Solvatochromic Behavior of Polarity Indicators in PILs and Their Mixtures with Molecular Solvents: Autoprotolysis and Its Relation to Acidity

Abstract

It is interesting to know the behavior of Protic Ionic Liquids (PILs) within the binary mixture of molecular solvents, since it is usual to carry out processes such as organic and inorganic synthesis, or liquid–liquid extractions in the presence of another solvent. Moreover, on certain occasions, the absence of water is strictly required. In this sense, it is important to note that the addition of small amounts of IL to the molecular solvent allows a fine adjustment in its microscopic properties, obtaining “new solvent systems” with particular properties. The solvatochromic dyes are traditionally used as microscopic descriptors to determine the molecular microscopic properties of solvents, so we aim to re-evaluate the behavior of these probes and reconsider the validity of traditional polarity scales such as ET(30) in alkylammonium-based PILs, as well as in their mixtures with molecular solvents, knowing that the real composition of these PILs depends on the equilibrium of autoprotolysis. The characterization of systems was completed in terms of a change in the ∆pKa of the precursor species of a PIL. A thermal analysis was also employed to determine the acid strength’s role in an ion’s complete formation in pure PILs.