Rare Events Calculations of Methane Clathrate Hydrate Nucleation From Solution At Moderate Supersaturations

Clathrate hydrates are crystalline ice-like inclusion compounds in which water molecules form cages around hydrophobic guest molecules.  Methane clathrate hydrates form at high pressures and low temperatures and are found in abundance on the ocean floor and under permafrost.  Additionally, their formation in oil and natural gas pipelines can severely affect pipeline economics and safety.  Past computational studies aimed at elucidating the mechanism of hydrate nucleation used “brute force” molecular dynamics (MD) trajectories.  These trajectories are run at extremely high driving force (i.e. supersaturation) compared to driving forces that exist in nature.  We used a coarse-grained model of methane hydrates¹ to perform phase coexistence calculations and to compute supersaturations.  Then we used rare events methods to examine methane hydrate nucleation at moderate supersaturation.  These calculations enable us to estimate nucleation rates and draw conclusions regarding the reaction mechanism. 

References

[1] L.C. Jacobson and V. Molinero, “A Methane-Water Model for Coarse-Grained Simulations of Solutions and Clathrate Hydrates”, J. Phys. Chem. B, 2010, 114, 7302-7311.