Comparison of physicochemical and thermal properties of choline chloride and betaine-based deep eutectic solvents: The influence of hydrogen bond acceptor and hydrogen bond donor nature and their molar ratios

Within the green chemistry area, deep eutectic solvents (DESs) are playing an increasingly prominent role thanks to their intriguing physicochemical properties. However, a comparative study encompassing a wide range of properties as a function of different parameters such as the nature of the hydrogen bond acceptor (HBA) and donor (HBD) and their molar ratio is still missing. In this work, six DESs based on the most used HBAs (choline chloride (ChCl) and betaine (Bet)) and HBDs (ethylene glycol (EG), glycerol (Gly) and levulinic acid (LevA)) combined in three different molar ratio (1:2, 1:3, 1:4) have been prepared and subjected to a series of analysis aimed at measuring different properties such as density, viscosity, refractive index, thermal stability and thermal behaviour. The most striking findings see the EG-based DESs displaying the lowest density and viscosity values, while the Gly-based DESs exhibited the highest. High viscosity and density have been obtained using Bet instead of ChCl as HBA. Increasing the amount of HBD in DESs caused lower viscosity in all cases, while density increased for all Gly-based DESs and decreased for EG and LevA-based DESs. The refractive index also decreased when the HBD portion was increased. However, higher refractive index was obtained using ChCl instead of Bet as HBA. The temperature and wavelength dependence of the refractive index is otherwise described pretty well by a Sellmeier model. The molar refractivity implied by density and refractive index data via the Clausius-Mossotti equation is consistent with that predicted by the empirical but well-established model of Wildman and Crippen. The short thermal stability of all investigated DESs is strictly related to the HBD used. EG-based DESs were less stable than LevA-based ones while Gly-based DESs were the most stable materials. Moreover, for all DESs three characteristic mass loss events have been identified. They can be attributed to the evaporation/degradation of the HBD, of the intimately interacting HBD-HBA and of the HBA, respectively. Finally, DSC analyses showed that all DESs can be used as solvents given that they are liquids at room temperature, and they maintain a liquid state in a broad range of temperatures (Tg < −50 °C or no thermal events are observed).