Physicochemical and Acid-base Properties of a Series of 2-Hydroxyethylammonium-based Protic Ionic Liquids

Abstract

Physicochemical properties such as a thermal behavior, ionic conductivity, viscosity and density, and acid-base properties of a new class of 2-hydroxyethylammonium-based protic ionic liquids (PILs) have been investigated. Thirty-six potential PILs were surveyed to find 32 salts with a melting point below 373 K. Among them, [(EtOH)_nEt_(3−n)NH⁺][TFS⁻] (Et, C₂H₅; n = 0 – 3) and [(EtOH)₂EtNH⁺][X⁻] (X = TFS, trifluoromethanesulfonate; TFSA, bis(trifluoromethanesulfonyl)amide; NO₃) were studied in terms of the Walden plots, molar volume and auto-protolysis reaction for effect of the number of 2-hydroxyethyl groups introduced in the cations and for dependence of the anion nature, respectively. With regard to [(EtOH)_nEt_(3−n)NH⁺][TFS⁻] (n = 0 – 3), the ion-ion interactions between cation-anion and cation-cation were enhanced with increasing the number of the 2-hydroxyethyl groups. In addition, the auto-protolysis constant K_s value for [(EtOH)₂EtNH⁺][TFSA⁻] is smaller than that for TFS⁻ based PIL, indicating that HTFSA behaves as a stronger acid than HTFS in the respective PIL. On the other hand, in [(EtOH)₂EtNH⁺][NO₃⁻], the emf jump was rather small, which suggests that the proton of HNO₃ does not easily transfer to (EtOH)₂EtN in the liquid state.