Role of Cations in Adsorption of Supercritical Carbon Dioxide at Smectite Mineral-Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Simulation Studies

The microscopic understanding of uptake and retention of supercritical carbon dioxide by expandable layered aluminosilicate minerals is of central importance for large-scale geological sequestration of CO,. At the molecular scale, the interlayer charge-balancing cations (CBCs) play a crucial role in CO, adsorption by these cost-effective and environmentally friendly materials. This article investigates the influence of CBCs on the structure, dynamics, and energetics of CO2- H2O mixtures nanoconfined in four different montmorillonites with different CBCs (Cs+, K+, Na+, and Ca2+) using molecular dynamics and enhanced sampling simulations. The results reveal that both CO, and H2O coexist in a single layer at the center of the interlayer, that CO, molecules orient parallel to the basal surface and form two-dimensional percolation paths that facilitate their lateral diffusion, and that the near-neighbor CO, molecules preferentially adopt a distorted slipped-parallel geometry in all of these systems. The calculated activation barriers for CO2 and H2O to diffuse away from the first coordination shells of cations show that Cs+-CO2 and Ca2+-H2O affinities are relatively higher than those of the other cations. The residence times in the cation coordination shells, relative orientations, and diffusion constants of intercalates are also quantified. The presented results can be useful for rational design of engineered layered minerals with improved CO2 retention capacity.