Transport Properties of Protic Ionic Liquids Based on Triazolium and Imidazolium: Development of an Air-Free Conductivity Setup

Abstract

The dynamical properties of four protic ionic liquids, based on the ethyltriazolium ([C2HTr124]) and the ethylimidazolium ([C2HIm]) cation, were investigated. The associated anions were the triflate ([TfO]) and the bistriflimide ([TFSI]). Ionic conductivity values and self-diffusion coefficients were measured and discussed, extending the discussion to the concept of fragility. Furthermore, in order to allow the measurement of the ionic conductivity of very small volumes (<0.5 mL) of ionic liquid under an inert and dry atmosphere, a new setup was developed. It was found that the cation nature strongly affected the transport properties, the [C2HTr124] cation resulting in slower dynamics than the [C2HIm] one. This was concluded from both conductivity and diffusivity measurements while for both properties, the anion had a lesser effect. By fitting the conductivity data with the Vogel–Fulcher–Tammann (VFT) equation, we could also estimate the fragility of these ionic liquids, which all fell in the range of very fragile glass-forming materials. Finally, the slower dynamics observed in the triazolium-based ionic liquids can be rationalized by the stronger interactions that this cation establishes with both anions, as deduced from the frequency analysis of relevant Raman signatures and density functional theory (DFT) calculations.

Evidence for NMR Relaxation Enhancement in a Protic Ionic Liquid by the Movement of Protons Independent of the Translational Diffusion of Cations

“…Based on this method, no separate movement of exchangeable protons in the −NH position from the diffusion of parent cations was detected for any of the protic ionic liquids studied so far. 10,12,13 Therefore, we believe that the observed low-frequency dispersion of the spin−lattice relaxation times of cationic protons is the first experimental evidence of this phenomenon in PILs. The observation of independent proton transport in…”

“…To date, most of the reported experimental evidence for cation-independent proton transport comes from PFG NMR measurements of the self-diffusion coefficient of hydrogen atoms. Based on this method, no separate movement of exchangeable protons in the −NH position from the diffusion of parent cations was detected for any of the protic ionic liquids studied so far. ,, Therefore, we believe that the observed low-frequency dispersion of the spin–lattice relaxation times of cationic protons is the first experimental evidence of this phenomenon in PILs. The observation of independent proton transport in PIL was possible due to the sensitivity of the FFC NMR method.…”

“…10 The cation-independent transport of protons in PILs has so far been studied using Pulse Field Gradient NMR (PFG NMR) methods, 11 which allow for the determination of the self-diffusion coefficients of protons of the alkyl chain, imidazolium ring and NH group of PIL. 10,12,13 The values were very similar for these three types of hydrogen atoms and based on this similarity, a separate movement of the exchangeable protons in the −NH position from the diffusion of the parent cations was excluded in pure PILs. On the other hand, evidence of a proton motion decoupled from molecular diffusion of ions was reported by Martinelli et al 14 in the mixture of protic ionic liquid and imidazole but in the low concentration range of imidazole.…”

“…34 So far, experimental evidence for cation-independent proton transport in PILs comes mainly from measurements of the self- diffusion coefficient of cation hydrogen atoms by PFG NMR. 12,13 Such evidence is considered to be a higher D NH value measured for the exchangeable proton than for other protons or the cation as a whole. The 1D proton NMR spectrum of [MIm][TFSI] at 343 K (liquid phase) along with the chemical structure and peak assignment is shown in Figure 6.…”

“…The studies of a wide series of 1-alkylimidazolium bis­(trifluoromethylsulfonyl)­imide ionic liquids with the different alkyl chain lengths (from n = 2 to 12) on the imidazolium cation have shown that −NH proton on the cation form stronger H-bonds with the anion for longer alkyl chain and that the ionic conductivity also depends on the alkyl chain length . The cation-independent transport of protons in PILs has so far been studied using Pulse Field Gradient NMR (PFG NMR) methods, which allow for the determination of the self-diffusion coefficients of protons of the alkyl chain, imidazolium ring and NH group of PIL. ,, The values were very similar for these three types of hydrogen atoms and based on this similarity, a separate movement of the exchangeable protons in the −NH position from the diffusion of the parent cations was excluded in pure PILs. On the other hand, evidence of a proton motion decoupled from molecular diffusion of ions was reported by Martinelli et al in the mixture of protic ionic liquid and imidazole but in the low concentration range of imidazole.…”