Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br^?) and complex (Al_nBr_3n+1]^?) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical ccsd(t)/MP2/DFT/COSMO‐RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pH_abs or μ_abs(H⁺, IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C₆MIM⁺AlBr₄]^? (C₆MIM=1‐hexyl‐3‐methylimidazolium) with or without the addition of basic (i.e. Br^?) or acidic (AlBr₃ and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.