Solubility of CO₂in 1-Butyl-3-methyl-imidazolium-trifluoro Acetate Ionic Liquid Studied by Raman Spectroscopy and DFT Investigations

The polarized and depolarized Raman spectra of 1-butyl-3-methyl-imidazolium-trifluoro acetate (Bmim TFA) ionic liquid and of the dense phase obtained after introduction of supercritical carbon dioxide (313K) under pressure (from 0.1 MPa up to 9 MPa) in the ionic liquid have been recorded. The spectrum of the pure ionic liquid has been assigned by comparison with the spectra of ionic liquids sharing the same cation and using literature data concerning the vibrational modes of the TFA anion. It was found that the spectra of the ionic liquid is almost unaffected by the CO(2) dilution. The only noticeable perturbation concerns a weak enhancement of the mode assigned here to the symmetric stretch vibration of the COO group of the TFA anion. The band shape analysis of the ν(CC) band in pure Bmim TFA shows that the carboxylate groups probe a variety of environments which are almost not affected by the dilution in carbon dioxide. The analysis of the Fermi dyad of carbon dioxide shows that this molecule is perturbed upon dilution in the ionic liquid. The spectra suggest the presence of carbon dioxide in two different environments. In the first one, carbon dioxide molecules interact with themselves, whereas in the second environment, this molecule interacts with the COO group of the TFA anion. This is supported by B3LYP-DFT calculations aimed at assessing the interaction between an ion pair dimer and a carbon dioxide molecule. It is shown that dissolved CO(2) molecules preferentially interact with the TFA anion through a weak charge transfer interaction taking place between the carbon atom of CO(2) (acting as a Lewis acid) and a oxygen atom of the COO group of TFA (as a Lewis base). The results show that Bmim TFA is able to accommodate a large amount of carbon dioxide without having its short-range local structure significantly perturbed. Most CO(2) is hosted in the voids existing among the ion pairs, while some also weakly interact with the anion. It is finally argued that the evolution of the local organization of the IL upon carbon dioxide dilution presents similarities with the microsegregation phenomena reported for IL upon increasing the alkyl chains lengths.