small cages. This gives an ideal formula of, for example, CHCl3 17H2O. Other molecules that form a Structure II hydrate include propane, ethyl chloride, acetone, and tetrahydrofuran. The conditions of pressure and temperature for hydrate formation are discussed. The statistical-mechanical treatment of hydrate stabilities shows that the cages are not completely occupied; thus the clathrate hydrates are non-stoichiometric compounds.
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