Excess Molar Enthalpies of Deep Eutectic Solvents (DESs) Composed of Quaternary Ammonium Salts and Glycerol or Ethylene Glycol

Molar enthalpies of mixing (HE) were measured for the following deep eutectic solvents (DESs): {choline chloride + glycerol}, {choline chloride + ethylene glycol}, {tetrabutylammonium chloride + glycerol}, and {tetrabutylammonium chloride + ethylene glycol} at 323.15 K and molar ratios of 1:4, 1:3, 1:2 and 1:1. Results show that all systems are endothermic, with H-E values ranging from 1.90 to 5.35 kJ.mol(-1). Results indicate that the intermolecular interactions between the molecules of the pure components are stronger than those of the DESs complexes. To shed some light on the mutual interactions between the molecules within the mixtures, effects of the hydrogen bond acceptor structure (HBA), hydrogen bond donor structure (HBD), and concentration (HBA:HBD molar ratio) were analyzed. The nature of the HBA salt is the most important: choline chloride-based systems required almost twice as much energy as tetrabutylammonium chloride-based systems in order to form the DES mixture, most likely because of a higher enthalpy of fusion of the choline-based HBA salt. Choline chloride is more stable than tetrabutylammonium chloride because of its hydroxyl group; consequently, more energy is needed to break the choline chloride interactions in order to form DES mixtures with glycerol or ethylene glycol. Other effects suggest a competition in the formation of hydrogen bonds among the pure species (like molecular interactions) and the DES complexes (unlike molecular interactions). Overall, this work reports a systematic evaluation of H-E for a series of representative DESs that elucidates the roles of HBD and HBA in the energy penalty required for DES formation, which is critical for assessing their potential in practical applications on an industrial scale.