Protic ionic liquids based on fatty acids: A mixture of ionic and non-ionic molecules

Fatty acids are long chain carboxylic acids naturally and abundantly found in oils and fats; therefore, they are interesting candidates for protic ionic liquids (PILs) synthesis. Mixtures of carboxylic acids and amines should lead to intermediate proton transfer from the acid to the amine to form PILs, with both ionic molecules (PIL) and non-ionic molecules observed in these mixtures, depending on the nature of the amine. This work studied the physicochemical properties of mixtures between fatty acids and amines, with phase transitions, thermal stability, viscosity, solvation (Kamlet-Taft parameters) and proton transfer evaluated. Our results show that like other carboxylic acids the proton transfer is highly dependent on the structure of the amine, being much higher for primary amines than for tertiary amines. Also, hydroxyl groups on tertiary amines increase the proton transfer. The extent of the proton transfer strongly influences the physicochemical properties of the mixtures, where a larger proton transfer leads to a higher melting point and higher viscosity. Also, mixtures based on highly volatile amines decomposed at low temperatures even when a high degree of proton transfer was observed. Surprisingly, mixtures based on butylamine or triethylamine exhibited high hydrogen bond acceptor capacity and low dipolarity-polarizability which is an unusual characteristics of ionic liquids, potentially opening new solvent application space for ionic liquids.