Free and Bound States of Ions in Ionic Liquids, Conductivity, and Underscreening Paradox

Abstract

Using molecular dynamics simulations and theoretical analysis of velocity-autocorrelation functions, we study ion transport mechanisms in typical room-temperature ionic liquids. We show that ions may reside in two states: free and bound with an interstate exchange. We investigate quantitatively the exchange process and reveal new qualitative features of this process. To this end, we propose a dynamic criterion for free and bound ions based on the ion trajectory density and demonstrate that this criterion is consistent with a static one based on interionic distances. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound "clustered states" and when they move quasifreely. Using this method, we evaluate the average portion of "free" ions as approximately 15%-25%, increasing with temperature in the range of 300-600 K. The ion diffusion coefficients and conductivities as a function of the temperature calculated from the velocity and electrical-current autocorrelation functions reproduce the reported experimental data very well. The experimental data for the direct-current conductivity (constant ionic current) is in good agreement with theoretical predictions of the Nernst-Einstein equation based on the concentrations and diffusion coefficients of free ions obtained in our simulations. In analogy with electronic semiconductors, we scrutinize an "ionic semiconductor" model for ionic liquids, with valence and conduction "bands" for ions separated by an energy gap. The obtained band gap for the ionic liquid is small, around 26 meV, allowing for easy interchange between the two dynamic states. Moreover, we discuss the underscreening paradox in the context of the amount of free charge carriers, showing that the obtained results do not yet approve its simplistic resolution.

Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications

“…Further investigations on the temperature-dependence of the lithium coordination and aggregation of ions might give further insight, if the ionicity interpretation can be related to the observed T-dependence. Other rationalizations of the ionicity, apart from the degree of unpaired ions, are for instance charge transfer form anion to cation [66] or different diffusional states [67]. For pure ionic liquids, the deviation between experimental and Nernst-Einstein conductivity is frequently interpreted by velocity anti-correlations between the different ions [51].…”

The effect of structural heterogeneity upon the microviscosity of ionic liquids

“…For diffusion to occur, the ion must ‘jump’ from this cage to a neighbouring cage. 50 – 52 It is this jump that provides a high activation barrier to translation, thus leading to slow diffusion of ions in ionic liquids. In their study of the size, shape, and charge of a solute rotating in [C 4 C 1 im][BF 4 ] Rumble et al demonstrated that such diffusive jumps were much more common and of greater amplitude for the smaller probes, where they significantly contributed to rotational diffusion coefficients, but rarer in the case of larger probes.…”

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

“…[126] The velocity of translocation of analytes is likely to be coupled with the clustering of ions. [128] However, the understanding of these processes are still in the nascent stage. [43] So far, it is relatively well accepted that for the majority of ILs, the cation is more diffusive than the anion.…”

“…[ 126 ] The velocity of translocation of analytes is likely to be coupled with the clustering of ions. [ 128 ] However, the understanding of these processes are still in the nascent stage. [ 43 ]…”