The Effect of the Nature of Cation on Transport Properties of Bis(trifluoromethylsulfonyl)imide Ionic Liquids

Abstract

The methods of viscosimetry, conductometry, densimetry, and differential scanning calorimetry are used for studying the physicochemical properties of cations (1-butyl-3-methylimidazolium [BuMeIm]⁺, N-butylmethylpyrrolidinium [BuMePyrr]⁺, and methytrioctylammonium [MeOc₃Am]⁺) and their effect on the transport properties of bis(trifluoromethylsulfonyl)imide ionic liquids (IL). It is shown that ILs formed by cyclic amines ([BuMeIm]⁺ and [BuMePyrr]⁺) as the cations demonstrate the close values and temperature dependences of viscosity and conductivity. The cation [MeOc₃Am]⁺ predetermines the difference in the IL structure and, thus, a substantial difference in the transport properties of the melt. For salts [BuMeIm][Tf₂N], [BuMePyrr][Tf₂N], and [MeOc₃Am][Tf₂N] at 293 К, the following transport characteristics are observed: viscosity 57.7, 88.1, and 726.5 mPa s; specific conductivity 0.376, 0.252, and 0.005 S m^–1; activation energy of conductivity 21, 21, and 35 kJ mol^–1; degree of “ionicity” 0.92, 0.94, and 1.00, respectively. Using the equation of Vogel–Fulcher–Tammann and based on the temperature dependences of specific conductivity and viscosity, the ideal glass transition temperatures are calculated for liquids under study. In the positive temperature range, a linear correlation is observed between the conductivity and the cation volume in these ionic liquids.