A Comparison of the Acidity Levels in Room-Temperature Ionic Liquids

The Bronsted acidity level was evaluated for several ionic liquids ([BMIm][BF4], [BMIm][PF6], [BMIm][SbF6], [BMIm][OTf], [BMIm][NTf2], [HNET3][NTf2], and [HBIm][NTf2]) to which a strong acid, such as HNTf2 [NTf2=N(CF3SO2)(2)] and HOTf (OTf=CF3SO3), has been added. The main purpose is to tentatively measure the influence on the resulting acidity of (i) the solvent anion or cation structure and (ii) the added acid nature. The evaluation method is based on the determination of the Hammett acidity functions H-0 using UV/visible spectroscopy. The acidity of protons is mainly determined by their solvation state, and consequently, the properties of protons depend on both the nature of the solvent and the nature and concentration of the acid. In practice, for the investigated ionic liquids, the cation as well as the added acid nature does not play a dominant role, whereas changing the anion nature may lead to very different acidities. Indeed, for a similar content of added acid, the measured acidity levels are in the order PF6->BF4->NTf2->OTf-. The problems of the influence of impurities on the final acidity and of the dissociating character of the ionic liquid are addressed.