Clathrate Hydrates

Abstract

The term “gas hydrate” has been applied for nearly a century to the solids which are formed by the combination of many gases and volatile liquids with a large excess of water. Their study goes back at least to 1810 when Humphry Davy, in the Bakerian lecture⁽³⁸⁴⁾ to the Royal Society, reported that an aqueous solution of oxymuriatic acid gas (which he suggested be renamed chlorine) froze more readily than water itself. In 1823 Faraday found⁽⁴⁷⁰⁾ the composition of the solid formed to be roughly Cl₂ • 10H₂O but recognized the possibility that incomplete drying of the crystals or loss of gas during the analysis could have led to underestimation of the chlorine content. This was indeed the case, and Faraday’s work illustrates the difficulties inherent in direct analysis of the gas hydrates which, along with the general assumption of simple whole-number stoichiometry, led to considerable controversy about the compositions of the 40 or so gas hydrates reported in the ensuing 100 years. The period up to 1925 is the subject of a detailed review by Schroeder.⁽¹²⁰⁶⁾ There was much speculation about the nature of the forces uniting the gas and water molecules. These were early recognized to be much weaker than those of primary chemical bonds. By 1880 it was clear that the vapor pressure of the hydrate system was in-dependent of its water content over a wide range and depended on temperature alone.^(697,1361) Pressure-temperature measurements along the three-phase equilibrium lines played an important role in Roozeboom’s elaboration⁽¹¹⁵⁷⁾ of the phase rule for heterogeneous equilibria during the period between 1884 and 1887.