Polarity Behaviour and Specific Interactions of Imidazolium-Based Ionic Liquids in Ethylene Glycol

Abstract

The molecular interactions of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate [C(4)mim][BF(4) ], 3-methyl-1-octylimidazolium tetrafluoroborate [C(8)mim][BF(4)] and 1-butyl-3-methylimidazolium octylsulfate [C(4)mim][C(8)OSO(3)] are investigated in ethylene glycol (EG) over the whole mole fraction range using fluorescence (steady-state and time-resolved), Fourier transform infrared and nuclear magnetic resonance (NMR) spectroscopy. The cybotactic region surrounding the pyrene fluorescent probe exhibits peculiar characteristics for different ILs in the EG-rich region. The extent of solute-solvent interactions is assessed by determining the deviations of experimentally observed vibronic band intensity ratios of peak 1 to peak 3 of pyrene fluorescence (I(1)/I(3)) from a composite I(1)/I(3) value obtained using a preferential solvation model. A distinct vibrational frequency shift for various stretching modes of EG (O−H) or ILs (C−H of ring protons, B−F and S=O of anions) indicates specific interactional preferences of EG toward the IL protons/anion. Splitting of the O−H vibration band of EG at 3000-3700 cm(-1) into three separate bands, and analysis of the changes in location and area of these bands as a function of concentration enable precise determination of the effect of ILs on hydrogen bridges of EG. NMR chemical shifts and their deviations from ideality show multiple hydrogen-bonding interactions of varying strengths between unlike molecules in the mixtures. A comparison of spectroscopic results with thermodynamic properties shows that the mixing microscopic behaviour of the investigated systems is completely different from the macroscopic behaviour, which is primarily governed by the difference in shape, size and nature of the molecules.