Predicting the Solubility and Diffusivity of Gases (CO2, CH4, H2, Noble Gases) in Nano-Confined Interlayer Water and Bulk Water Using Molecular Dynamics Simulations

The chemistry of water films with dissolved gases confined between clay mineral surfaces has applications for a number of areas in energy and environment. For example, gas adsorption in solid-water interfaces impacts the formation of CH₄ hydrates in fine-grained sediments, the behavior of CH₄ in shale, CO₂ leakage across caprocks of geologic CO₂ sequestration sites, H₂ leakage across engineered clay barriers of high-level radioactive waste repositories, and the use of noble gases as tracers for subsurface hydrocarbon migration. Here, we present molecular dynamics (MD) simulations of the solubility and diffusivity of a range of gases (CH₄, CO₂, H₂, noble gases) in bulk water and in nano-confined interlayer water in clays. Our results indicate that the affinity of dissolved gases for clay mineral surfaces has a non-monotone dependence on the hydrated radius of the gas molecules. This non-monotone dependence arises from a combination of two effects: the enthalpic affinity of the gas solutes for the surrounding species and the entropic penalty of hydrophobic solvation.