Information About Clathrate

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What does Clathrate mean? What are the characteristics, composition, features and uses of Clathrate? Information on Clathrate.

clathrate

CLATHRATE, a type of inclusion compound in which one component ( the “guest” ) is completely enclosed within the crystalline lattice structure of the other component ( the “host”). The name is derived from the Latin word clathratus, meaning enclosed or protected by a grating or cage; it was proposed in 1948 by the British chemist H. M. Powell, who performed much of the fundamental work on the structure of clathrates.

The usual laws of chemical stoichiometry are not applicable in the formation of clathrates because the two components do not react chemically with each other. Clathration does not result from formation of chemical bonds; at most, the interaction between guest and host is a weak one involving van de Waals’ forces or dipole attractions. In keeping with their complex nature, clathrates possess some properties of the host molecule, some of the guest molecule and some resulting from mutual effects of the two.

The primary factor determining clathrate formation is a spatial and geometric one. The guest molecule must not be so large that it cannot be accommodated in, nor so small that it can escape from, the enclosure formed by the crystalline framework of host molecules. In addition to possessing the proper shape, the guest must also be properly oriented at the moment the host molecules are crystallizing, or it will fail to be trapped within the cage, and empty holes will result.

The host molecules are linked together in such a manner that an open, rather than a close-packed, structure results. The guest molecules cannot escape from their positions in the lattice unless the strong forces holding the surrounding molecules together are overcome. Thus the presence of a volatile guest component in a clathrate cannot be detected by odor because none of its molecules can escape. However, the guest may be released by simple means such as heating, dissolving, or sometimes by merely grinding the clathrate, which is always a single-phased crystalline solid.

In 1849 the German chemist Friedrich Wohler prepared the first clathrate compound, by the reaction of hydrogen sulfide (guest) with hydroquinone (host). Other investigators later found that hydroquinone formed similar compounds with certain volatile molecules such as sulfur dioxide, hydrogen cyanide, and hydrogen chloride, and with argon, krypton, and xenon, which are inert gases and rarely form chemical compounds. The hydroquinone clathrates contain one molecule of the guest for each three host molecules.

Next in importance to the hydroquinone clathrates are the “gas hydrates,” in which water is the host and the guests are gases or volatile liquids. These hydrates, which are crystals witii low melting points, can be divided into two groups: those with small guest molecules, such as chlorine, hydrogen sulfide, or methane, have 6 guest molecules to 46 host molecules; and those in which the guest components are slightly larger molecules, such as methyl iodide, chloroform, or propane, contain 17 host molecules for each guest molecule. In both hydroquinone and gas hydrate clathrates the host molecules are linked together through hydrogen bondiiig. Clathrates may be formed with many other substances as hosts.

Clathrates are generally prepared by a relatively simple process—slow crystallization from a solution of the components. In cases where the guest component is a gas, high pressures and low temperatures may be required to maintain the maximum ratio of guest to host molecules.

Since clathrate formation requires a highly selective fitting together of guest and host, it has been used, particularly in the petroleum industry, to carry out separations, purifications, identifications, and analyses that are difficult to accomplish by other means. For example, benzene has been purified from its major contaminant, thiophene, by selective clathration.

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