Hydrate clathrates

Fig. 5. Constitution of a typical hydrate clathrate (tri-n-butylsulfonium ion as guest). The guests are accommodated within H-bond-mediated water polyhedrons (apexes are equivalent to water oxygens) (Adapted from Ref. 6)...

The same applies to the historic gas-hydrates (hydrate clathrates, Fig. 5)17,18). However, on principle, only such molecules are suited for inclusion into the complicated H-bridge networks of gas-hydrates which do not interfere with the H-bridges of water, but have a hydrophobic nature. More recent hosts related to this inclusion principle are given in Chapter 3 of this book. 

Hydrate clathrates of organic compounds are thought to be responsible for the behavior of ice in the heads of comets and in wet methane under pressure.22 Unless methane is carefully dried, high-pressure lines may become clogged with the ice-like gas hydrate. There may be large deposits of methane hydrates, the ice that burns. beneath the ocean floor. 

This is also the optimum size for guests in clathrate hydrates, confirming the close link between methane solutions and methane hydrate discussed by Glew (1962). Indeed the mechanism of solid hydrate clathrate formation may involve the formation of this structure in the aqueous solution (Ewing and Ionescu, 1974).,... 

Solvation, folding of biological molecules Gas hydrates (clathrates)... 

A consideration of thermodynamic properties of the aqueous solution of rare gases and hydrocarbons led to the iceberg model for water structure around nonpolar molecules [139], which later had to be abandoned (see Part IV, Chap. 23.4). The gas hydrate clathrate structures described in Part IV, Chap. 21 provided... 

The model of icebergs around nonpolar solute molecules in aqueous solution is clearly not a very realistic one. However, if solutions of hydrocarbons (or noble gases) are cooled, then the solid phase that sometimes separates out consists of a so-called gas hydrate (clathrate), in which water provides a particular kind of hydrogen-bonded framework containing cages that are occupied by the nonpolar solute molecules. Obviously, such gas hydrates (clathrates) represent more realistic models for the phenomenon of hydrophobic hydration [176]. 

Quinol (hydroquinone) crystal was the first composite to be called a clathrate. Nowadays, this term has been adopted for many complexes which consist of a host molecule (forming the basic frame) and a guest molecule (set in the host molecule by interaction). The clathrate that is of interest to this study is the clathrate hydrate, also referred to as gas hydrate. Clathrate hydrates were discovered in 1810 by Sir Humphrey Davy. In his lecture to the Royal Society in 1810, he said that he had found, by several experiments, that the solution of chlorine gas in water freezes more readily than pure water [9]. 

Fig. 17.1 The structure of the xenon hydrate clathrate. The xenon atoms occupy the centers of regular pentagonal dodecahedra of water molecules (cf. Fig. 8.8).

Gas hydrate clathrates are formed by cooling water in presence of the guest species. The conditions of existence of the hydrates are conveniently represented by means of a pressure-temperature diagram sueh as Fig. 2. The domain of stability of the hydrate is located on the left part of curves II and III. Three data must be known in order to construct this diagram ... 

Liquid hydrate clathrates are formed in conditions similar to those for gas hydrates. The corresponding physicochemical data are given in Table 1. Liquid... 

Steric criteria are also valuable to explain the formation of liquid hydrate clathrates (Fig. 3, 16.2.2.1). They appear for molecules having dimensions between the free diameters of the largest voids of types I and II structures, d, 2 and dn,2- The situation is the same for double liquid hydrates, but here a type I structure is theoretically possible in some cases, e.g., CHjBr, COS and CH3I. For molecules with dimensions >690 pm, no hydrates are formed. This selectivity in encaging certain molecules but not others has been used for fractionation of natural gas by clathration and for desalination of sea water "... 

Ice-like gas hydrates (clathrates) also have been found at numerous sites along the convergent plate margins (e.g., Suess et al. 1997). Methane hydrates are stable in solid form only in a narrow temperature-pressure window (Dickens and Quinby-Hunt 1994 Fig. 14.3 in chapter 14). In theory, Im of methane hydrate can contain up to 164 m of methane gas at standard conditions (Kvenvolden 1993). It has been estimated that the amount of carbon in gas hydrates considerably exceeds the total of carbon occurring in all known oil, gas and coal deposits worldwide (Kvenvolden and McMenamin 1980 Kvenvolden 1988). This raises the possibility that gas hydrates may be a future energy source of global importance. 

Hesse, R., and Harrison, W.E., 1981. Gas hydrates (clathrates) causing pore-water freshening and oxygen istope fractionation in deep-water sedimentary... 

R. Radhakrishnan and B. L. Trout, A new approach for studying nucleation phenomena using molecular simulations application to CO2 hydrate clathrates. J. Chem. Phys. 117 (2002), 1786-1796 Nucleation ofcrystalline phases of water in homogeneous and inhomogeneous environments. Phys. Rev. Lett., 90 (2003), 158301-158304 Nucleation of hexagonal ice (Ih) in liquid water. /. Am. Chem. Soc., 125 (2003), 7743-7747. 

Gas hydrates (clathrates) may technically be considered as an alternative form of ice that has the ability to entrap relatively large volumes of gas within cavities in the hydrate crystal matrix. The entrapped guest molecules (gas) stabilize the structure by means of van der Waals interactions, and combinations of the different unit cells give rise to structures I, II (175-177), and H (178). The most common gas to form gas hydrates is methane, but ethane, propane, butane, carbon dioxide, nitrogen, and many other types of gases may also give rise to gas hydrates. 

Smirnov, G.S., Stegailov, V.V. Toward determination of the new hydrogen hydrate clathrate structures. J. Phys. Chem. Lett. 4, 3560-3564 (2013)... 

The reaction of PH3 with H2O yields H3PO3, H3PO4, and H2 at elevated temperature and is already described in Phosphor C, 1965, p. 34. PH3 in air and in N2 forms only traces of H3PO4 when stored over water. The reaction becomes faster in the presence of moist oxide clay soils, but is still slow and incomplete [29]. The formation of the solid hydrate (clathrate) PH3 5.9 H2O under pressure is also already covered in Phosphor C, 1965, p. 49. The entropy change for the reaction PH3(g) + 6 H20(l)- PH3 6 H20(s) was estimated from the entropies of solidification and condensation of the two reactants to be ArS = -52.3 cal mol" K". An entropy change of ArS = -61.0 cal-mol" -K" was calculated from the reaction enthalpy ArH = -16.4 kcal/mol and published thermodynamic data [30]. 

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