Abstract

Pseudo-protic ionic liquids have recently become interesting candidates for potential application as proton-conducting electrolytes in fuel cells. These form a unique class of ionic liquids which owing to the low pKa difference between the involved species undergo partial ionization, leading to a complicated mixture inherently consisting of neutral molecules along with the dissociated ions. This affords the formation of extensive hydrogen bonding networks within the ionic liquids allowing for proton shuttling through the system, thus, inducing a Grotthuss diffusion enhanced conductivity.

In previous studies, the rich acid-base equilibrium in the 1-methyimidazolium acetate [C1HIm][OAc] system has been investigated through classical and ab initio molecular dynamics simulations to reveal factors influencing the conductivity, including the degree of ionization and introduction of neutral molecules [1, 2]. In this contribution, we are extending the study to the [C2HIm][OAc] ionic liquid – using ab initio MD (AIMD) simulations to investigate the effect of the increased side chain length on the hydrogen bond network formation and therefore, on the expected facile proton transport mechanism.

Since conductivity in such systems is sensitive to the introduction of amphoteric molecules, we explore the influence of the simple α-amino acid – L-alanine in its neutral and zwitterionic forms on the ionization and Grotthuss diffusion. Moreover, a similar investigation is performed with the neutral and protonated forms of an interesting non-canonical analogue of alanine, namely, 2-aminopropionamidine (N-alanine). These nitrogen-based analogues of α-amino acids known as α-amino amidines were brought into focus by J.B.S. Haldane’s proposition of ammonia as a possible life-supporting solvent and differ from native amino acids solely in the replacement of oxygen-based functionalities by nitrogen-based counterparts. In our previous investigations [3, 4], the solvation behaviours of these compounds in water and ammonia have been studied, contrasting them with those of α-amino acids. Based on findings from these studies, they could be considered potential candidates for aiding proton conduction in ionic liquids – a question that is explored in the present investigation.

References

[1] L. Wylie, M. Kéri, A. Udvardy, O. Hollóczki, and B. Kirchner, ChemSusChem, 16, e202300535 (2023).

[2] J. Ingenmey, S. Gehrke, and B. Kirchner, ChemSusChem, 11, 1900 (2018).

[3] H. Niemöller, J. Blasius, O. Hollóczki, and B. Kirchner, Journal of Molecular Liquids, 367, 120282 (2022).

[4] H. Niemöller, J. Ingenmey, O. Hollóczki, and B. Kirchner, The Journal of Physical Chemistry B, 129, 3007–3017 (2025).