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

Abstract

Molar enthalpies of mixing (H E) 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.

Eutectic Mixtures Based on Polyalcohols as Sustainable Solvents: Screening and Characterization

“…In that work it was observed that the interactions with the chloride anion seem to be favored by some sugar configurations, as demonstrated by the difference in the values of the activity coefficient between the two isomers reported there, arabinose and xylose. 34 16 Although the systems studied are often reported as DES, 4,5,12,13,16,[35][36][37][38] it is here demonstrated that in some mixtures the eutectic temperature is very close to that predicted assuming ideality ([Ch]Cl+ethylene glycol or propanediol). The absence of favourable interactions on these systems is further supported by the excess enthalpy measurements reported by Lopez-Porfiri et al 37 showing these mixtures to be endothermic.…”

Phase behavior of aqueous solutions of ethaline deep eutectic solvent

“…Choline chloride (>99%) and ethylene glycol (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Two DES were prepared by weighting and adding ethylene glycol and choline chloride in a molar ratio of 2:1 and 3:1, which is in the region of maximum freezing depression [20,21].The DES were mixed by mechanical agitation at about 60°C for 30 min until a clear homogeneous liquid phase was obtained and served as stock solutions. Two series of 10 working solutions were prepared by pipetting and addition of deionized water.…”

Viscosity of Choline Chloride-Based Deep Eutectic Solvents: Experiments and Modeling

“…All the DES used in this work have a higher density than the pure HBD used in their preparation and at a fixed temperature, the density increases with a higher molar fraction of the HBA. Other workers 45,46 have shown that the DES in general exhibit a non ideal behavior. For the systems studied in this work we cannot confirm this finding as the density of solid choline chloride has not been reported in literature and the first derivative of molar volume remains constant over the limited range of mole fractions studied.…”