Possible Proton Conduction Mechanism in Pseudo-Protic Ionic Liquids: A Concept of Specific Proton Conduction

Abstract

In a previous work, we have found that the pseudo-protic ionic liquid N-methylimidazolium acetate, [C1HIm]­[OAc] or [Hmim]­[OAc], mainly consists of the electrically neutral molecular species N-methylimidazole, C1Im, and acetic acid, AcOH, even though the mixture has significant ionic conductivity. This system was revisited by employing isotopic substitution Raman spectroscopy (ISRS) and pulsed field gradient (PFG) NMR self-diffusion measurements. The ISRS and PFG-NMR results obtained fully confirm our earlier findings. In particular, the self-diffusion coefficient of the hydroxyl hydrogen atom in AcOH is identical to that of the methyl hydrogen atoms within the experimental uncertainty, consistent with very little ionization. Therefore, a proton conduction mechanism similar to the Grotthuss mechanism for aqueous acid solutions is postulated to be responsible for the observed electrical conductivity. Laity resistance coefficients (rij ) are calculated from the transport properties, and the negative values obtained for the like-ion interactions are consistent with the pseudo-ionic liquid description, that is, the mixture is indeed a very weak electrolyte. The structure and rotational dynamics of the mixture were also investigated using high-energy X-ray total scattering experiments, molecular dynamics simulations, and dielectric relaxation spectroscopy. Based on a comparison of activation energies and the well-known linear free energy relationship between the kinetics and thermodynamics of autoprotolysis, we propose for [C1HIm]­[OAc] a Grotthus-type proton conduction mechanism involving fast AcOH/AcO− rotation as a decisive step.

The unseen evidence of Reduced Ionicity: The elephant in (the) room temperature ionic liquids

“…These results correlate well with those from Infrared and NMR spectroscopies (vide infra). Focusing on TEATFA, the νCC band between 770 cm -1 and 870 cm -1 is diagnostic of the formation of the anion for acetate-based systems, as shown by Umebayashi et al [22,24] Specifically, in the case of dissociation, the signal is substantially red-shifted if compared to the pure acid one. Figure S6 reports the specific Raman region for the TEATFA system.…”

“…While most AILs have almost zero vapor pressure, PILs precursors generally have low boiling points, enabling their distillation thanks to the Le Châtelier's principle [17]. Their ability to establish extended 3D hydrogen-bonded networks [18] gives rise to a series of exciting properties like enhanced proton mobility and peculiar solvation character as observed both experimentally [19][20][21][22][23][24] and by theoretical calculations [21,25,26]. PILs share morphologically similar hydrogen bond networks with water [27], although they differ in almost every aspect.…”

Structure Effects on the Ionicity of Protic Ionic Liquids

“…Watanabe et al. proposed a Grotthuss‐type proton conduction mechanism in the PIL composed of N ‐methylimidazole and acetic acid involving fast rotation of acetic acid/acetate as a key step . A similar mechanism may contribute in [HA][Ac], accounting for the higher conductivity observed compared to the other PILs.…”

Electrolytes for Lithium (Sodium) Batteries Based on Ionic Liquids: Highlighting the Key Role Played by the Anion

“…This is a flawed argument (see below, Section 3.1.4). To date, the only known examples appear to be the protic 1‐methyl‐2‐oxopyrrolidinium tetrafluoroborate, [PyrOMe][BF 4 ], 1,8‐diazabicyclo‐[5,4,0]‐udec‐7‐eneium methanesulfonate, [DBUH][CH 3 SO 3 ], (which both need experimental confirmation given some uncertainties in the quality of the data), the pseudo‐protic ionic liquid N ‐methylimidazolium acetate where association is more certain, and certain lithium salts (see discussion below, Section 3.2.2). Normal aprotic ionic liquids all have positive like‐ion r ii that increase with increasing viscosity and decrease with increasing temperature .…”