Electrical conductivity and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

Abstract

Broadband dielectric and terahertz spectroscopy (10(-2)-10(+12) Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce--using the Einstein-Smoluchowski equation--the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel-Fulcher-Tammann and for the latter, an Arrhenius temperature dependence is obtained. There is no indication of a mode arising from the reorientation of stable ion pairs.

Dynamics of RTILs: A comparative dielectric and OKE study

“…Ref. [44], as well as the charge-hopping times deduced from low-frequency dielectric measurements [45,46,17] are comparable to the sub-α relaxation time, which is in the order of a few hundred picoseconds at room temperature. …”

Spotlight on ionic liquids

“…A specific test comparing self-diffusion from NMR measurements compared with electrical mobilities from dielectric spectroscopy and conductivity measurements claims that there is no evidence for any mode arising from reorientation of stable ion pairs, indicating that lifetimes for ion clusters must be shorter than reorientational time scales. 65 Molecular dynamics simulations have provided a wealth of information regarding transport in ILs, yet there does not seem to be a general rule for understanding ion transport in these systems. Certainly, Stokes-Einstein hydrodynamic scaling is frequently not observed.…”

Electric field induced deformation of hemispherical sessile droplets of ionic liquid

“…The charge is also sufficiently mobile; the conductivity of the dry liquid increases with frequency and a relaxation which has been observed in the ionic motions is in the range 10 4 to 10 5 Hz [25] (at 280 K) is well above the value of 1 kHz used in our experiments at 293 K. The height change values, ∆h = h(E) -h(0), in response to D.C. and A.C. voltages were found to agree to within ±1% over the full range of voltage, cell gap and drop heights used in the studies. Using transparent electrodes enabled the drops to be viewed both from above and from the side during the experiments.…”

“…The estimated charge density of the liquid, at 10 26 to 10 27 m -3 [25] is sufficient to screen and exclude electric fields many orders of magnitude higher than used in the experiment from the inside of the liquid droplet.…”