GAS HYDRATES - REVIEW OF PHYSICAL CHEMICAL PROPERTIES/

An overview is provided of time-independent physical/chemical properti es as related to crystal structures. The following two points are illu strated in this review: (1) Physical and chemical properties of struct ure I (sI) and structure II (sII) hydrates are well-defined; measureme nts have begun on sH. Properties of sI and sII are determined by the m olecular structures, described by three heuristics: (i) Mechanical pro perties approximate those of ice, perhaps because hydrates are 85 mol % water. Yet each volume of hydrate may contain as much as 180 volumes (STP) of the hydrate-forming species. (ii) Phase equilibrium is set b y the size ratio of guest molecules within host cages, and three-phase (L-W-H-V) equilibrium pressure depends exponentially upon temperature . (iii) Heats of formation are set by the hydrogen-bonded crystals and are reasonably constant within a range of guest sizes. (2) Fundamenta l research challenges are (a) to routinely measure the hydrate phase ( via diffraction, NMR, Raman, etc.), and (b) to formulate an acceptable model for hydrate formation kinetics. The reader may wish to investig ate details of this review further, via references contained in severa l recent monographs.