Experimental Evidence for Azeotrope Formation from Protic Ionic Liquids

Herein, we present experimental evidence that protic ionic liquids (PILs), derived from 1 : 1 liquid mixtures of the organic superbases 1,5‐diazabicyclo[4.3.0]non‐5‐ene (DBN) and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) with carboxylic acids, form azeotropic mixtures with acid/base molar fractions different from 1 : 1. The ability of the carboxylic acids to form strong hydrogen bonds with the PIL ion pair leads to an azeotropic composition richer in the acid component. The results show that the azeotropic composition is ruled by the extent of acid‐base equilibrium and the relative volatility of the neutral species in the PIL medium. The PILs show marked negative deviations from Raoult's Law with the stronger superbase (DBU) leading to an azeotropic composition closer to the equimolar 1 : 1 ratio. Acid over base! Protic ionic liquids readily form azeotropic mixtures with non‐stoichiometric compositions. The ability of carboxylic acids to form hydrogen bonds with the ion pair explains why these azeotropes are richer in the acid component.