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Molecular Behavior of Binary SF6-H2 Hydrate

Molecular Behavior of Binary SF6-H2 Hydrate
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Sulfur hexafluoride (SF6), one of the most potent greenhouse gases, is known as a hydrate former and has been studied at the high pressure up to 1.3 GPa with gas mixtures and with aqueous surfactant. Since we regard SF6 as a potential promoter molecule that can stabilize hydrate structure more effectively compare to the other promoters, further investigation is required to verify the stabilizing ability of SF6 in the hydrate structure. However, the insoluble nature of SF6 in water or gases hinders fine scale analyses. In this study, we discuss the data obtained by using molecular dynamics simulations of structure II (sII) clathrate hydrates containing SF6 and H2. The simulations were performed using the TIP4P/Ice model for water molecule and a previously reported SF6 molecular model (optimized at the pure SF6 single phase system (Olivet and Vega, 2007)), and a H2 molecular model (adapted from the THF+H2 hydrate system (Alavi et al., 2006)). The simulations are performed to observe the stability of SF6 and H2 in the sII clathrate hydrate system with varying temperature (273 and 300 K) and pressure (1, 10, 100, and 500 bar) conditions, and occupancies of SF6 and H2, which cannot be easily tuned experimentally. First of all, we calculated density profiles and F4 order parameters. The density profiles showed that hydrogen molecules were occupied in the gas hydrate cages at moderate temperature and pressure even with the low occupancy of promoter SF6. Additionally, to figure out the unexpected dissociations at several conditions, we calculated F4 order parameters. However, we couldn’t find the indication that can explain these unexpected dissociations. Finally, we identified cage types which enclathrate the guest molecules. In general, the number of large cages and small cages of structure II hydrate decreased or increased proportionally, whereas certain configuration assumed as a metastable phase didn’t. In summary, we examined the potential of SF6 as a promoting agent for gas hydrate formation using molecular dynamics simulation. This study is the first report of molecular behavior of SF6 in gas hydrate structure. We expect that this study contribute to designing new system for hydrogen storing clathrate. Further research on spatial F4 parameters and free energy calculations would clarify the behavior of SF6 and H2 in clathrate.
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