Interactions of CO2 with the homologous series of СnMIMBF4 ionic liquids studied in situ ATR-FTIR spectroscopy: spectral characteristics, thermodynamic parameters and their correlation

In this work, in situ ATR-FTIR spectroscopy was used to study the interaction of CO2 and a series of 1-alkyl-3-methylimidazolium tetrafluoroborate СnMIMBF4 (n = 2, 4, 6, 8, 10) ionic liquids. A detailed analysis of the infrared spectra acquired from ionic liquids and sorbed СО2 was performed as ionic liquids with different the alkyl chain lengths were subjected to changing pressures of СО2 and temperature. With a longer alkyl chain length, an increase in the shift of the CH stretching vibrations bands of alkyl groups and BF stretching vibrations of BF4− anions is observed during CO2 sorption. This indicates the disaggregation of alkyl chains and anions. There is a correlation with the position of the wavenumber of the ν3 asymmetric stretching CO2 band and the length of the alkyl chain. It was found that some of the CO2 adsorbed by ionic liquids does not interact with the ionic liquid but is “free” in the bulk of the alkyl chains. For the first time, ATR-FTIR spectroscopy was used to determine the thermodynamic parameters of CO2 sorption in ionic liquids. It was demonstrated that the values of enthalpy and entropy obtained by analysis of the ATR-FTIR spectra are consistent the data obtained by other methods. A correlation was found between the enthalpy of sorption of CO2 and the wavenumber of the ν3 band. This opens up the possibility of using CO2 as an IR-sensitive probe molecule to characterize the acid-base properties of ionic liquids and determine the enthalpy of CO2 sorption.