Do group 1 metal salts form deep eutectic solvents?

• Published in: Physical chemistry chemical physics : PCCP 2016-09, Vol.18 (36), p.25528-25537
• Main Authors: Abbott, A. P, D'Agostino, C, Davis, S. J, Gladden, L. F, Mantle, M. D
• Format: Article
• Language: English
• Subjects: Behavior; Dynamics; Eutectics; Glycerols; Liquids; Sodium salts; Solvents; Viscosity
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp05880a

Mixtures of metal salts such as ZnCl 2 , AlCl 3 and CrCl 3 ·6H 2 O 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·3H 2 O and Na 2 B 4 O 7 ·10H 2 O. 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. 1 H and 23 Na 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 1 H species, probed by means of PFG NMR diffusion coefficients, is in line with the viscosity of these liquids. However, 1 H and 23 Na T 1 relaxation measurements suggest that the Na-containing species also play a crucial role in the structure of the liquids. Group 1 salts were compared with quaternary ammonium chlorides for their ability to form deep eutectic solvents and it was found that while some formed liquids the sodium ions caused the liquids to become structured and increased their viscosity.