Determination and Prediction for the Polarity of Ionic Liquids

Ionic liquids (ILs) have attracted remarkable attention as a new class of novel reaction medium and soft functional materials in the framework of Green Chemistry, and the polarity of ionic liquids is one of the primary concerns in the scientific community. Here, we introduce a new and simple scale of polarity, δμ, for ILs. Initially, two kinds of representative ionic liquids, {1-alkyl-3-methyllimidazolium acetate} [Cnmim][AcO] (n = 2,4,5,6) and {1-alkyl-2,3-dimethyimidazolium-N, N-bis(trifluoromethyl sulfonyl)imide} [Cnmmim][NTf2] (n = 2,4) are prepared and confirmed by 1H NMR spectroscopy and 13C NMR spectroscopy, respectively. Furthermore, a spectroscopic method based on solvatochromic probes is used to obtain the polarity of room temperature ILs (RTILs). In this work we mainly introduce that the scale of solvent polarity, ET(30) values, is determined by means of Reichardt’s dye (30), and π* values are determined by N,N-diethyl-4-nitroaniline. Simultaneously, in terms of the values of enthalpy of vaporization, δμ values of these ILs are predicted. Moreover, the trend of δμ values of the ionic liquids with the different alkyl chains are the same as the ET(30) and π* values. In addition, the new scale of polarities δμ of some molecular liquids are also predicted, and the results are in good agreement with relative permittivity, εr.