Influence of the Hammett Base Concentration on Acidity Levels in Ionic Liquids Using Raman Spectroscopy and Hammett Acidity Functions

ABSTRACT Room‐temperature ionic liquids (RTILs) are a class of solvents with remarkable properties, including specific solvation properties and tuneable acidity levels. The Hammett acidity functions, commonly used to assess the acidity levels of ILs, rely on monitoring the protonation of colour indicators (especially nitroanilines) via UV‐visible spectroscopy. However, this method possesses certain limitations, prompting our group to adapt it in Raman spectroscopy in a previous study. Yet the influence of the indicator concentration on the acidity functions has never been thoroughly examined. In this article, we investigated the effect of the 2,4‐dichloro‐6‐nitroaniline concentration (from 7 to 50 mM) on the Hammett acidity functions estimated by Raman spectroscopy in 1‐butyl‐3‐methylimidazolium bistriflimide, [BMIm]NTf 2 . The acidity measured with this method was higher than the acidity evaluated via UV‐visible spectroscopy but lower than the acidity evaluated via Strehlow acidity functions, suggesting specific solvation effects of the nitroaniline in [BMIm]NTf 2 and the formation of ion pairs in this solvent, which are discussed in this paper.