How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties

Abstract

Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Lambda(imp)/Lambda(NMR)), calculated from the molar conductivity measured by the electrochemical impedance method (Lambda(imp)) and that estimated by use of pulse-field-gradient spin-echo NMR ionic self-diffusion coefficients and the Nernst-Einstein relation (Lambda(NMR)). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), E(T)(30), hydrogen bond donor acidity (alpha), and dipolarity/polarizability (pi), as well as NMR chemical shifts. The Lambda(imp)/Lambda(NMR) well illustrates the degree of cation-anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and cationic backbone structures with fixed counterparts, and by the inductive and dispersive forces for the various alkyl chain lengths in the cations. As a measure of the electrostatic interaction of the RTILs, the effective ionic concentration (C(eff)), which is a dominant parameter for the electrostatic forces of the RTILs, was introduced as the product of Lambda(imp)/Lambda(NMR) and the molar concentration and was compared with some physical properties, such as reported normal boiling points and distillation rates, glass transition temperature, and viscosity. A decrease in C(eff) of the RTILs is well correlated with the normal boiling point and distillation rate, whereas the liquid-state dynamics is controlled by a subtle balance between the electrostatic and other intermolecular forces.

1H NMR and Molecular Dynamics Evidence for an Unexpected Interaction on the Origin of Salting-In/Salting-Out Phenomena

“…38 It has been argued that this interaction leads to significant anion-cation pair formation with concomitant loss of the IL ionic character. 38 Although an important interaction between H(2) and the salt ions was expected, the signal of H(2) does not appear in any of the 1 H NMR spectra. This is believed to be caused by the exchange of this acidic hydrogen with deuterium afforded by the solvent.…”

Hydrogen-bond acidity of ionic liquids: an extended scale

“…22 This situation is even more problematic since different dyes and experimental approaches result in significantly different values of solvatochromic parameters for the same ILs. 13,15,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] For instance, the reported α value for the wellcharacterized IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 4 C 1 im] [NTf 2 ]) varies from 0.51 to 0.86 (cf. Fig.…”

“…13,15,17,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][47][48][49][50][51] It should be highlighted that the hydrogen-bond acceptor strength of an IL is dominated by its anion while the hydrogen-bond donor ability is essentially controlled by the cation and only slightly depends upon the anion of the IL. 18,19 Thus, and since we were focused on developing an extended series for the hydrogen-bond acidity of ILs, ILs comprising a fixed anion ([NTf 2 ] − ) and a wide range of cations (imidazolium-, pyridinium-, piperidinium-, pyrrolidinium-, ammonium-, sulfonium-, and phosphonium-based) were selected.…”

Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile

“…Generally, ILs are described in the literature as Newtonian fluids. [42][43][44][45] Nevertheless, non-Newtonian behaviors have already been reported for some AILs [46][47][48] 49,50 Non-Newtonian shear thinning phenomena have recently been described in several PILs by Burrell et al 50 In this paper, Burrell et al have also investigated the effect of water on this property, and they showed that by increasing the water concentration the rheological behavior of the (PIL þ water) binary system changes from non-Newtonian shear thinning, to Newtonian, to non-Newtonian shear thickening behaviors. 50 More recently, a publication of Guo and Guo reports also that ionic liquid induced structure transition of nonionic wormlike micelles composed of Tween 80 and Brij 30.…”