FIGURE 2.1 The hydrate crystal structures seen in the middle column are shown as their smallest repeating, or unit, structures. Given at the right of each crystal is the reported number of water molecules necessary to form the unit crystal. The “guest” molecules referred to in the right-hand column indicate the pure guests which will form in each structure under normal conditions, with methane the primary molecule of interest in this report. In the left-hand column, the cavity types are pictured with the number of pentagonal and hexagonal faces indicated as 5n6n where n refers to the number of faces. For example, 51264 indicates that the cavity has 12 pentagonal faces and 4 hexagonal faces. In the left-hand column, the n For example, the figure shows that pure propane will form in the structure II unit crystal, which has a host composed of 136 water molecules, with 16 of the 512 cavities and 8 of the 51264 cavities.
SOURCE: Reprinted with permission of Nature (Sloan, 2003) Macmillian Publishers, Ltd.