Ion pairing in ionic liquids

Abstract

In the present article we briefly review the extensive discussion in literature about the presence or absence of ion pair-like aggregates in ionic liquids. While some experimental studies point towards the presence of neutral subunits in ionic liquids, many other experiments cannot confirm or even contradict their existence. Ion pairs can be detected directly in the gas phase, but no direct method is available to observe such association behavior in the liquid, and the corresponding indirect experimental proofs are based on such assumptions as unity charges at the ions. However, we have shown by calculating ionic liquid clusters of different sizes that assuming unity charges for ILs is erroneous, because a substantial charge transfer is taking place between the ionic liquid ions that reduce their total charge. Considering these effects might establish a bridge between the contradicting experimental results on this matter. Beside these results, according to molecular dynamics simulations the lifetimes of ion-ion contacts and their joint motions are far too short to verify the existence of neutral units in these materials.

Predictions of Physicochemical Properties of Ionic Liquids with DFT

“…Note, a detailed assessment of ionic charges by several computational methods, including CHELPG, is given in Reference [28].…”

From Solvent-Free to Dilute Electrolytes: Essential Components for a Continuum Theory

“…[5][6][7] The optimized design of these devices requires the mechanistic spatiotemporal understanding of ionic arrangement and charge transport in electrolytes. Although the physicochemical aspects of electrolyte solutions have been extensively studied, a series of recent experimental and computational results [8][9][10][11][12][13][14][15][16][17][18][19] reflects knowledge gaps even in the context of basic science. 3,16,[20][21][22][23][24] A robust spatiotemporal framework is thus required to advance electrochemically-based industrial applications of highlyconcentrated electrolytes, e.g., fuel cells ∼1M (mol/liter) and batteries ∼10M.…”

Ion pairing and phase behaviour of an asymmetric restricted primitive model of ionic liquids

“…In future work, fractional charges may also feature, given the possibility that charge transfer may play a role in RTILs. 15,19 In this context, we note that varying the ionic charge is essentially equivalent to changing the dielectric constant or the temperature. In contrast to the standard RPM, for all ions, the charge will be located a distance, b, away from the hard-sphere centres of the particles.…”

“…There are surface force, conductivity and voltammetry measurements that do support the existence of ion pairs, or higher order clusters, in ionic liquids. [11][12][13][14][15][16] For example, Gebbie et al 11,12 used the Surface Force Apparatus, SFA, to measure interactions between charged surfaces, separated by ILs. They found long-ranged forces, persisting to separations of 10-12 nm.…”

“…It is true that the conductivity is generally lower than one would anticipate in a fully dissociated fluid, but a degree of association of about 50 percent normally suffices to explain the measured values. While it is possible that charge transfer 15,19 can at least offer a partial explanation for these discrepancies, there seems to be little overall consensus about this issue in the presence of seemingly contradictory experimental results.…”