Enthalpic Driving Force for the Selective Absorption of CO₂ by an Ionic Liquid

Molecular dynamics (MD) simulations validated against two-dimensional infrared (2D-IR) measurements of CO₂ in an imidazolium-based ionic liquid have revealed new insights into the mechanism of CO₂ solvation. The first solvation shell around CO₂ has a distinctly quadrupolar structure, with strong negative charge density around the CO₂ carbon atom and positive charge density near the CO₂ oxygen atoms. When CO₂ is modeled without atomic charges (thus removing its strong quadrupole moment), its solvation shell weakens and changes significantly into a structure that is similar to that of N₂ in the same liquid. The solvation shell of CO₂ evolves more quickly when its quadrupole is removed, and we find evidence that solvent cage dynamics is measured by 2D-IR spectroscopy. We also find that the solvent cage evolution of N₂ is similar to that of CO₂ with no atomic charges, implying that the weaker quadrupole of N₂ is responsible for its higher diffusion and lower absorption in ionic liquids.