Clathrate hydrates cavities

Before proceeding, it is important to recall the significant feature which appears to distinguish the cluster model from the two other prominent mixture models—i.e., the broken-down ice lattice and the clathrate hydrate cage structures. The latter two theories allow for the existence of discrete sites in water, owing to the cavities present either in the ice... 

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).

Because it is impossible for all cavities to be occupied (an analog would be a perfect crystal) simple hydrates always have more water molecules than the ideal composition. Usually the ratios range from G -5(3/4) H20 to G 19H20, with typical fractional occupancies of the smaller cavities of 0.3-0.9, based on size restrictions. This variation causes clathrate hydrates to be called nonstoichiometric hydrates, to distinguish them from stoichiometric salt hydrates. 

Figure 7.3 Cavity types found in structures I, II and H clathrate hydrates and the numbers of each that goes into each structure unit cell along with number of water molecules and typical guests. Each vertex denotes a water molecule connecting lines represent hydrogen-bonded contacts.

Assuming the 512 cavities in structure I clathrate hydrates to be approximately spherical (radius 3.91 A), calculate the occupancy factor and notional pressure for a methane molecule included in such a cavity. How does this compare with a 51262 cavity (radius 4.33 A) Which cavity do you think methane is most likely to occupy ... 

Because in the cyclodextrin hydrates the cavity is occupied by water molecules, they can be considered as the inverse of the clathrate hydrates discussed in Part IV, Chapter 21. In these, the water molecules form the host structure and the organic molecule is the guest. 

In fact, more recent detailed studies of H2-THF clathrate hydrate directly contradict the findings of Lee et al. [20]. Strobel et al. [21] similarly utilized volumetric measurements in conjunction with Raman and NMR data for the binary system of H2-THF. They concluded that small cavities can only accommodate single H2 molecules, and that, irrespective of initial aqueous THF concentration and/or formation conditions, large cavities are always fully occupied by THF. They reported, therefore, that a maximum hydrogen content of around 1 mass% was possible for pressures less than 60 MPa, a value considerably lower than that claimed by Lee et al. [20]. The result of such contradictory findings is that the true phase behavior of binary H2-TH F hydrates remains to be identified through future studies on the topic. 

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen. /. Phys. Chem. B, 112, 1885. 

Figure 7. The potential energy curves of a propane molecule in a large cavity of the clathrate hydrate II around three orthogonal axes. Solid line x-axis, dashed line y-axis, dash-dot line z-axis, dotted line spherical guest.

Clathrate hydrates form when small (<0.9 nm) non-polar molecules contact water at ambient temperatures (typically <3(X) K) and moderate pressures (typically >0.6 MPa). On a molecular scale, single small guest molecules are encaged (enclathrated) by hydrogen-bonded water cavities in these non-stoichiometric hydrates. Guest repulsions prop open different sizes of water cages, which combine to form three well-defined unit crystals shown in Figure 1. 

When all hydrate cavities are filled, the three crystal types (I, n, and H) have remarkably high and similar concentrations of components 85 mol % water and 15 mol % guest(s). Hydrate formation is most probable at the interface of the bulk guest and aqueous phases because the hydrate component concentrations greatly exceed the mutual fluid solubilities. The solid hydrate film at the interface acts as a barrier to further contact of the bulk fluid phases, and fluid interface renewal is required for continued, rapid clathrate formation. Three phase interfacial hydrate formation occurs with equilibrium of gas, liquid, and hydrate phases in artificial situations such as laboratories or in man-made processes. 

Clathrate hydrates (known also as gas hydrates) belong to a large class of crystalline, non-stoichiometric, inclusion-compound materials that are stable within a certain range of pressure and temperature. The host solid framework structure is made up of water molecules, connected through hydrogen bonds that form cavities (cages) . The cavities can be stabilized by the inclusion of small molecules such as CH4, CaHg, CO2, N2, Ar, etc. Over 100 different molecules are known to form hydrates. 

Clathrate hydrates are crystalline inclusion compounds in which the host framework is formed by water molecules linked through hydrogen bonds. The cavities of the framework are occupied by guest molecules of appropriate size and shape. Hydrates composed of gas molecules as guests and water molecules as the host are called gas hydrates. As a rule, there are only van der Waals interactions between the guest and host subsystems. 

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