Critical nucleation of nanoconfined methane hydrate: Theory and molecular modeling

The thermodynamics of methane hydrate in porous media remains poorly understood with important questions left unanswered regarding its nucleation when confined at the nanometer scale. In particular, key aspects such as the shape and size of the critical nucleus in confinement challenge existing theories which fail to capture experimental observations. Here, a combination of molecular modeling tools and classical nucleation theory is used to predict the critical nucleus of methane hydrate confined within a slit-shaped nanopore. Owing to capillary/curvature effects and the associated Laplace pressure, the hemispherical nucleus at a single surface or bridge nucleus between two parallel surfaces are found to form at thermodynamic conditions that differ from that for a spherical nucleus in bulk solution. Using such thermodynamic modeling, we predict the temperature dependence of the critical nucleus size in nanoconfinement. Moreover, by estimating the energy barriers along different nucleation paths, we identify a crossover between surface nucleation through a hemispherical nucleus to confined nucleation through a bridge nucleus for methane hydrate in nanopores.