Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate Probed by Quasielastic Neutron Scattering

Abstract

Quasielastic neutron scattering (QENS) in combination with deuterium labeling allows for studying protonated "highlighted" species and extracting detailed information about tangled stochastic processes. This approach has been applied to examine proton dynamics in the protic ionic liquid, triethylammonium triflate. The temperature range covered during the experiments (2-440 K) included two melting transitions correspondingly reflected in the global and localized dynamics of the cation. To focus on the dynamics of the acidic proton, QENS spectra of the sample with the deuterated alkyl side chains were analyzed. The remaining hydrogen atom served as a tagged particle for investigating both global long-range motion of the cation and specific dynamics of the proton and, thus, provided insight into the transport properties of triethylammonium triflate, which is important for designing electrochemical devices.

Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions

“… 15 analyzed the temperature dependent efficiency of proton transfer events. In our recent work 14 we investigated triethylammonium triflate (TEA-TF, or [NH(C 2 H 5 ) 3 ][SO 3 CF 3 ]) with the prevailing vehicular mechanism of the acidic proton transport as it has been shown by pulsed field gradient NMR (PFG-NMR) 26 , 27 and also proved by our computer simulations 28 . Using D/H-isotope substitution in the ethyl chains of the cation we left the proton attached to the triethylamine unchanged to be used as a tagged particle.…”

“…Due to the large neutron incoherent cross section of hydrogen, the method is predominantly sensitive to single-particle (uncorrelated) motions of protons ubiquitous in organic cations of ILs 13 – 16 . Selective deuterium/hydrogen (D/H) isotope substitution can be applied to intensify coherent scattering 14 , 17 , 18 and to shift the research focus to collective (correlated) processes governed by the charge alteration in ILs 19 , 20 .…”

“…During the last years, several publications on single-particle dynamics in PILs have appeared in the literature 14 – 16 . In general, the PIL spectra are interpreted in an analogous way as those of more extensively studied aprotic ILs 17 , 18 , 24 , 25 , assuming long-range diffusion and various subdiffusive (localized) motions.…”

Transport properties and intermolecular interactions in binary mixtures based on the protic ionic liquid ethylimidazolium triflate and ethylene glycol

“…2 Nevertheless, whether or not PILs can provide a long-range proton transport mechanism decoupled from molecular diffusion is an issue of current debate. 3,4 Along with studies focused on the behaviour of neat PILs, there is now an increasing interest in investigating the effect of adding a second compound. In this context, PIL/water binary mixtures have been studied to measure changes in thermodynamic properties such as phase transition temperatures, density, and viscosity.…”

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

“…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.…”