Effect of water addition on choline chloride/glycol deep eutectic solvents: Characterization of their structural and physicochemical properties

In the present study the effect of water on the physicochemical properties and on the structural features of three deep eutectic solvents (DESs) were evaluated. The DESs are formed by choline chloride (ChCl) and glycols with a different number of oxyethylene units: diethylene glycol (DEG), triethylene glycol (TEG) and poly(ethylene gly- col) 200 (PEG 200). From the dilution effect on the physicochemical properties of DESs it has emerged that even a small amount of water could increase fluidity, conductivity and polarity of the DESs. The effect of water addition on the nanostructural changes of DESs was also investigated by FTIR and NMR spec- troscopies. The results clearly demonstrated that there are strong interactions between the two components of the DES and dilution with water caused the interactions weaken gradually up to 50% (w/w) water added, but with the supramolecular structures to some extent preserved. Subsequent water additions led to the disappear- ance of such interactions until they disappeared completely at around 75% (w/w). Moreover, NMR studies showed that the H-bonding interactions between ChCl and DEG are less strong than those between ChCl and both TEG and PEG 200, indicating that the number of oxyethylene units may be important. The changes observed in the physicochemical properties and on the nanostructural features of the mixtures can lead to the realization of liquid designed for tailored applications