Anomalous Wien Effects in Supercooled Ionic Liquids

Abstract

We have measured conductivity spectra of several supercooled monocationic and dicationic ionic liquids in the nonlinear regime by applying ac electric fields with large amplitudes up to about 180 kV/cm. Thereby, higher harmonic ac currents up to the 7th order were detected. Our results point to the existence of anomalous Wien effects in supercooled ionic liquids. Most ionic liquids studied here exhibit a conductivity-viscosity relation, which is close to the predictions of the Nernst-Einstein and Stokes-Einstein equations, as observed for classical strong electrolytes like KCl. These "strong" ionic liquids show a much stronger nonlinearity of the conductivity than classical strong electrolytes. On the other hand, the conductivity-viscosity relation of the ionic liquid [P_{6,6,6,14}][Cl] points to ion association effects. This "weak" ionic liquid shows a strength of the nonlinear effect, which is comparable to classical weak electrolytes. However, the nonlinearity increases quadratically with the field. We suggest that a theory for explaining these anomalies will have to go beyond the level of Coulomb lattice gas models.

Transient Nonlinear Response of Dynamically Decoupled Ionic Conductors

“…The first observation is that the general characteristics exhibited in the nonlinear spectra of the dimer (practically an ionic liquid) agree with previous studies of (monomeric) liquid electrolytes. 21,22,23 Most importantly, at first glance the long-chain system seems to reveal a response which is also similar to that reported for some ionic liquids. 28 However, it is important to emphasize a critical aspect regarding the experimental procedure leading to these results: For both materials the frequency sweeps were performed in a conventional manner, with an integration time of maximum 0.5 s or one period for each data point and no time delays inserted between the probing intervals.…”

“…In this nonlinear regime higher-order conduction effects start playing a significant role. 17 Although nonlinear effects in viscous liquids have recently gained a large interest in the glass community, 18 their investigation for the case of conducting materials is scarce, with a focus on ionic liquids 19,20,21,22,23 and solid (glassy) ionic conductors 24,25 These studies reveal fascinating phenomena, not accessible via linear investigations: dynamically heterogeneous charge transport, 24 compliance with the reverse calorimetry concept, 19 accessibility of effective ionic jump lengths, 20 the emergence of "hump" features in the third-order conductivity spectra, 23 and anomalous Wien effects 22,26 for ionic liquids. To the best of our knowledge, such investigations are lacking for the scientifically and technologically highly relevant polymeric materials.…”

Investigation of nonlinear effects in glassy matter using dielectric methods

“…The nonlinearity can be quantified, e.g., by the difference of high and low-field permittivity [9,16,21,24]. ii) Application of a high ac field and determination of the higher harmonics of the response, leading to higher-order permittivities [12,13,18,20,21,22,76,77]. iii) Application of a high dc bias voltage and measurement of the permittivity, usually with a smaller ac signal [78,79,80,81,82].…”

“…However, in recent years a completely different approach is attracting increasing interest: In various works, both experimental and theoretical, it was revealed that valuable additional information on the glass transition can be gained by applying excessively high electrical fields (using voltages up to the kV range), thus driving the investigated material into a nonlinear regime (see, e.g., [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]). This nonlinear dielectric response is monitored, e.g., by determining the dielectric permittivity at high fields and comparing it to * measured in the linear regime.…”

Correlations from Ion Pairing and the Nernst-Einstein Equation

“…This approximation holds however rather well for dilute systems, especially when the dielectric constant of the solvent is high. For concentrated solutions, the assumption of non-interacting species breaks down, and failures of the Nernst-Einstein relation have been reported [7][8][9][13][14][15]. Ionic transport shifts from a single-particle to a many-particles picture.…”