Proton Transfer Equilibrium in Pseudoprotic Ionic Liquids: Inferences on Ionic Populations.

Nonstoichiometric pseudoprotic ionic liquids (NPPILs) are an emerging class of ionic liquids with interesting physical properties and intriguing prospects for technological applications. However, fundamental questions remain about the proton transfer equilibria that underlie their ionic character. We use a combination of nuclear magnetic resonance spectroscopy, infrared spectroscopy, and small-angle X-ray scattering to characterize the equilibria of trihexylamine/butyric acid and water/butyric acid mixtures. This combination of techniques offers considerable insight into how proton transfer changes with the composition of the mixture. Further, we construct a model based on information from H and C NMR, which yields numerical values for the concentrations of all ions present in the trihexylamine/butyric acid mixture, and demonstrate that the results of the model are supported by data from other physical measurements. This is the first quantitative calculation of ionic concentrations in an NPPIL.