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

In 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 obtai...