Molecular Simulation Study of Water-Rock Interfaces During Supercritical CO2 Sequestration.

Understanding how supercritical CO2 and water interact with mineral surfaces is essential for predicting the stability and sealing performance of geological storage formations. Yet, the combined effects of mineral surface chemistry and confined pore geometry on interfacial structure and fluid dynamics remain insufficiently resolved at the molecular scale. In this study, molecular dynamics simulations were employed to quantify how methylated SiO2, hydroxylated SiO2, and kaolinite regulate CO2-H2O interfacial behavior through variations in wettability and electrostatic interactions. The results show a clear hierarchy in water affinity across the three minerals. On methylated SiO2, the water cluster remains spherical and poorly anchored, with a contact angle of ~140°, consistent with the weakest water-surface Coulomb attractions (only -400 to -1400 kJ/mol). Hydroxylated SiO2 significantly enhances hydration, forming a cylindrical water layer with a reduced contact angle of ~61.3° and strong surface-water electrostatic binding (~-18,000 to -20,000 kJ/mol). Kaolinite exhibits the highest hydrophilicity, where water forms a continuous bridge between the two walls and the contact angle further decreases to ~24.5°, supported by the strongest mineral-water electrostatic interactions (-23,000 to -25,000 kJ/mol). Meanwhile, CO2-water attractions remain moderate (typically -2800 to -3500 kJ/mol) but are sufficient to influence CO2 distribution within the confined domain. These findings collectively reveal that surface functionalization and mineral type govern interfacial morphology, fluid confinement, and electrostatic stabilization in the sequence methylated SiO2 < hydroxylated SiO2 < kaolinite. This molecular-level understanding provides mechanistic insight into how mineral wettability controls CO2 trapping, fluid segregation, and pore-scale sealing behavior in subsurface carbon-storage environments.