Clathrate-hydrate cage structures

Clathrate-Cage Model. The final water model which is of major interest is based on clathrate hydrate cage structures. It was originally proposed by Pauling (116), who noted the existence of clathrate hydrates of many inert gases and suggested, by analogy to the chlorine hydrate,... 

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... 

The importance of carefully considering anomalies when studying the behavior of solute-solvent interactions has been stressed. For aqueous solutions, many anomalous results presented in the literature suggest the existence of some type of discreteness in water structure. Discreteness is consistent with a view of water structure providing distinct sites such as those found in the models of water, implying a broken down ice lattice structure or clathrate hydrate cage-like structures. 

CO2 hydrate is perhaps one of the best understood clathrate hydrate regarding structural, thermodynamic, as well as other physical properties. Figure 7(a) and (b) show the CO2 molecules with full symmetry in small and large cages in cubic structure I analyzed by the Rietveld refinement with the direct-space technique. In the large cage, the best... 

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

Structure identification and relative cage occupancies. The hydration number and relative cage occupation for pure components and guests were measured by Sum et al. (1997), Uchida et al. (1999), and Wilson et al. (2002). Raman guest spectra of clathrate hydrates have been measured for the three known hydrate crystal structures si, sll, and sH. Long (1994) previously measured the kinetic phenomena for THF hydrate. Thermodynamic sl/sll structural transitions have been studied for binary hydrate systems (Subramanian et al., 2000 Schicks et al., 2006). 

A help gas stabilizes double and mixed hydrates. There is some uncertainty whether these inclusion hydrates are in fact stable without a guest species in the small 512 cages. All the clathrate hydrate structures are known to be stabilized and their melting points raised by including H2S with the guest compounds, which for this reason is called the helpgas [782-786]. Since we know of no experiments where the gas hydrates were prepared in the absence of air or some other gas, this help-gas phenomenon may occur to a more or less degree in all laboratory... 

Udachin, K.A., Ratcliffe, C.I., and Ripmeester, J.A. (2001) A dense and efficient clathrate hydrate structure with unusual cages. Angew. Chem. Int. Ed., 40, 1303. 

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