Ionic Liquid-Enhanced Interfaces to Boost Reactive CO₂ Capture

The addition of ionic liquids (ILs) to a mixture containing a molecular solvent and other ionic species can induce the heterogeneous redistribution of cations and anions at the gas-liquid interface. This nonuniform redistribution of cations and anions driven by the differences in the solvophilicity of ions can improve the thermophysical and interfacial properties of such mixtures, creating a local chemical environment that is conducive to some reactions. In this work, ILs are added to a mixture of potassium hydroxide (KOH) and ethylene glycol (EG), used as a reactive absorbent and electrolyte in the migration-assisted moisture-gradient (MAMG) process for CO₂ capture. Molecular dynamics (MD) simulations are employed to probe into the effects of complex ion-ion and ion-solvent interactions and to examine the chemical composition at the gas-liquid interface. A total of 12 systems are investigated using molecular simulations to identify trends in the performance of IL additives based on the choice of cation, anion, and IL concentration. The cation effects are studied using IL additives based on 1-ethyl-3-methylimidazolium ([EMIM]⁺) and 1-butyl-3-methylimidazolium ([BMIM]⁺), while the impact of anions is examined using additives based on dicyanamide [DCA]^-, triflate [TfO]^-, bistriflimide [NTf₂]^-, and hexafluorophosphate [PF₆]^- anions, respectively. The influence of the IL concentration is also evaluated at molar concentrations between 1% and 4%. The simulation results indicate that the use of IL additives can affect the physical CO₂ solubility, surface tension, and the localization of CO₂ around the [OH]^- ions at the gas-liquid interface. It is also evident that the choice of cations, anions, and IL concentration determines the extent to which the IL additives impact the local physicochemical properties. Physical dissolution, diffusive transport, and interaction with [OH]^- are critical intermediate steps toward reactive CO₂ capture using a liquid absorbent. Hence, the improvement in one or more of these properties, aided by IL additives, is expected to improve the overall CO₂ capture performance. Experiments reaffirmed the impact of IL additives on CO₂ capture performance and the sensitivity to the choice of the cation, anion, and concentration of the IL additive.