Hydrophilicity/hydrophobicity driven CO2 solubility in kaolinite nanopores in relation to carbon sequestration

Abstract

CO2 sequestration into underground aquifers is a viable option to alleviate greenhouse gas emissions. CO2 solubility trapping in nanoporous media saturated with saline water may play an important role. In this process, CO2 solubility in nanoporous media is determined by the mutual interactions among CO2, water, and nano-confinement, which ultimately determines the CO2 sequestration amount. However, the effects of nano-confinement surface characteristics are not studied yet. In this study, we use molecular dynamics simulations to study the effects of surface characteristics on CO2 solubility under nano-confinement by deploying kaolinite nanopores as a model under typical geological conditions (373 K and up to 400 bar). We find that, compared to the bulk solubility, CO2 under-solubility is observed in hydrophilic kaolinite nanopores, while over-solubility is seen in hydrophobic kaolinite nanopores. From the microscopic view, in the hydrophilic kaolinite nanopores, water forms strong adsorption layers on the pore surfaces, which repel CO2 molecules. However, CO2 and water can co-adsorb on the pore surfaces in the hydrophobic kaolinite nanopores. In addition, CO2 molecules align parallel to the pore surface in hydrophobic kaolinite nanopores, while water molecules align perpendicular to the hydrophilic one. CO2 is less hydrated under the nano-confinement than in bulk, especially under the hydrophobic one. In conclusion, hydrophobic strata are more capable to sequestrate CO2 than hydrophilic strata in terms of solubility trapping mechanism.