Molecular simulation on CO₂ adsorption heterogeneity in montmorillonite nanopores with different surface charges in presence of water

Understanding the CO₂ adsorption mechanism in clay nanopores is crucial for enhancing carbon capture and storage in shale reservoirs. Nonetheless, the impact of surface charge on the heterogeneity of CO₂ adsorption and its subsequent effects on adsorption capacity within clay nanopores are not fully understood. By employing molecular dynamics simulations and grand canonical Monte Carlo techniques, we examine the effects of surface charge, pressure, and nanopore water on the adsorption capacity, density distribution, and free energy profile of CO₂ in montmorillonite (Mt) nanopore. Utilizing density entropy as an innovative metric for adsorption heterogeneity, we shed light on how isomorphous substitution, which results in a non-uniform distribution of negative charges within Mt layers, triggers heterogeneous distribution of CO₂ and water in Mt nanopore. Our findings highlight a unique correlation between the density entropy of CO₂ and its adsorption capacity. The maximum adsorption capacity is strongly associated with surface charge heterogeneity in dry Mt nanopores and is governed by the heterogeneity of water distribution in wet Mt nanopores. We find that a greater heterogeneity in the CO₂ distribution within the Mt nanopore system aligns with an increased maximum adsorption energy for CO₂, consequently resulting in an enhanced overall adsorption capacity.