ATR-IR spectroscopic observation on intermolecular interactions in mixtures of imidazolium-based ionic liquids CnmimTFSA (n = 2–12) with DMSO

Abstract

The intermolecular interactions in mixtures of ionic liquids (ILs) of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnmimTFSA) and dimethyl sulfoxide (DMSO) have been elucidated by measurements of refractive indexes, densities, and ATR-IR spectra as a function of the alky chain length (n=2–12) of Cnmim+ and DMSO molar fraction xDMSO. The excess quantities of refractive indexes and molar volumes showed that in all CnmimTFSA+DMSO systems the packing of IL and DMSO is the loosest around 1:3 molar ratio of IL to DMSO. However, the significant alkyl chain dependence was not observed in both quantities. This suggests that the hydrogen bonding of the three imidazolium hydrogen atoms with DMSO molecules mainly contributes to the loose packing. From the ATR-IR spectra measured for CnmimTFSA+DMSO mixtures, the excess molar absorbance in the wavenumber ranges of the imidazolium ring CH vibrations and the SO vibrations of TFSA− anion and DMSO was depicted as a function of DMSO content. The excess molar absorbance of the CH vibrations revealed that the three imidazolium C2,4,5H sites, particularly the C2H, are progressively hydrogen-bonded with DMSO as the molar fraction increases to xDMSO≈0.6. The SO vibrations of TFSA− anion suggested that TFSA− anions are eliminated from the imidazolium rings with increasing xDMSO to the same. Hence, TFSA− anions are replaced by DMSO around the imidazolium rings with increasing DMSO content. However, the hydrogen bonding between them is not significantly influenced by the cation's alkyl chain. The partial molar absorbance of the SO vibrations of DMSO showed that DMSO molecules self-aggregate in the mixtures above xDMSO≈0.6. However, the aggregation of DMSO molecules may be suppressed with the longer alkyl chain of Cnmim+.