Abstract:
Methane gas, being a hydrophobic hydrocarbon, has low solubility in water. Yet at moderately high pressures and low temperatures, such as those occurring at the ocean floor or in oil and gas pipelines, these two species crystallize together into a molecular lattice of hydrogen bonded water molecules with polyhedral cages containing individual methane molecules. Given the ubiquity of these two compounds, these methane “clathrate hydrates” are found in enormous quantities on earth and form easily in pipelines, and thus have significant energy and environmental implications. In addition to being a generic behavior of water and hydrophobic molecules, gas hydrates have been of scientific interest as a model system for multicomponent multiphase nucleation. However, the nucleation of methane hydrates has been difficult to study, since it is a rare stochastic event that occurs in nanoseconds at a random location on a nanometer length scale. By analyzing ensembles of large-scale microseconds molecular dynamics simulations, we captured the mechanism of spontaneous hydrate nucleation, revealing an intricate cooperative dance between methane and water molecules. These observations and resulting insights overturned leading hypotheses for methane hydrate formation, and have recently led to the discovery of surprising new order-transmission mechanisms for heterogeneous nucleation in general.