Supercritical CO2/ionic liquid systems: what can we extract from infrared and Raman spectra?

Infrared (IR) spectroscopy has been successfully applied to study the solubility of supercritical (sc) CO2 in an ionic liquid (IL), the swelling of the IL under scCO2, the diffusion of CO2 in the IL, and the molecular interaction between the IL and scCO2 using a single defined experimental setup. The study has been performed using 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and scCO2 with a pressure of up to 150 bar at 313 K. The solubility of scCO2 in the IL was calculated via the intensity of the CO2 antisymmetric stretching mode, which was measured using the attenuated total reflection (ATR) method. This approach allowed to determine also the swelling of the IL under scCO2 using several IL bands. The knowledge gained about the solubility and swelling allowed the calculation of the molar fraction of CO2 in the IL and the attainment of similar data as previously determined by other methods. The kinetic measurement of the infrared spectra also offered the opportunity to observe the evolution of the concentration of CO2 in the IL and allowed an estimation of the diffusion coefficient of CO2 in IL under high pressure. Finally, information about the molecular interactions between the IL and scCO2 could also be gained in situ using both infrared and Raman spectroscopy.