Methane Clathrate Hydrate Nucleation Mechanism by Advanced Molecular Simulations
The nucleation mechanisms of methane hydrates are studied using well-tempered metadynamics and restrained molecular dynamics. The collective variables we used to follow the process are the methane-methane and methane-water coordination numbers, from which we computed the corresponding Landau free energy surface. This surface is characterized by two minima, corresponding to the two-phase methane bubble/water solution and clathrate crystal, and a transition state. The clathrate crystal is of type II, while in the simulation conditions (T = 273 K and P = 500 atm) the most stable phase should be type I. We constructed the steepest ascent/descent path connecting the two-phase methane bubble/water solution to the clathrate state and passing through the transition state. We interpret this path as the nucleation path, which shows four phases. First, the concentration of solvated methane increases in the aqueous domain via diffusion through the methane-water interface. Second units of methane molecules solvated in water meet to form an unstructured cluster. Third, the water content of the nucleus decreases to a value compatible with the type II methane clathrate hydrate composition. Finally, a reordering process of solvated methane and water molecules occurs in a manner consistent with the "blob" hypothesis (Jacobson, L. C.; Hujo, W.; Molinero, V. J Am. Chem. Soc. 2010, 132, 11806-11811).