Researchers at the University of Liège have used Raman spectroscopy to determine the acidity of ionic liquids for the first time, outperforming traditional methods. This discovery is significant as ionic liquids contain more active protons due to lower proton solvation, making them much more acidic than water. Ionic liquids are composed of organic salts and are liquid at room temperature.
Traditional pH measurement is not applicable in these solvents, so Hammett acidity functions, based on the protonation of weak bases in acidic solutions, are used instead. However, UV-visible spectroscopy has limitations, such as requiring transparent media and colored indicators that can interact with ionic liquids, distorting measurements.
A recent article in Physical Chemistry Chemical Physics describes an innovative method using Raman spectroscopy to measure Hammett acidity functions in three aprotic ionic liquids based on the cation 1-n-alkyl-3-methylimidazolium and the anion bistriflimide.
Raman spectroscopy eliminates potential sources of error as it does not require transparent media or colored indicators, unlike UV-visible spectroscopy. The study’s results confirmed the possibility of determining acidity functions in these ionic liquids, providing a better understanding of acidity-sensitive chemical reactions and new prospects for characterizing pH-sensitive processes in real-time.
This pioneering research enables experimental estimation of the energy required to transfer protons from water to ionic liquids, which can be compared with computational chemistry developments. The potential applications include understanding super-acidity for acid-catalyzed processes, stable electrolytes in batteries, and lignocellulosic waste depolymerization.
Aurelie Rensonnet, a researcher at University of Liège, explains that “this study marks a significant advance in the field of ionic solvent chemistry, paving the way for new methods of measuring acidity and opening up promising industrial applications.” (Physical Chemistry Chemical Physics. DOI: 10.1039/D3CP04741E)
Source: https://phys.org/news/2024-07-raman-spectroscopy-insights-ionic-liquid.html