Ionicity of Protic Ionic Liquid: Quantitative Measurement by Spectroscopic Methods

Area of Science:

• Chemistry
• Materials Science

Background:

• Ionic liquids are salts that are liquid at ambient temperatures.
• Protic ionic liquids (PILs) are formed by the proton transfer between Brønsted acids and bases.
• Determining the ionicity of PILs is crucial for understanding their properties and applications.

Purpose of the Study:

• To determine the ionicity of n-propylammonium acetate, a protic ionic liquid.
• To evaluate the suitability of different methods for measuring ionicity in PILs.
• To establish spectroscopic techniques as reliable methods for ionicity determination in PILs.

Main Methods:

• Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to observe molecular and ionic species.
• Potentiometric titration was assessed but found unsuitable due to a small ΔpK_a.
• Nuclear magnetic resonance (NMR) spectroscopy was employed to verify the ionicity results.

Main Results:

• Spectroscopic methods, specifically IR and NMR, were successfully applied to determine the ionicity of n-propylammonium acetate.
• The IR method, using characteristic absorption peaks calibrated with a standard curve, indicated 93% ionic species.
• The NMR method, analyzing the weighted average of ¹H chemical shifts, corroborated the IR findings.

Conclusions:

• Spectroscopic techniques (IR and NMR) are effective for determining the ionicity of protic ionic liquids, especially those with a small ΔpK_a.
• Potentiometric titration is not suitable for PILs with a small ΔpK_a.
• This study validates the use of IR and NMR for quantitative ionicity measurements in PILs.