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

• Published in: Physical chemistry chemical physics : PCCP Vol. 17; no. 23; pp. 15297 - 1534
• Main Authors: D'Agostino, Carmine, Gladden, Lynn F, Mantle, Mick D, Abbott, Andrew P, Ahmed, Essa, I, Al-Murshedi, Azhar Y. M, Harris, Robert C
• Format: Journal Article
• Language: English
• Published: England 21.06.2015
• Subjects: Cations; Chlorides; Eutectics; Hydrogen bonds; Liquids; Nuclear magnetic resonance; Solvents
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c5cp01493j

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. Diffusional studies in deep eutectic solvent-water mixtures suggest that in many cases the fluids do not act as homogeneous mixtures and there is a complex behaviour between the cation, hydrogen bond donor and water.