Supercritical Carbon Dioxide at Smectite Mineral-Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Investigation of CO2/H2O Mixtures Nanoconfined in Na-Montmorillonite

The carbon dioxide (CO2) retention capacity and adsorption/desorption energetics of layered nanoporous oxide materials depend critically on the hydration level and the nature of molecular interactions among H2O, CO2, charge-balancing cations, and the oxide/hydroxide layers. Molecular-scale understanding of the structure, dynamics, and interfacial energetics of H2O/CO2 binary mixtures confined in the interlayer nanopores is paramount to geological CO2 storage efforts in clay-rich materials. This Article investigates the effects of supercritical CO2 (scCO(2)) in the hydrated interlayer galleries of the hydrophilic smectite mineral (Na-montmorillonite) under geochemically relevant conditions using classical molecular dynamics simulations and enhanced sampling free energy methods. For the compositions investigated, the interactions among the cations, intercalated fluid species, and the basal surfaces result in structures with H2O and CO2 coexisting in a single layer at the center of the interlayer. The water molecules in this central H2O/CO2 layer cluster around and hydrate Na+ ions desorbed from the basal surfaces, whereas CO2-CO2 hydrophobic interactions favor mutual clustering of CO2 molecules. This arrangement results in dynamic percolation paths that facilitate single file-like anisotropic lateral diffusion of CO2. The water clusters around the Na+ ions act as two-dimensional nanopores for the diffusion of Na+ between the basal surfaces and across the central H2O/CO2 layer, whereas the CO2-rich regions are not permeable to Na+ The near-surface Na+ ions occur in two distinct types of coordination environments with distinct NMR spectral fingerprints. Type-I near-surface Na+ ions are coordinated by two basal oxygen atoms and four water molecules, whereas for type-II one of the coordinating water molecules is replaced by a CO2 molecule. The activation energies for a H2O and a CO2 molecule to move out of the first coordination shell of a near-surface Na+ are similar to 2.75 and similar to 0.5 kcal/mol, respectively. The activation barriers for site-hopping of a H2O molecule within the first coordination shell of near-surface and displaced Na+ ions are similar to 1.6 kcal/mol whereas those for site-hopping of CO2 around the near-surface and displaced Na+ ions are similar to 1.8 and similar to 3.5 kcal/mol, respectively. The results provide a detailed picture of the interlayer structure and energetics of diffusional motion of cations and intercalates.