P3:GAS-WATER INTERACTIONS IN GAS HYDRATES

Werner F KUHS
GZG Abt.Kristallographie, Universität Göttingen, GERMANY

    A general picture of the interactions between gas and water molecules will be drawn as it emerges from the established crystal structures. Notably, the cubic structure sI is the best polyhedral solution to the so-called Kelvin problem which tries to divide space with minimal partitional area. Thus, this structure can be considered as the arrangement with a maximal contact of gas molecules to the water polygons with a minimum amount of water molecules. In other words, gas hydrates structures can be considered as the result of a systematic hydrophobic hydration in which the water molecules arrange around gas molecules without sacrificing H-bonds. Attractive van der Waals interactions as well as excluded-volume effects strongly influence the physical properties of gas hydrates. Thermal conductivity, thermal expansion and compressibility show that guest-specific signatures exist. Understanding these interactions will lead to better predictions of the thermodynamic properties of gas hydrates. Bridging the gap between the entropically driven low solubility of most gases in liquid water and the systematic hydrophobic hydration in crystalline gas hydrates is a crucial point for nucleation and growth of gas hydrates from liquid water and of considerable importance for questions related to flow assurance. Finally, some intriguing questions concerning the nature of the gas hydrate interface with water and silicates will be addressed which play a central role in the formation of natural gas hydrates in sedimentary matrices.