The Polarity of Ionic Liquids: Relationship between Relative Permittivity and Spectroscopic Parameters of Probe

Polarity is one of the most important properties of ionic liquids (ILs) and an essential requirement when choosing an IL for a specific industrial application. Up to now, several empirical solvent polarity scales, such as Hildebrand solubility parameter, relative permittivity, the electronic transition energy of the longest-wavelength Vis absorption band of betaine dye no. 30 (E_T(30) value) and the hyperfine coupling constant (A_N) etc. have been applied to ILs to provide quantitative evaluation of the polarity of ILs. Among them, the E_T(30) value is widely used to reflect the polarity of ILs, however, it cannot be determined for opaque solvents or for solvents in which betaine dye no. 30 is insoluble. To broaden its scope of application and uniform the polarity standard, many approaches were used to predict the values of E_T(30). Herein, we managed to apply a modified semiempirical reaction field of molecular solvents to predict the E_T(30) and A_N values of spin probe in ILs. Based on the experimental and estimated E_T(30) values of 791 data entries, 240 ILs, 108 cations, and 34 anions of ILs, an “overall” polarity sequence of ILs can be obtained: primary, secondary, and tertiary alkylammonium salts > heterocyclic salts > quaternary alkylammonium salts ≈ phosphonium salts ≈ guanidinium salts.