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

Protic ionic liquids (PILs) form through proton transfer from a Bronsted acid to a Bronsted 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 prima...