Clathrate hydrates properties

Sum, A., Measurements of Clathrate Hydrate Properties Via Raman Spectroscopy, M.S. Thesis, Colorado School of Mines, Golden, CO (1996). 

Sugahara, K., Tanaka, Y., Sugahara, T., Ohgaki, K, J. Supramol. Chem. 2, 365 (2002). Sum, A., Measurements of Clathrate Hydrate Properties Via Raman Spectroscopy, M.S. Thesis, Colorado School of Mines, Golden, CO (1996). 

Finally, for aqueous nonelectrolyte solutions much of the available evidence suggests the involvement of discreteness in the water structure in determining the properties of such mixed solvents. This is consistent with a mixture model, especially a clathrate hydrate model. 

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

Clathrate hydrates are inclusion compounds formed by the enclosure of a small guest molecule within a hydrogen bonded cage of solid-state water. Clathrate hydrates are co-crystals and are thus distinct from ice, which is made of pure water, and hence can have different physical properties to ice such as a different melting point. The classic example of a clathrate hydrate is the burning snowball of methane clathrate hydrate. The combustion of the methane in the clathrate is self-sustaining, Figure 7.1. Many... 

Solid clathrate hydrates are formed under very specific conditions of temperature and pressure. It is one of their most important and remarkable properties that they are often stable solids well above the 0 °C melting point of the most common form of pure ice (symbol Ih, meaning ice-hexagonal). Indeed, some gas hydrates are stable up to 31.5 °C. This property was noted by Davy, who commented on his chlorine (then called oxymuriatic gas) hydrate ... 

N.J. English, J.M.D. Macelroy, Structural and dynamical properties of methane clathrate hydrates. J. Comput. Chem. 24, 1569-1581 (2003)... 

Many gases, such as Ar, Kr, Xe, N2, O2, CI2, CH4 and CO, can be crystallized with water to form ice-like clathrate hydrates. The basic structural components of these hydrates are the (H2O)20 pentagonal dodecahedron and other larger polyhedra bounded by five- and six-membered hydrogen-bonded rings, which can accommodate the small neutral molecules. The inclusion properties of water imply that such polyhedra are likely to be present in liquid water as its structural components. 

Where explanations of the properties of the liquid mixtures have been couched in terms of either clathrate-hydrate structures or... 

Gas hydrates usually form two crystallographic stmctures - stmcture Type I and structure Type II. Rarely, a third structure may be observed (Type H). Figure 3.1 shows these three stmctures of clathrate hydrates and Table 3.1 presents their stmctural properties. 

Table 1. Some properties of unit cells of clathrate hydrate I and II.

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

Air bubbles and clathrate hydrates in polar ice cores have attracted considerable interest because they provide the most direct record of past atmospheric gas compositions (e.g., Raynaud et However, the processes of air clathration in polar ice sheets should be taken into account when considering gas analyses. It is known that extreme fractionation of gases in air inclusions occurs when air bubbles change into clathrate hydrates by the diffusive mass transfer of air molecules between bubbles and hydrates. To estimate the effects of hydrate formation on the distribution of atmospheric gases in deep ice, it is essential to understand the structure and physical properties of natural air hydrates. 

In this study we performed experiments to investigate the incorporation rate of gas molecules in hydrates and the formation rate of clathrate hydrates from a liquid water phase in absence and in presence of a free gas phase. In case of a present free gas phase we observed the hydrate formation process in an optical cell. We also analysed the gas composition of the gas which was encased in the hydrate after the decomposition of the hydrates (Figure 2, Table 3). Due to the fact that the aim of this study was to investigate the influence of molecular properties of the guest molecules such as dimension (— diffusivity) and water solubility (—> concentration/fugacity) we focused our observation on the content of isomers which do have the same molecular weight but different molecular dimensions (see Table 2) and solubilities. 

Davidson, D. Gas hydrates as clathrate ices. In Natural Gas Hydrates—Properties, Occurrence and Recovery, Cox, J., Ed. Butterworth Woburn, MA, 1983 I-I6. 

H20). The properties of the three main structural types and cages are summarised in Figure 7.3. The bromine clathrate hydrate is unique in being the only one to form a 15-hedral cage, which generally has an unfavourable strain relative to the other three cavities. 

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