Connecting precursors to a protic ionic liquid: Effects of hydrogen bond synergy in acid-base binary mixtures on the solvent-solute interactions

Abstract

How does the solvation behaviour of a protic ionic liquid (PIL) relate to their precursor Brønsted acid and base? By studying how the solvent-solute interactions in a binary mixture of a Brønsted acid and base changes with respect to composition, a rational connection between precursor and PIL can be made. Using acetic acid and N,N-dimethylethanolamine as model precursors, we demonstrate here that the change in solvent-solute interactions, as inferred from the Kamlet-Taft solvatochromic parameters, are drastically different from additivity approximation based on the simple sum of precursors. Indeed, the apparent hydrogen bond basicity of these mixtures can either be greater than or lesser than either precursor, and the apparent hydrogen bond acidity of these mixtures are dependent on the solvatochromic probe used. Nevertheless, the change in polarizability correlates well with the extent of proton transfer taking place, as inferred from infrared spectroscopy, and can be used as a measure for precursor non-additivity in PILs. Our findings shed light on how the solvation properties of PILs connect to their precursor materials and showcase a new strategy to expand the tuneability of the solvent properties in PIL-based systems.