Interfacial water structure effect on CO₂ solubility in water-saturated silica confinements: A molecular perspective

Solubility trapping is a key mechanism in geological carbon sequestration (GCS), yet CO₂ solubility in water-filled nanopores often deviates markedly from bulk behavior. We hypothesize that variations in CO₂ solubility within silica nanopores originate from differences in interfacial water structure that are controlled by surface chemistry. In particular, specific Si-OH arrangements on Q2, Q3, and Q4 silica surfaces (defined by the number of Si atoms bonded through oxygen to a central Si atom) modulate hydrogen-bonding (HB) environments and adsorptive volumes that regulate CO₂-water-solid interactions.

We conducted molecular dynamics simulations of water-saturated Q2, Q3, and Q4 silica confinements under representative GCS conditions. Interfacial density profiles, HB distributions, and CO₂ spatial probability maps were analyzed to quantify fluid-solid interactions and to evaluate CO₂ solubility relative to bulk water.

Hydrophilic Q3 surfaces exhibit enhanced CO₂ solubility compared to the bulk liquid, arising from CO₂-water co-adsorption facilitated by a dense interfacial HB network. Hydrophobic Q4 confinements, by contrast, show pronounced over-solubility dominated by strong direct CO₂ adsorption within enlarged low-HB regions. Q2 surfaces display intermediate behavior reflecting mixed hydrophilic-hydrophobic character. We introduce two mechanistic descriptors, adsorptive volume and HB site density. High HB site density promotes hydrophilicity and co-adsorption, whereas large adsorptive volume favors direct CO₂ adsorption and over-solubility. Overall, these results demonstrate that CO₂ solubility in silica nanopores is jointly governed by interfacial water structure and surface chemistry. The findings provide molecular-scale insights into solubility trapping in silica-rich formations and inform the design of engineered materials for CO₂ capture and storage.