How Proton Transfer Equilibria Influence Ionic Liquid Properties: Molecular Simulations of Alkylammonium Acetates

Protic ionic liquids (PILs) form through proton transfer from a Brønsted acid to a Brønsted base. In this work we use molecular dynamics simulation to study how PIL properties vary with χ, the extent of the proton transfer reaction. Three PILs are considered: N-propylammonium acetate, [N3]­[Ac], N-butylammonium acetate, [N4]­[Ac], and N,N-dimethylbutylammonium acetate, [N114]­[Ac]. In all cases density and viscosity increase with increasing χ, while diffusivities of all species decrease with increasing χ. In each PIL the ionic conductivity exhibits a maximum at intermediate χ due to competition between increasing ion concentration and decreasing ion mobility. Ionicity analysis suggests that strongly correlated behavior is present at all χ. Finally, we determine the χ for which the properties of each simulated PIL best agree with experimental data; these are χ = 0.86, 0.80, and 0.18 for [N3]­[Ac], [N4]­[Ac], and [N114]­[Ac], respectively. These results suggest that proton transfer is nearly complete in the primary ammonium PILs but not in the tertiary ammonium PIL, consistent with recent experimental observations. We propose that this difference is due to cooperative production of hydrogen bonds with increasing χ in the primary ammonium PILs, which does not occur in the tertiary ammonium PIL.