Temperature-dependent conductivity in room temperature ionic liquids. The relative importance of diffusional and exchange mechanisms

Abstract We report on the translational diffusion behavior of the chromophore Rhodamine B (RB) in a series of alkylimidazolium room temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIM TFSI), 1- n -butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIM TFSI), 1-methyl-3- n -octylimidazolium bis(trifluoromethylsulfonyl) imide (OMIM TFSI), and 1- n -butyl-3-methylimidazolium tetrafluoroborate (BMIM BF 4 ). We compare these results to published data on the electrical conductivity of the same RTILs. This comparison underscores the inability of conductivity in RTILs to be accounted for solely on the basis of constituent ion translational diffusion as described in the Nernst-Einstein model. The dependence of the “excess” conductivity on RTIL cation structure and system temperature points to the important role of exchange-mediated ion transport in RTILs, and from the temperature-dependence of the excess conductivity, we determine the activation energy for the exchange process. The activation energy for the exchange process is less than a typical hydrogen bond, suggesting a facile process, and the cation structure dependence of the activation energy is consistent with the intrinsically heterogeneous nature of RTILs.