Understanding the role of temperature in structural changes of choline chloride/glycols deep eutectic solvents

Deep eutectic solvents (DESs) represent a new "green" alternative to classic organic solvents; however, their high viscosity often limits their utilization. The physicochemical properties of DESs can be tuned by adding cosolvents, by varying the operating temperature, or both, but their structure has to be preserved. In this work, three pure DESs based on choline chloride (ChCl) mixed with diethylene glycol (DEG), triethylene glycol (TEG) or polyethylene glycol 200 (PEG 200), as well as ChCl/TEG/water mixtures were studied. Particular attention was paid to the effect of temperature on some properties, i.e. density, conductivity, and viscosity, and on the structural characteristics of pure DESs and DES/water blends in the range 303-353 K. All the pure eutectic solvents followed both Arrhenius law and Walden's rule and showed very similar activation energy and poor ionicity, indicating the formation of intimate ion pairs. Multinuclear NMR analysis, such as the determination of the chemical shift of 1H, 13C, 35Cl and linewidths at half height of 14N and 35Cl, as a function of temperature highlighted the conservation of the characteristic supramolecular interactions up to 353 K. However, the temperature effects on the physicochemical properties of DES/water mixtures changed differently depending on the water content. Excess properties and NMR investigations highlighted the strengthening of the hydrogen bond interactions up to a 10% level of added water and their progressive weakening at a higher degree of hydration. These results also showed that temperature only slightly affects the structure of both DES and DES/water systems but can drastically increase their transport properties.