Clathrates, noble gases

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 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]. 

The historically first experiment of this kind for clathrates was an attempt to synthesize a noble gas clathrate XegGe46 by chemical transport of Ge with GeLt under high Xe gas pressure. While the existence of the noble gas clathrates is still under discussion, the first inverse clathrate IgGe43.33I2.67 [95] was obtained in this way. The technique was also successfully applied for the preparation of phases such as GesgAsgIg [18]. Later it was developed into a fruitful synthetic tool with the possibility to obtain well-developed single crystals [19, 20]. 

What about neutral helium-containing compounds Atomic helium has been trapped inside dodecahedrane and buckminsterfullerene, analogous to other endohe-dral species to be discussed later. Are the resulting species C2oH2oHe and CeoHe organohelium compounds, or are they better understood as related to the long-known noble gas clathrates ... 

Probably the most familiar of all clathrates are those formed by Ar, Kr and Xe with quinol, l,4-C6H4(OH)2, and with water. The former are obtained by crystallizing quinol from aqueous or other convenient solution in the presence of the noble gas at a pressure of 10-40 atm. The quinol crystallizes in the less-common -form, the lattice of which is held together by hydrogen bonds in such a way as to produce cavities in the ratio 1 cavity 3 molecules of quinol. Molecules of gas (G) are physically trapped in these cavities, there being only weak van der Waals interactions between... 

Strictly speaking, the title noble-gas chemistry should be an oxymoron. But the noble gases are not literally and completely noble in the sense that they fail entirely to interact chemically with other forms of matter. Under appropriate conditions in the laboratory they can form real compounds with other elements, although there is no evidence that actual noble-gas compounds are relevant in cosmochemistry (possibly excepting ice clathrates in comets). StiU, planetary materials do contain noble gases that were somehow incorporated into them, and at least some of these appear to have involved some form of chemical interaction. The issue of chemical interactions is a venerable topic in noble-gas cosmochemistry, but there are still questions that have been unanswered for a long time. 

If an aqueous solution of hydroquinone is cooled while under a pressure of several hundred kilopascals (equals several atmospheres) of a noble gas [X = Ar. Kr. Xe], a crystalline solid of approximate composition [QHjtOHI JjX is obtained. These solids are /)-hydroquinone clathrates with noble gas atoms filling most of the cavities. Similar noble gas hydrates are known (Fig. 17.1). These clathrates are of some importance since they provide a stable, solid. source of the noble gases. They have also been used to eftect separations of the noble gases since there is a certain selectivity exhibited by the clathrates. 

Hydrates of Ar, Kr, and Xe were first synthesized by Villard in 1896 [141]. They were further studied, as well as hydrates of krypton and xenon, by de Forcrand [142]. Several structures for noble gas hydrates are known [143-146]. All the hydrate structures are different from that of ordinary hexagonal ice. In the two fundamental structures, the water molecules form pentagonal dodecahedra which are stacked with different degrees of distortion from their ideally regular forms [146]. The two types of structures are shown in Fig. 26a and 26b [140]. One structure contains 46 water molecules in the unit cell with 2 small and 6 larger cavities. The other structure has 136 water molecules in the unit cell with 16 small and 8 larger cavities. The formation of the two fundamental types of hydrates depends mainly on the size of the guest species. More detailed data for the two principal clathrate hydrate structures are available from the literature [147]. 

Of particular interest is the effect of noble gases in biological systems. For example, xenon has an anesthetic effect. This is somewhat surprising in that the conditions present in biological systems are obviously not sufficiently severe to effect chemical combination of the noble gas (in the ordinary sense of that word). It has been proposed that the structure of water might be altered via a clathrate-type interaction. 

Until 1962 only physical inclusion compounds were known. Argon, krypton, and xenon form cage or clathrate compounds with water (clathrate hydrates) and with some organics such as quinol. The host molecules are arranged in such a way that they form cavities that can physically trap the noble gas atoms, referred to as guests. The noble gas will be released upon dissolution or melting of the host lattice. 

The guest atoms or molecules exchange with the host lattice only weak intermolecu-lar forces and no specific chemical bonds. Clathrates are known with noble gas atoms as guests. 

Chemically, radon is a noble gas. As such, it is colorless, odorless, and almost chemically inert. Although radon is not chemically active, it is interesting to note that radon is not a totally inert gas either. Studies on radon chemistry have been reported in which compounds such as clathrates and complex fluorides have been formed. Compared with the other noble gases, radon is the heaviest and has the highest melting point, boiling point, critical temperature, and critical pressure. 

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