Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation

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Abstract
The alkyl chain length of 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Rmim][(CF3SO2)2N], R = methyl (m), ethyl (e), butyl (b), hexyl (C6), and octyl (C8)) was varied to prepare a series of room-temperature ionic liquids (RTILs), and the thermal behavior, density, viscosity, self-diffusion coefficients of the cation and anion, and ionic conductivity were measured over a wide temperature range. The self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity change with temperature following the Vogel−Fulcher−Tamman equation, and the density shows a linear decrease. The pulsed-field-gradient spin−echo NMR method reveals a higher self-diffusion coefficient for the cation compared to that for the anion over a wide temperature range, even if the cationic radius is larger than that of the anion. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order [emim][(CF3SO2)2N] > [mmim][(CF3SO2)2N] > [bmim][(CF3SO2)2N] > [C6mim][(CF3SO2)2N] > [C8mim][(CF3SO2)2N], which greatly contrasts to the viscosity data. The ratio of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst−Einstein equation quantifies the active ions contributing to ionic conduction in the diffusion components, in other words, ionicity of the ionic liquids. The ratio decreases with increasing number of carbon atoms in the alkyl chain. Finally, a balance between the electrostatic and induction forces has been discussed in terms of the main contribution factor in determining the physicochemical properties.