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 CO 2 and a series of 1-alkyl-3-methylimidazolium tetrafluoroborate С n MIMBF 4 (n = 2, 4, 6, 8, 10) ionic liquids. A detailed analysis of the infrared spectra acquired from ionic liquids and sorbed СО 2 was performed as ionic liquids with different the alkyl chain lengths were subjected to changing pressures of СО 2 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 BF 4 − anions is observed during CO 2 sorption. This indicates the disaggregation of alkyl chains and anions. There is a correlation with the position of the wavenumber of the ν 3 asymmetric stretching CO 2 band and the length of the alkyl chain. It was found that some of the CO 2 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 CO 2 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 CO 2 and the wavenumber of the ν 3 band. This opens up the possibility of using CO 2 as an IR-sensitive probe molecule to characterize the acid-base properties of ionic liquids and determine the enthalpy of CO 2 sorption. Unlabelled Image • In situ ATR-FTIR spectroscopy was used to study CO 2 sorption in a homologous series of ionic liquids. • ATR-FTIR spectroscopy was used to determine the thermodynamic parameters of CO 2 sorption in ionic liquids. • A correlation was found between the enthalpy of CO 2 sorption and the wavenumber of the ν 3 band of CO 2. • CO 2 can be used an IR-sensitive probe molecule to characterize the properties of ionic liquids. [ABSTRACT FROM AUTHOR]