Physicochemical and acid-base properties of a series of 2-hydroxyethylammonium-based protic ionic liquids

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)(n)Et((3-n))NH(+)][TFS(-)] (Et, C(2)H(5); n = 0 - 3) and [(EtOH)(2)EtNH(+)][X(-)] (X = TFS, trifluoromethanesulfonate; TFSA, bis(trifluoromethanesulfonyl)amide; NO(3)) 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)(n)Et((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)(2)EtNH(+)][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)(2)EtNH(+)][NO(3)(-)], the emf jump was rather small, which suggests that the proton of HNO(3) does not easily transfer to (EtOH)(2)EtN in the liquid state.