Mechanisms and Nucleation Rate of Methane Hydrate by Dynamical Nonequilibrium Molecular Dynamics

We investigate the effects of high solvated-methane concentration on methane-hydrate nucleation at 250 K and 500 atm. We consider solutions at four levels of methane molar fraction in the initial H2O-CH4 solution, chi CH4 = 0.038, 0.044, 0.052, and 0.058, which are higher than (metastable) bulk supersaturation. chi CH4 is controlled independently of the temperature and pressure thanks to the use of special simulation techniques [Phys. Chem. Chem. Phys. 2011, 13, 13177]. These conditions mimic a possible increase of local methane concentration beyond supersaturation induced, for example, by freeze concentration or thermal fluctuations. The nucleation mechanism and kinetics are investigated using the dynamical approach to nonequilibrium molecular dynamics. We demonstrate a hydrate-forming/-ordering process of solvated methane and water molecules in a manner consistent with both the blob hypothesis and cage adsorption hypothesis: the system initially forms an amorphous nucleus at high methane concentration, which then gets ordered, forming the clathrate crystallite. We evaluate nucleation rates using both the methods of the mean first-passage time, i.e., the curve of the average time the system takes to reach a crystalline nucleus of given size, and survival probability, i.e., probability that up to a given time the system has not nucleated yet. We found a dependence of the nucleation rate on initial methane concentration of a form consistent with the dependence of classical nucleation theory rate on supersaturation and determined the relevant parameters of this relation. We found a very rapid increase of nucleation rate with solvated-methane concentration, proving that methane molar fraction, even beyond bulk supersaturation, is key at triggering the homogeneous nucleation of clathrate. We derive a kinetic equation that allows for estimation of the nucleation rate over a wide range of concentration conditions.