Deep eutectic solvents: A structural point of view on the role of the anion

Highlights: • Reline and DESs with fluoride, nitrate or acetate instead of chloride were studied. • The effect of the different anions on the DES structure was highlighted. • Strong anion-urea hydrogen bonds are found in all DES systems. • The DES melting point order is not related to the urea-anion hydrogen bond strength. • DES properties depend on anion ability to build favourable networks of interactions. The structural properties of four deep eutectic solvents (DESs), namely a 1:2 mixture of choline chloride and urea, and three analogous DESs containing different anions in place of chloride, namely fluoride, nitrate or acetate, were investigated by using Molecular Dynamics. The order of the DES melting points was found not to correlate with the strength of urea-anion hydrogen bonds. However, the DES low melting points are related to the anion ability to build favourable networks of interactions with both choline and urea, in such a way as to maximize the hydrogen bonds among all the different moieties of the system.