Understanding CO₂ capture kinetics and energetics by ionic liquids with molecular dynamics simulation

Room temperature ionic liquids (ILs) are recognized to be potential media for CO₂ capture, but the interaction nature is less documented which hinders the future development of ILs with high CO₂ solvation capability. Here, through all atom molecular dynamics (MD) simulations, the solvation process of CO₂ with four representative ILs, [EMIM][BF₄], [BMIM][BF₄], [EMIM]CL and [BMIM]CL was systematically studied. Our data clearly reflect the fact that hydrophobic components from both cations and anions dominate CO₂ solvation because IL-CO₂ attraction is mainly driven by the van der Waals attractions. Consequently, cations with longer alkyl chain (for instance, [BMIM]⁺ than [EMIM]⁺) and anions with higher hydrophobicity (for instance, [BF₄]⁻ than CL⁻) effectively enhance CO₂ solvation. For all the ILs under study, addition of water into ILs abates CO₂ solvation through regulating the anion-CO₂ interactions. Moreover, it is worth mentioning that ILs with a hydrophobic anion ([BF₄]⁻ in this study) are more resistant to the existence of water to capture CO₂ than ILs with a hydrophilic anion (Cl⁻ in this study). Free energy decomposition analyses were conducted which support the above findings consistently. Generally, it is predicted that cations with long alkyl chain, anions with high hydrophobicity, and ILs with smaller surface tension are potentially effective CO₂ capturing media. Our present study helps the deep understanding of the CO₂ capturing mechanism by ILs and is expected to facilitate the future design and fabrication of a novel IL medium for gas capture and storage.