Clathrate hydrates

Clathrate hydrates are in general any hydrates, in which one of the components is enclosed by the other in a cage like fashion. The name originates from the Greek klethra meaning ‘bars’ (in the sense of a lattice), which in Latin became clathratus meaning ‘latticed’. In a more specialised meaning, clathrates are inclusion compounds, in which a small guest molecule is surrounded by a cage of water molecules without any strong or specific directional interaction between the two species. Clathrate hydrates are very important in the modern oil industry, as the ambient conditions in an oil pipeline facilitate the growth of gas hydrates. These hydrate crystals can lead to a complete blockage of the pipeline causing serious damage, and thus unsurprisingly, research into this type of hydrates has been mainly promoted by the oil industry.

Three common clathrate hydrate structures can be distinguished. Structure I and II are cubic, while structure III shows hexagonal symmetry. These structures differ in the number and size of the water cage and in the dimensions of their unit cells. Thus, the structure obtained for a clathrate hydrate mainly depends on the size of the guest molecule, wherein smaller guests such as methane (CH4) or ethane (C2H6) crystallise as structure I, while larger guest molecules such as cyclopentane crystallise in the hexagonal structure III. Other crystal systems have been identified for clathrate hydrates, which form under elevated pressure of several giga Pascals. For further details see (http://dx.doi.org/10.1039/b008672j).