Pulsed-Gradient Spin−Echo ¹H and ¹⁹F NMR Ionic Diffusion Coefficient, Viscosity, and Ionic Conductivity of Non-Chloroaluminate Room-Temperature Ionic Liquids

Abstract

Room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), 1-butylpyridinium tetrafluoroborate (BPBF4), and 1-butylpyridinium bis(trifluoromethylsulfonyl)imide (BPTFSI), were prepared and characterized. The thermal property, density, self-diffusion coefficient of the anions and cations, viscosity, and ionic conductivity were measured for these ionic liquids in wide temperature ranges. A pulsed-gradient spin−echo NMR method was used to independently measure self-diffusion coefficients of the anions (19F NMR) and the cations (1H NMR). The results indicate that the cations diffuse almost equally to the anion in EMIBF4 and BPBF4, whereas they diffuse faster than the anion in EMITFSI and BPTFSI. The summation of the cationic and anionic diffusion coefficients for each ionic liquid follows the order EMITFSI > EMIBF4 > BPTFSI > BPBF4, under an isothermal condition. The order of the magnitude of the diffusion coefficient well contrasts with that of the viscosity for each ionic liquid. The temperature dependencies of the self-diffusion coefficient, viscosity, and ionic conductivity obey the Vogel−Tamman−Fulcher (VTF) equation, and the VTF parameters were presented. Relationships among the self-diffusion coefficient, viscosity, and molar conductivity were analyzed in terms of the Stokes−Einstein equation and the Nernst−Einstein equation. The most interesting feature of the relationships is that the ratios of the molar conductivity, determined by complex impedance measurements, to that calculated from the NMR diffusion coefficients, range from 0.6 to 0.8 for EMIBF4 and BPBF4, whereas the ratios range from 0.3 to 0.5 for EMITFSI and BPTFSI. This difference could be understood by taking the ionic association into consideration for EMITFSI and BPTFSI.