any of the substances intermediate between interstitial solid solutions and true chemical compounds. Inclusion compounds are formed as a result of the inclusion of one type of molecule in crystal-lattice cavities or molecules of another type. The entering molecules are referred to as foreign substances and the molecules or lattices entered are the host substances. The capacity of a substance to form inclusion compounds depends on the presence of cavities of molecular size. The included molecules, which must have a shape corresponding to that of the cavity, are held in place by intermolecular forces. Since it is not individual molecules but rather groups of molecules that are bound to one another in inclusion compounds, a whole-number ratio between the foreign and host substances, as a rule, does not exist. Inclusion compounds constitute a special type of complex.

A distinction is made between lattice inclusions and molecular inclusion compounds. In lattice inclusions, the cavities arise during the formation of a crystal lattice and sometimes only in the presence of the foreign substance. Lattice inclusions can be of the channel type (cavities in the shape of a channel) or the clathrate type (cavities in the shape of a cage). Molecular inclusion compounds are formed when cavities exist in individual molecules (hosts) of relatively low molecular weight. High-molecular-weight substances permit the formation of inclusion compounds owing to the cavities between macromolecular chains. Inclusion compounds are often formed by mixing and, less often, triturating the components. They are unstable and in solutions usually decompose to yield the original substances.

Inclusion compounds are used for the separation of mixtures. Urea, for example, permits the separation of unbranched hydrocarbons, which it binds in inclusion compounds, while thiourea binds branched hydrocarbons. Zeolites prepared with various given cavity dimensions are used industrially for the drying of gases and the separation of substances in ion-exchange processes. Methods for storing and separating many gases and low-boiling liquids are based on the capacity of these substances to form inclusion compounds (clathrates) with water. Inclusion is a means of protecting certain unstable compounds from oxidation in the air. Inclusion compounds are also used analytically, for example, in adsorption and partition chromatography and in the separation of nitrophenols and nitramines.