Luke WYLIE1, Barbara KIRCHNER1
1University of Bonn, Bonn, Germany
The ability for protons to readily diffuse throughout a solution independent of being attached to associated molecules is a phenomena known as Grotthuss diffusion and is prominent in [C1Him][Ace] as studied previously by Ingenmey et al. It is also known that [C1Him][Ace] acts as somewhat of a “pseudo-ionic liquid”, due to the ionization of its ions being reported at only between 10% and 40% in solution. In this study we use molecular dynamics, both classical and ab initio, to investigate both the effect of the partial ionization of [C1Him][Ace] and the addition of separate 1-methylimidazole and acetic acid on the physical properties and Grotthuss diffusion of the solution.
It was found from this investigation that higher ionicity correlated to an increased density with a decrease in diffusion coefficients, likely due to the ions forming stronger intermolecular interactions and thus hindering movement. Among the substituents it was found that acetate is capable of forming stronger and more localized hydrogen bonds with hydrogen bond acceptors than imidazole, while also indicating that as ionization increases, the coordination number of acetate hydrogen bonding with imidazolium and acetic acid approaches 1, whilst the coordination number of imidazole with the same groups approaches 0, as determined for analysis of radial distribution functions.
Addition of imidazole and acetic acid were shown to provide inverse effects for Grotthuss diffusion. In this instance it was shown that imidazole addition increases the Grotthuss diffusion of protons, while acetic acid lowers it. This is shown by examining the lifetimes of the covalent bonds, wherein addition of excess acetic acid was shown to cause the 28.1% of O-H bonds remain after 10 ps when acetic acid is in a 1:1 ratio to [C1Him][Ace], whereas 0.7% remains after the same duration when imidazole is added in the same ratio. This is found to come due to the addition of acetic acid allowing for stable H-bond networks to stabilize covalent bonds through an abundance of H-bond acceptors. While in imidazole addition the labile hydrogen is able to jump between multiple H-bond donors freely. This insight provides the potential to tailor Grotthuss diffusion through addition of H-bond donors such as imidazole to improve on the desirable properties which high Grotthuss diffusion can enable.