Ionicity of Protic Ionic Liquid: Quantitative Measurement by Spectroscopic Methods

The ionicity value, which is a key property of protic ionic liquids, was obtained by attenuated total reflection Fourier transform infrared spectroscopy and nuclear magnetic resonance, respectively, for a protic ionic liquid: n-propylammonium acetate. The method of potentiometric titration is found to not be suitable for such a kind of ionic liquid, as the ΔpKa of the compositing acid and base is relatively small. In the IR spectrum, molecular species can be directly observed in the range 1200–1800 cm–1, and the ratio of ionic and molecular species can be quantitatively calculated by the area of characteristic absorption peaks calibrated by a standard curve from NaAc/HAc solutions. The results show that 93% components in n-propylammonium acetate are ionic species. The NMR method was also introduced to test and verify the result. Despite that only one mixed peak can be observed for molecular and ionic species, the observed 1H chemical shift can be assumed to be the weighted average of them. In this way, the ionicity can be calculated and it fits well with the IR approach. It indicates that, for a protic ionic liquid with a relatively small ΔpKa value, spectroscopic methods such as IR and NMR could be applied to determine the ionicity.