Molecular Understanding of CO₂ Absorption by Choline Chloride/Urea Confined within Nanoslits

Clarifying the potential relationship between the microstructure of nanoconfined choline chloride/urea (ChClU) and CO₂ absorption performance is key to understanding the abnormal increase in CO₂ under nanoconfinement. In this study, we used molecular dynamics simulations and grand canonical Monte Carlo (GCMC) to systematically study the mechanism underlying the absorption of CO₂ by ChClU within nanoslits. According to the spatial distribution, ChClU can form two different laminar regions within nanoslits, namely, the interfacial region (region I) and beyond region I (region II). In region II, the interface induces rearrangement of ChClU, resulting in an increase in free volume and subsequent increase in CO₂ solubility. In region I, changing the interface from hydrophobic to hydrophilic (e.g., S_I to S_IV) by setting the appropriate charge patterns, the urea molecules gradually change from "disordered" to "ordered standing" relative to the solid surface. The preferential orientation of the urea molecules causes competition between the ChClU's free volume and urea molecules, resulting in a non-monotonic change in CO₂ solubility. Specifically, from S_I to S_III, the increase in urea molecules enhances the CO₂ solubility. In S_IV, space for CO₂ absorption is insufficient due to the accumulation of urea molecules, and thus CO₂ solubility decreases.