Clathrate decomposition temperature

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). 

The data on the decomposition temperatures of clathrate hydrates of pure hydrogen within the pressure range 90 - 360 MPa were already published in the graphical form. The digital data summarized from these experiments are shown in Table 1. 

Table 1 Decomposition temperatures for clathrate hydrogen hydrates at hipressure.

Clathrate Starting material Condition of decomposition Temperature range (°C) Reaction time (days)... 

Some data on the phases, forming in these systems, have been obtained (Table III). The difference in the pressures of the evolving gas for the DTA method(a sealed ampoule) and TG-analysis (a labyrinth sample holder) accounts for the difference in the decomposition temperatures for [NiBi X2] and [NiB X2] B. X-Ray powder diffractograms showed that Ni- and Co-clath-rates are isostructural and differ from Zn and Cd-clathrates, differing from each other. 

In addition to temperature and pressure, the stability of the clathrate hydrate phase depends strongly on the size and chemical nature of the guest species. Table 4 shows the decomposition temperature at a pressure of 100 kPa of the clathrate hydrates with a number of typical guest molecules. The decomposition temperature at this pressure spans a temperature range of 130 K. 

The family of true clathrates based on hydrocarbons only is further enriched by the inclusion compounds of 48 with benzene (1 1) and p-xylene (2 1) 90> (Table 19). Figure 28 illustrates the structure of 48 benzene (1 1). The structure of the p-xylene clathrate shows intercalated guest molecules at centers of symmetry in the crystal lattice. Both clathrates are rather unstable at ambient temperatures and decompose easily, e.g. on exposure to X-rays (even in the presences of mother liquor). The 48 p-xylene clathrate is unstable to such a degree that decomposition occurs at low temperature. 

The model is formulated on the premise that the decomposing hydrate particle is surrounded by a cloud of the product gas hence the driving force for the decomposition process is expressed in terms of the fugacity difference given in Eq. (1). The process of decomposition possibly involves (1) destruction of the clathrate host (water) lattice at the surface of the particle, and (2) and desorption of the guest (hydrate former) molecules from the surface. The particle size distribution was incorporated in the calculations for the determination of the intrinsic rate constants.The following Arrhenius type equation is used to represent the effect of temperature on the intrinsic rate constant ... 

A different problem arises when simple monomers like ethylene and propylene are subjected to polymerization. All efforts made so far have been unsuccessful, mainly because the correct conditions for inclusion were not known and for the widely diffused erroneous opinion that these molecules cannot act as guests for structural reasons. We directed our work to a knowledge of the decomposition curve of their PHTP clathrates by assuming that the inclusion compound exists in the temperature range in which the equilibrium pressure of the solid phase is lower them that of the pure monomer. 

Table 7.2 Compositions and superconducting transition temperature Tc) of silicon clathrate compounds obtained by decomposition of Na2BaSi4 at different temperatures [14]...

The publication of this series, interestingly enough, coincides with the 200th anniversary of the first report of the synthesis of a clathrate hydrate, that of chlorine, by Humphrey Davy. He reported that a solution of chlorine in water froze at a temperature higher than the ice melting point. Upon decomposition, this unique new material returned unchanged to the starting materials. Clathrate hydrates were likely the first synthetic materials that could be classified as supramolecular. 

The complete characterization of clathrate hydrates is a difficult task. The materials are crystalline however, they are nonstoichiometric. Sample handling must always be carried out where hydrate decomposition is negligible, for example, at low temperatures or at pressures where the hydrate is stable. 

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