Do group 1 metal salts form deep eutectic solvents?

Abstract

Mixtures of metal salts such as ZnCl₂, AlCl₃ and CrCl₃·6H₂O form eutectic mixtures with complexing agents, such as urea. The aim of this research was to see if alkali metal salts also formed eutectics in the same way. It is shown that only a limited number of sodium salts form homogeneous liquids at ambient temperatures and then only with glycerol. None of these mixtures showed eutectic behaviour but the liquids showed the physical properties similar to the group of mixtures classified as deep eutectic solvents. This study focussed on four sodium salts: NaBr, NaOAc, NaOAc·3H₂O and Na₂B₄O₇·10H₂O. The ionic conductivity and viscosity of these salts with glycerol were studied, and it was found that unlike previous studies of quaternary ammonium salts with glycerol, where the salt decreased the viscosity, most of the sodium salts increased the viscosity. This suggests that sodium salts have a structure making effect on glycerol. This phenomenon is probably due to the high charge density of Na⁺, which coordinates to the glycerol. ¹H and ²³Na NMR diffusion and relaxation methods have been used to understand the molecular dynamics in the glycerol–salt mixtures, and probe the effect of water on some of these systems. The results reveal a complex dynamic behaviour of the different species within these liquids. Generally, the translational dynamics of the ¹H species, probed by means of PFG NMR diffusion coefficients, is in line with the viscosity of these liquids. However, ¹H and ²³Na T₁ relaxation measurements suggest that the Na-containing species also play a crucial role in the structure of the liquids.