Ion Conduction in a Protic Glass-Forming Ionic Liquid with Long Carbon Chains

A simple model for the analysis of electrode polarization was used to extract the number density (charge carrier density) and the mobilities of the charge carriers separately from the dielectric spectra. The observed Vogel-Fulcher-Tammann (VFT) dependence of the DC conductivity can be traced back to a similar temperature dependence of the mobility, while the number density of mobile charges shows only a marginal temperature dependence. However, the fraction of available (mobile) charge carriers was found to be surprisingly low (6.38 % at 25 $^{\circ}\mathbf{C}$), a fact that was attributed to charge immobilization in molecular aggregates as discussed in recent literature. A subtle step in the mobility and (mobile) charge density around −10°C was discussed in terms of a “hidden” phase transition possibly affecting the molecular organization in such aggregates. Increasing the fraction of mobile ionic charges by preventing aggregation would be an effective strategy to improve ionic conductivity of the protic glass-forming ionic liquid trioctylammonium methanesulfonate ([TOA][OMs]).