Ionic conduction within non-stoichiometric N-Methylimidazole-Acetic Acid Pseudo-Protic ionic liquid mixtures

Abstract

We recently suggested that proton conduction mechanisms, such as the Grotthuss mechanism, could occur in an equimolar mixture of N-methylimidazole (C1Im) and acetic acid (CH3COOH), which is a pseudo-protic ionic liquid (pPIL). Appropriate amounts of C1Im and CH3COOH were mixed in various molar ratios to investigate the concentration dependence of proton conduction. At a mole fraction of xC1Im = 0.2, the ionic conductivity was the highest, despite the highest viscosity. Proton dynamics and carriers are crucial in rapid proton conduction. To clarify the proton conduction mechanism and the identities of the chemical species within the non-equimolar pPILs [C1Im]x[CH3COOH](1 –x), pulsed-field gradient nuclear magnetic resonance and Raman spectroscopy were performed. C1HIm+ predominantly existed at xC1Im = 0.2 due to proton transfer from CH3COOH, and CH3COO–···CH3COOH hydrogen bonding occurred. With an increasing molar ratio of C1Im, the molecular complex C1Im-CH3COOH formed, with the species distribution function exhibiting a maximum at approximately xC1Im = 0.5. Although the self-diffusion coefficients were small and the viscosity increased with decreasing C1Im concentration, the ionic conductivity increased. In addition, the self-diffusion coefficients of the labile hydrogen atoms were the largest among all of the hydrogen atoms at xC1Im = 0.2, suggesting that a specific proton conduction mechanism occurred via hydrogen bonding. Furthermore, the proton conduction mechanism could be controlled by altering the composition of the pPIL.