Study of accessible acidity levels in ionic liquids by Raman spectroscopy

Abstract

Ionic Liquids (ILs) are molten salts at room temperature (<100°C) with particular physico-chemical properties of interest for the chemical industry. They exhibit properties that classify them as green solvents, such as non-volatility. Additionally, they possess super-acidity, enabling them to replace conventional molecular solvents and paving the way for various applications like catalysis and synthesis. Given the current European legislations, such as REACH, ionic liquids were identified as an alternative to polluting volatile organic solvents. Therefore, to gain a deeper understanding of these unique solvents, particularly their super-acidic nature, this Master thesis focuses on the experimental estimation of proton solvation energy in the ionic liquid [BMIm][OTf] using Raman and UV-visible spectroscopy, employing the Hammett acidity function. The first part explores the use of different nitroanilines as pH indicators in the [BMIm][OTf] to determine the Hammett acidity function values with the gradual addition of a strong acid (HOTf). The Results indicate that the determined acidity is apparent due to the influence of basicity and the concentration of pH-reporter on the proton solvation energy, along with the formation of ion pairs in the medium. The second part of this thesis addresses initial attempts to study proton transfer from a pH-sensitive molecule adsorbed on silver-coated gold nanoparticles using surface-enhanced Raman spectroscopy (SERS) directly in [BMIm][OTf]. The use of SERS aims to mitigate the impact of interactions between pH indicators and improve acidity characterization.