The nucleation and growth of methane hydrates have been investigated by molecular dynamic simulations.
Gas hydrates are inclusion compounds — also known as clathrates — in which hydrogen-bonded water molecules are organized into cages that encapsulate gas molecules. These compounds hold great promises in diverse areas, including as gas-storage media and energy sources, but can also block oil and natural gas pipelines. Despite their far-reaching implications, however, their formation mechanism has remained elusive; rapid, localized nucleation processes are difficult to observe experimentally, and long induction times have hindered theoretical approaches.
Amadeu Sum, David Wu and co-workers at the Colorado School of Mines in the USA have now extended1 classic molecular dynamics simulations from nanoseconds to microseconds, and observed the trajectories of methane and water molecules during the formation of methane hydrates. Upon cooling and pressurizing a methane–water mixture, spontaneous hydrate nucleation and subsequent rapid growth occurred after roughly 1.2 μs. Sum, Wu and co-workers observed a cooperative organization of two methane and five water molecules. The two methane molecules, adsorbed on a pentagonal water face, retained their relative positions during the subsequent formation and dissolution of small water cages.
These small cages were in turn converted into larger ones, owing to steric constraints. The resulting hydrates showed a structure in which two main clathrates lattices co-existed, linked by cages of unusual shape.
References
Walsh, M. R., Koh, C. A., Sloan, E. D., Sum, A. K. & Wu, D. T. Microsecond simulations of spontaneous methane hydrate nucleation and growth. Science 10.1126/science.1174010 (2009).
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Pichon, A. A longer look. Nature Chem (2009). https://doi.org/10.1038/nchem.461
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DOI: https://doi.org/10.1038/nchem.461