Aqueous Methane in Slit-Shaped Silica Nanopores: High Solubility and Traces of Hydrates

Equilibrium molecular dynamic simulations were employed to investigate the methane solubility in water confined between two parallel silica surfaces. The solid substrate was obtained from beta-cristobalite; all nonbridging oxygen atoms were protonated. The resultant surface density of OH groups was similar to 4.54 sites per nm(2). The simulations were conducted at constant temperature, 300 K, and at increasing bulk methane pressure for pores of width 1.0 nm. For bulk systems, these thermodynamic conditions are outside the window of methane hydrates stability. Methane solubility in confined water was found to far exceed that in bulk systems. The increase in tangential pressure, observed under confinement, cannot solely explain the marked increase in solubility predicted by our simulations. Most likely, the structure of confined water favors the sequestration of methane. The excess chemical potential for methane was found to significantly decrease within the confined water compared with that in the bulk phase. On the basis of the cage adsorption hypothesis for hydrate nucleation, the predicted solubility of methane in the confined water (up to similar to 0.05 mol fraction) is large enough to suggest the possible formation of methane hydrates. Indeed, analysis of simulation data shows the presence of amorphous cages of hydrogen-bonded water that host a single methane molecule. Within the limits of our simulations, these amorphous cages last for only short times. Perhaps the pores considered are too narrow to allow the formation of stable methane hydrates, and perhaps longer simulations would allow us to observe the formation of a hydrate nucleus. The large methane solubility in confined water predicted by our simulations might have consequences for hydraulic fracturing and other technological processes.