Abstract
This work uses density functional theory (DFT) to investigate the poorly characterized structure II gas hydrates, for various guests (empty, propane, butane, ethane-methane, propane-methane), at the atomistic scale to determine key structure and mechanical properties such as equilibrium lattice volume and bulk modulus. Several equations of state (EOS) for solids (Murnaghan, Birch-Murnaghan, Vinet, Liu) were fitted to energy-volume curves resulting from structure optimization simulations. These EOS, which can be used to characterize the compressional behaviour of gas hydrates, were evaluated in terms of their robustness. The three-parameter Vinet EOS was found to perform just as well if not better than the four-parameter Liu EOS, over the pressure range in this study. As expected, the Murnaghan EOS proved to be the least robust. Furthermore, the equilibrium lattice volumes were found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has significant implications for the widely used van der Waals and Platteeuw thermodynamic model for gas hydrates. Also, hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be used predictively to determine the bulk modulus of various structure II gas hydrates. Taken together, these results fill a long existing gap in the material chemical physics of these important clathrates.
Highlights
Gas hydrates are crystalline solids, physically similar to ice, created by a network of hydrogenbonded water molecules which form cages that can entrap small gas molecules or volatile liquids.[1]
The equilibrium lattice volumes were found to increase with guest size, with double-guest hydrates showing a larger increase than single-guest hydrates, which has significant implications for the widely used van der Waals and Platteeuw thermodynamic model for gas hydrates
Hydrogen bonds prove to be the most likely factor contributing to the resistance of gas hydrates to compression; bulk modulus was found to increase linearly with hydrogen bond density, resulting in a relationship that could be used predictively to determine the bulk modulus of various structure II gas hydrates
Summary
Gas hydrates are crystalline solids, physically similar to ice, created by a network of hydrogenbonded water molecules which form cages that can entrap small gas molecules or volatile liquids.[1]. Gas hydrates are significant in many other areas, from flow assurance in the oil and gas industry, to carbon dioxide sequestration, global climate change, and planetology.[3,4,5,6,7,8] In virtually all of these applications, including hydrate formation and stability models, hydrate deposit detection and concentration estimation, their material properties are of critical importance Despite their importance, gas hydrate material properties are generally lacking or not well characterized in current literature, and our understanding of these unique structures at a fundamental level is still inadequate. The bulk modulus of structure I (sI) gas hydrate has been reported as being
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