Dual Effects of Water on CO2 Adsorption in Silica Mesopores: Interfacial Promotion versus Volume-Limited Inhibition

Abstract Water is ubiquitous in natural and engineered porous media and plays a critical role in regulating CO2 adsorption in carbon capture, utilization, and storage (CCUS) systems. Previous studies report contradictory effects of water, ranging from suppression to enhancement of CO2 uptake, indicating that the underlying mechanisms remain unclear. Here, we investigate CO2 adsorption in silica mesopores with varying water contents using molecular simulations under thermodynamic conditions relevant to three typical CCUS scenarios. The results suggest two distinct adsorption regimes controlled by the thermodynamic state of CO2. Under gaseous conditions, CO2 uptake is governed by an interfacial-area-controlled regime and exhibits a pronounced nonmonotonic response to water content, with enhancement arising from abundant water interfacial area within low-density regions. In contrast, under supercritical conditions, CO2 uptake is governed by an accessible-volume-controlled regime and decreases monotonically with increasing water content. Similar trends are observed in both hydrophilic and hydrophobic pores, although the spatial distribution of water differs substantially between them. These findings provide molecular-scale insights into the contrasting effects of water on CO2 adsorption and may inform future investigations of water management in CO2 capture, mineral carbonation, and geological storage.