Thermophysical Properties and Proton Transport Mechanisms of Trialkylammonium and 1-Alkyl-1H-imidazol-3-ium Protic Ionic Liquids

Abstract

We investigated the thermophysical and electrochemical properties of eight model protic ionic liquids (PILs) primarily because of our interest in their proton conductivity and low volatility. The chemical structures of the cations (ammonium vs imidazolium) and anions (mesylate vs triflate) were found to strongly govern properties such as density, viscosity, ionic conductivity, thermal and electrochemical stability, and phase transition behaviors. Structure–property relations were analyzed on the basis of charge delocalization, cation π-stacking, van der Waals interactions of alkyl tails, and hydrogen bonding interactions between cations and anions. The diffusion coefficients of the free proton, the cation, and the anion were determined by using NMR spectroscopy, and were used to differentiate between the vehicular and Grotthuss mechanisms of diffusion of protons. A correlation, based on the Sutherland–Einstein equation, was developed to predict ionic conductivity by using the room temperature molar volume and the VFT equation for viscosity.

Transport Properties, Local Coordination, and Thermal Stability of the Water/Diethylmethylammonium Methanesulfonate Binary System

“…This was also observed by Judenstein et al 35 and Nebane et al. 12 for the ionic liquids triethylammonium triflate, [TEA][TfO], and triethylammonium methanesolfunate, [TEA][OMs], respectively. This behavior is, however, different from what we have previously observed for imidazolium based ionic liquids, in which the cation diffuses faster than the anion for all water contents investigated 17.…”

“…These properties in turn determine the nature of ionic diffusion, with the acidic protons being hypothetically able to diffuse via a Grotthuss mechanism and thus faster than the cationic molecule. A proton (H + ) transport decoupled from the diffusion of the cation has been discussed for PILs such as di‐ethyl‐methylammonium triflate ([DEMA][TfO]) 10, triethylammonium triflate ([TEA][TfO])11, and 1‐butylimidazolium methanesulfonate ([CH 4 Im][OMs]) 12 based on diffusion‐NMR and quasi elastic neutron scattering (QENS) experiments, but ambiguities remain on whether a fast proton exchange may be the probed event instead 13. Moreover, the presence of even the smallest traces of water may complicate the interpretation of the collected data.…”

Physicochemical study of diethylmethylammonium methanesulfonate under anhydrous conditions

“…20 18 ) as received without further purification while other reports also indicated significant water content (e.g. about 2000 ppm 21 ). Nevertheless, the influence of even lower water content on the physicochemical properties of [DEMA][OMs] has so far been neglected.…”

Characterization of the Bridged Proton Structure in HTFSI Acid Ionic Liquid Solutions

“…Furthermore, although the applications of AILs are well-known, microscopic evidence about the ionic structures and solvation mechanisms is not available. 46 The proton solvation structure in an IL is a key unknown in understanding the solution's acidity and proton-transport properties. In this current work, we investigated AIL solutions of HTFSI dissolved in the IL 1-butyl-3-methylpyrrolidinium bis(trifloromethanesulfonyl)imide (PyrrTFSI) with HTFSI concentration up to 5.0 M. We used conductivity measurements, UV−vis, fluorescence, Raman, and infrared spectroscopy (FTIR) and a "mini-clusters" computational simulation together to characterize the proton-solvation structures.…”