CO₂ Molecular Behaviors at Kaolinite–Geofluid Interfaces: The Role of Edge Defects in Relation to Geologic Carbon Sequestration

Geologic carbon sequestration (GCS) represents a promising approach to achieve global net-zero carbon targets. Kaolinite, a common component in natural sediments, plays critical roles as a caprock in GCS projects. Herein, molecular dynamics simulations are employed to investigate CO₂ behaviors at the kaolinite-water interface, focusing on natural defects such as nanovalleys. Our findings reveal that the diffusion behavior of CO₂ molecules varies depending on their proximity to different kaolinite surfaces. Specifically, CO₂ near the hydrophobic siloxane surface, which exhibits a stronger affinity for CO₂, demonstrates lower diffusion coefficients compared with molecules near the hydrophilic gibbsite and edge surfaces. This reduced mobility leads to prolonged residence times at the siloxane surface. Additionally, edge sites exhibit greater CO₂ retention than gibbsite surfaces, likely due to their higher surface roughness and reactivity. Furthermore, our results indicate that CO₂ primarily penetrates the nanovalley through the central region and areas adjacent to the siloxane surface. At elevated CO₂ concentrations, nanobubbles begin to form; however, these nanobubbles are unable to enter the nanovalleys due to the presence of substantial breakthrough pressure barriers. This study enhances our understanding of CO₂ behaviors in clay-rich environments and its application within the scope of sustainable chemistry related to the carbon neutrality strategy.