By employing atomistic molecular dynamics (MD) simulations, we first explored the behavior of pure water and hydrogen gas within kaolinite nanopores. Our investigations focused on slit-shaped pores of 10 and 20 Å, where we found that pure water develops pronounced hydration layers. Molecular hydrogen preferentially resides in these hydration layers, resulting in solubilities that can reach up to 25 times higher than those observed in bulk water. Notably, hydrogen accumulation is pronounced near the siloxane surface, which experiences significant fluctuations in water density. Conversely, the dense hydration layer at the gibbsite surface exhibits a depletion of hydrogen. A key finding is that while confinement impedes water mobility, hydrogen diffusion is enhanced in narrower pores, which can be attributed to these density fluctuations.
To examine the influence of salinity, we also investigated NaCl brines at concentrations ranging from 5% to 15% within 10-Å pores. Our results indicated non-homogeneous ion distributions, with Na+ cations demonstrating a preference for the siloxane surface and accumulating in regions characterized by lower water density. In contrast, Cl- anions are primarily found within hydration layers adjacent to the gibbsite surface. Ion pairing is notably more significant in confined environments compared to bulk solutions under analogous conditions. The presence of ions markedly modifies the properties of confined water, with variations in salinity affecting the stability of hydrogen bonds and the propensity for density fluctuations near kaolinite surfaces. The observed diffusion coefficients exhibit non-monotonic trends with respect to salt concentration, likely linked to the tendency for ion pairing, with cations displaying higher mobility than anions—contrary to trends seen in bulk brines.
While MD simulations provide detailed insights into the atomistic behavior of involved species, scaling up the system size presents considerable computational challenges. To overcome these constraints, our ongoing work employs Kinetic Monte Carlo (KMC) simulations to extrapolate these MD findings to larger systems. The implications of these multi-scale results are significant for understanding hydrogen permeability in UHS sites, with potential applications extending to other geo-energy initiatives, such as carbon capture and sequestration (CCS).