Gas clathrate

Nada, H. (2006). Growth mechanism of a gas clathrate hydrate from a dilute aqueous gas solution A molecular dynamics simulation of a three-phase system. J. Phys. Chem. B, 110 (33,1, 16526-16534. 

Stem, L.A. Kirby, S.H. Circone, S. Durham, W.B. (2004). Scanning Electron Microscopy investigations of laboratory-grown gas clathrate hydrates. American Mineralogist, 89, 1162-1175. 

A (gas) clathrate hydrate is a crystalline compound which can be obtained by the formation of a hydrogen-bonded host lattice around one or more species of guest molecules. It s a pleasing thought to a crystallographer that when it snows on the outer planets it might snow gas hydrates. (Jeffrey and McMullan, 1967.)... 

Figure 2.5 Three cavities in gas clathrate hydrates (a) pentagonal dodecahedron (512), (b) tetrakaidecahedron (51262), (c) hexakaidecahedron (51264), (d) irregular dodecahedron (435663), and (e) icosahedron (51268).

Natural gas clathrate hydrates normally form either in the primitive cubic structure I, in the face-centered cubic structure II, or in the hexagonal structure H. 

Hydrate research has expanded substantially over the past decade, resulting in more than 4,000 hydrate-related publications. Collating this vast amount of information into one source, Clathrate Hydrates of Natural Gases, Third Edition presents a thoroughly updated, authoritative, and comprehensive description of all major aspects of natural gas clathrate hydrates. 

Provides a historical perspective of natural gas clathrate hydrates existing from 1790 to 2006... 

Noble gas clathrates will not now form on the Earth, as can be seen from the air pressure decomposition temperatures in Table 2.7. They might, however, form in cooler regions of the primitive solar nebula (see Limine Stevenson, 1985). Sill and Wilkening (1978) note that for pressures in a plausible model nebula, pure ice clathrates of Ar, Kr, and Xe could form at 40, 45, and 62 K, respectively. 

As with other compounds, solution effects can elevate the condensation temperatures of clathrate guest species. Sill and Wilkening calculated that in a gas of solar composition the major clathrate, and the first to form, will be ice-methane, and that noble gases can substitute for the methane at temperatures higher than decomposition temperatures for noble gas clathrates. They calculate, for example, that in a total nebular pressure of 2 x 10 atm (high in comparison with most model pressures currently considered of about 10 4 atm ), ice-methane clathrate at 80 K will have dissolved 99% of the available Xe (and substantially smaller amounts of the other noble gases). 

Table 2.7. Decomposition vapor pressure data for ice-noble gas clathrates...

The original X-ray studies of the gas clathrate hydrate structures were based on powder diffraction data, since the gas hydrates are notoriously difficult to obtain as single crystals. Clathrate hydrates formed by compounds which are liquids above 0°C, such as those of ethylene oxide and tetrahydrofuran, can be readily grown as large single crystals, and X-ray single crystal studies of both these hydrates have been carried out [787, 788]. 

Although inert-gas clathrates have been described, this compound is believed to be the first xenon charge-transfer compound which is stable at room temperatures. Lattice-energy calculations for the xenon compound, by means of Kapustinskii s equation, give a value 110 kcal. mole", which is only 10 kcal. mole" smaller than that calculated for the dioxygenyl compound. These values indicate that if the compounds are ionic the electron affinity of the platinum hexafluoride must have a minimum value of 170 kcal. mole". ... 

Solid hydrate research covers various classes of chemical compounds, which possess different importance and practical use. There are stoichiometric hydrates and those with varying water content as zeolitic hydrates. There are true hydrates and pseudohy-drates The latter contain water as hydroxide or hydroxonium ions or as -OH and -H groups ( water of constitution ). The true hydrates with separable H2O molecules ( water of crystallization ) include inorganic salts, i.e., the so-called salt hydrates, hydrates of organic compounds, and the clathrates, as, e.g., the novel gas clathrates. This article is mainly concerned with the salt hydrates. 

Answers to such difficult questions can be found in applied thermodynamics - in terms of measured, macroscopic values of pressures, temperatures, compositions, volumes, enthalpies, etc. This chapter provides an overview of natural gas clathrate hydrates - structures, phase diagrams, and thermodynamic predictions/measurements that guide our understanding in dealing with such questions. The hydrate historical perspective provides an example of how knowledge advances in a technical field. At the conclusion of the chapter, future thermodynamic challenges are presented. 

The best-known noble gas clathrates are hydrates, hydroquinone and phenol clathrates, which have found an increasing number of uses [131]. Clathrates may serve as convenient storage for noble gases. Because of the different affinity hydroquinone clathrate prepared from an equal mixture of krypton and xenon liberates 3 times the amount of Xe than Kr [132]. Clathrates are also of interest for nuclear technology. Radioactive isotopes of argon, xenon and krypton can more easily be handled in the compact form of a solid rather than in gas form [133-136]. 

Liquid clathrates, analogues of the gas clathrates but present in the liquid phase, are unusual two phase systems in which an upper layer of solvent lies above a denser layer of solvent saturated with ionic species. Some interesting examples are found in Atwood s work. Initially a salt, such as sodium chloride, is added to an aromatic solvent in which it is insoluble. Another reagent, an aluminium alkyl in one example, is added and the salt is solublized to form a dense phase [1] which may be interpreted as ... 

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