TABLE OF CONTENTS The simplest of the hydrate structures is the Type I. It is made from two types of cages: (1) dodecahedron, a 12-sided polyhedron where each face is a regular pentagon; and (2) tetrakaidecahedron, a 14-sided polyhedron with 12 pentagonal faces and two hexagonal faces. The dodecahedral cages are smaller than the tetrakaidecahedral cages; thus the dodecahedra are often referred to as small cages, whereas the tetrakaidecahedra are referred to as large cages.
Type I hydrates consist of 46 water molecules. If a guest molecule occupies each of the cages, then the theoretical formula for the hydrate is: X .5 3/4 H 2O, where X is the hydrate former.
Often in the literature you will find oversimplifications for the hydrate crystal structure. For example, it is common that only the dodecahedron is given as the unit crystal structure. This is incorrect. The correct structures are given here.
One of the reasons why it took a long time to establish the crystal structure of hydrates is because hydrates are nonstoichiometric. That is, a stable hydrate can form without a guest molecule occupying all of the cages. The degree of saturation is a function of the temperature and the pressure. Therefore, the actual composition of the hydrate is not the theoretical composition given in the previous paragraph.
Some of the common Type I hydrate formers include methane, ethane, carbon dioxide, and hydrogen sulfide. In the hydrates of CH 4, CO 2, and H
