Dynamics Slowdown Induced by Gas Oversolubility in Nanoconfined Fluids

Oversolubility refers to the observation in nanoconfined liquids of significant gas solubilities that largely surpass the bulk solubility. While this thermodynamic effect is now well-documented, its impact on the dynamics of fluids confined in nanoporous materials has not been explored. Yet, by affecting adsorption and wetting at solid/liquid interfaces, oversolubility is expected to be a key phenomenon in separation and catalysis but also in geological applications such as pollutant migration in soils, carbon capture/storage in natural environments, and underground/atmosphere exchanges. Here, we employ atom-scale simulations and NMR experiments to show that gas oversolubility is expected in hydrated nanoporous materials and that it reduces both water and ion diffusivities [by 10% up to 60% depending on thermodynamic conditions]. Despite the complexity of adsorption/transport coupling in such gas/liquid/solid systems, we establish that diffusivities in the presence of small gases such as CO₂, CH₄ and H₂ can be rationalized by accounting for the increase in the confined fluid viscosity (which is found to be directly linked to the decrease in the free volume accessible to the liquid upon solubilization). Moreover, in agreement with the reported data, by invoking Stokes-Einstein relation between the viscosity and diffusivity, we predict that the dynamics slowdown is identical for the confined water molecules and ionic species. We also show that this oversolubility-induced dynamical effect becomes more pronounced as the strength of the molecular interactions between the solubilized gas and the liquid/solid increases. This approach provides a robust formalism to fluid diffusion in nanoconfined environments subjected to gas solubility and potential oversolubility effects.