Molecular and ionic diffusion in aqueous – deep eutectic solvent mixtures: probing inter-molecular interactions using PFG NMR

Abstract

Pulsed field gradient (PFG) NMR has been used to probe self-diffusion of molecular and ionic species in aqueous mixtures of choline chloride (ChCl) based deep eutectic solvents (DESs), in order to elucidate the effect of water on motion and inter-molecular interactions between the different species in the mixtures, namely the Ch + cation and hydrogen bond donor (HBD). The results reveal an interesting and complex behaviour of such mixtures at a molecular level. In general, it is observed that the hydroxyl protons ( 1 H) of Ch + and the hydrogen bond donor have diffusion coefficients significantly different from those measured for their parent molecules when water is added. This indicates a clear and significant change in inter-molecular interactions. In aqueous Ethaline, the hydroxyl species of Ch + and HBD show a stronger interaction with water as water is added to the system. In the case of Glyceline, water has little effect on both hydroxyl proton diffusion of Ch + and HBD. In Reline, it is likely that water allows the formation of small amounts of ammonium hydroxide. The most surprising observation is from the self-diffusion of water, which is considerably higher that expected from a homogeneous liquid. This leads to the conclusion that Reline and Glyceline form mixtures that are inhomogeneous at a microscopic level despite the hydrophilicity of the salt and HBD. This work shows that PFG NMR is a powerful tool to elucidate both molecular dynamics and inter-molecular interactions in complex liquid mixtures, such as the aqueous DES mixtures.

The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution

“…Therefore, above this point, it is inappropriate to describe the system as a DES, and it should instead be considered as a solution of DES components in water. Importantly, this nanostructure transition point correlates with trends in the physical properties of cholinium DES/water mixtures 8, 12, 23…”

“…This explains the tolerance of the DES nanostructure and properties towards hydration, and can be related to the solvophobic accommodation of solutes in ILs;2 “solvent‐separated ion pairs” are seen in many IL/cosolvent mixtures 21. A transient water‐rich sequestered domain around choline helps to rationalize the unusual properties of hydrated DESs, such as low water activity23 and a high water self‐diffusion coefficient 12. The retention of the DES “pseudo‐IL” character at such low ionic strengths is remarkable.…”

“…All DES interactions are weakened upon addition of 1 molar equivalent of water, except for the strengthened choline–urea hydrogen‐bonding interaction (OH⋅⋅⋅NH 2 ), which is reflected in the coordination numbers shown in the Supporting Information. This unexpected increase in intermolecular interaction strength explains why DES‐1 w systems do not undergo the anticipated viscosity reduction 8, 12. Whilst 80 % (on average) of the original nanostructure is retained at 1 w , even low‐level DES/water mixtures clearly differ from pure DES, with water contributing to the nanostructure and hence altering the physicochemical properties.…”

“…Trends in these properties suggest that there is an upper limit to this hydration above which DESs are more like aqueous solutions 9, 10, 11. However, it is not known how far such mixtures can be hydrated before they cease to be DESs on a nanostructural level because only a limited compositional range has been probed experimentally in detail by NMR spectroscopy,12 which has also been used extensively for ionic liquid (IL)/water mixtures 13, 14. The effect of water on DESs, and hence their classification, therefore remains one of the most significant unanswered questions in the field; do they resemble ILs, ionic mixtures, or merely solutions of ions?…”

Do group 1 metal salts form deep eutectic solvents?

“…12 Recent diffusion studies on aqueous choline chloride-based DES were able to show that some of these liquids are inhomogeneous at a macroscopic level and it was observed that the addition of water significantly changes the molecular and ionic interactions between the different species. 13 For example, in some cases, the hydroxyl protons of the hydrogen bond donor behave differently from its parent molecule when water is added to the system. Electrical conductivity, density and viscosity measurements of relatively dilute alkali metal halides and glycerol have been previously carried out.…”