Enhanced CO₂ Capture in Binary Mixtures of 1‑Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids with Water

Absorption of carbon dioxide and water in 1-butyl-3-methylimidazoliun tricyanomethanide ([C₄C₁im]­[TCM]) and 1-octyl-3-methylimidazolium tricyanomethanide ([C₈C₁im]­[TCM]) ionic liquids (ILs) was systematically investigated for the first time as a function of the H₂O content by means of a gravimetric system together with in-situ Raman spectroscopy, excess molar volume (<i>V</i>^E), and viscosity deviation measurements. Although CO₂ absorption was marginally affected by water at low H₂O molar fractions for both ILs, an increase of the H₂O content resulted in a marked enhancement of both the CO₂ solubility (ca. 4-fold) and diffusivity (ca. 10-fold) in the binary [C_<i>n</i>C₁im]­[TCM]/H₂O systems, in contrast to the weak and/or detrimental influence of water in most physically and chemically CO₂-absorbing ILs. In-situ Raman spectroscopy on the IL/CO₂ systems verified that CO₂ is physically absorbed in the dry ILs with no significant effect on their structural organization. A pronounced variation of distinct tricyanomethanide Raman modes was disclosed in the [C_<i>n</i>C₁im]­[TCM]/H₂O mixtures, attesting to the gradual disruption of the anion–cation coupling by the hydrogen-bonded water molecules to the [TCM]⁻ anions, in accordance with the positive excess molar volumes and negative viscosity deviations for the binary systems. Most importantly, CO₂ absorption in the ILs/H₂O mixtures at high water concentrations revealed that the [TCM]⁻ Raman modes tend to restore their original state for the heavily hydrated ILs, in qualitative agreement with the intriguing nonmonotonous transients of CO₂ absorption kinetics unveiled by the gravimetric measurements for the hybrid solvents. A molecular exchange mechanism between CO₂ in the gas phase and H₂O in the liquid phase was thereby proposed to explain the enhanced CO₂ absorption in the hybrid [C_<i>n</i>C₁im]­[TCM]//H₂O solvents based on the subtle competition between the TCM–H₂O and TCM–CO₂ interactions, which renders these ILs very promising for CO₂ separation applications