Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species

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Abstract

Room-temperature ionic liquids (RTILs) based on 1-butyl-3-methylimidazolium ([bmim]) with a variety of fluorinated anions were prepared, and the thermal behavior, density, viscosity, self-diffusion coefficients of the cations and anions, and ionic conductivity were measured over a wide temperature range. The temperature dependencies of the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been fitted to the Vogel−Fulcher−Tamman equation, and the best-fit parameters for the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been estimated, together with the linear fitting parameters for the density. The self-diffusion coefficients determined for the individual ions by pulsed-field-gradient spin−echo NMR method exhibit higher values for the cation compared with the anion over a wide temperature range, even if its radius is larger than that of the anionic radii. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order [bmim][(CF3SO2)2N] > [bmim][CF3CO2] > [bmim][CF3SO3] > [bmim][BF4] > [bmim][(C2F5SO2)2N] > [bmim][PF6] at 30 °C, and the order of the diffusion coefficients greatly contrasts to the viscosity data. The ionic association is proposed from the results of the ratios of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst−Einstein equation. The ratio for the ionic liquids follows the order [bmim][PF6] > [bmim][BF4] > [bmim][(C2F5SO2)2N] > [bmim][(CF3SO2)2N] > [bmim][CF3SO3] > [bmim][CF3CO2] at 30 °C and provides quantitative information on the active ions contributing to ionic conduction in the diffusion components.

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