Molecular Insights into the Effects of Inorganic Salt on CO ₂ Hydrate Formation in Clay Nanopores

Marine sediments provide favorable geological conditions for long-term CO2 sequestration, yet mineral surface properties and dissolved inorganic salts in pore water significantly modulate CO2 adsorption and hydrate nucleation. In this study, molecular dynamics simulations were employed to systematically investigate the formation mechanisms of CO2 hydrates in kaolinite slit nanopores under seawater like conditions. The results show that different ions exhibit pronounced inner-sphere and outer-sphere adsorption behaviors on SiO4 and AlO6 surfaces. Ion adsorption on mineral surfaces substantially inhibits the formation of hydrate cage structures at the interface. Inner-sphere adsorbed ions partially desolvate and occupy surface coordination sites, displacing water molecules that would otherwise form hydrogen bonds with the surface and competing with CO2 for adsorption sites, thereby markedly reducing interfacial CO2 adsorption stability. In contrast, outer-sphere adsorbed ions retain intact hydration shells and induce ordered orientation of near surface water molecules, densifying and stabilizing the hydrogen bond network, which indirectly regulates CO2 enrichment and adsorption kinetics at the interface. Inorganic salts significantly enhance the migration ability of CO2 and water molecules, leading to increased local disturbances, which in turn inhibit the formation of hydrate structures, especially significantly suppressing the generation quantity and growth rate of the 51262 large cage. The concentration of salts in the liquid phase and the adsorption behavior of ions on mineral surfaces, together with their regulation of interfacial molecular distribution, collectively influence the formation of CO2 hydrates in kaolinite nanopores. This study reveals the critical role of mineral surface solvation microstructure and salt ions in controlling CO2 hydrate nucleation pathways, providing molecular level insights for optimizing hydrate-based CO2 sequestration strategies in marine sediments.