Green Solutions: Deep Eutectic Solvents

Interactions between DES molecules and water.

Snapshot of the nanostructure of the choline chloride-urea-water 1:2:10 mixture, showing the interactions between the DES molecules and the clusters of water, which are depicted as blue surfaces. Neutron diffraction data were taken using SANDALS and NIMROD, and resolved using EPSR modelling. Credit: STFC.
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First reported in 2001 (by Abbott et al), Deep Eutectic Solvents (DESs) are part of an extended class of ionic liquids. Frequently made from cheap, benign and biodegradable components, they offer the enticing possibility of being designer solvents without the inherent toxicity of many room temperature ionic liquids.

The defining characteristic of DESs is that - at a unique percentage ratio between their two components - they remain liquid at a temperature lower than the melting point of either component. Lower temperature liquid state chemistry is not just more energy efficient; it allows the synthesis of products that would degrade under more extreme conditions.

Oliver Hammond’s paper on these intriguing solutions, ‘Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution’ is the latest in a triumphant trio for this 2nd-year PhD student from ISIS and the University of Bath. Awarded a ‘Very Important Paper’ rating from prestigious journal Angewandte Chemie, it follows on from a paper on the structure of a DES in Green Chemistry, and one on the deep eutectic-solvothermal synthesis of nanostructured Ceria, in Nature Communications. Together these three papers tell us a story about a group of chemicals that could be as commercially interesting as room temperature ionic liquids, and which are made from much more environmentally friendly components.

We're just beginning to explore the possibilities of synthesis using DESs, but Hammond’s Nature Communications paper explains one application - using the DES reline (1:2 ChCl:Urea) for a more energy efficient synthesis of nanostructured Ceria, a technologically important material used in catalysts, vehicle emission control and solid-oxide fuel cells.

One of the problems with DESs is their high viscosity, which makes them challenging liquids to use in the lab and beyond. Their rheology and molecular scale morphology can be tailored by adding water, making them much easier to handle, but it is important (for both reproducibility and chemical engineering) that the water content is accurately reported. Hammond’s latest paper investigated the structural changes in one DES as water content increased, and found an unexpectedly sharp change - a point at which it ceased to behave as a DES, and became more like an aqueous solution. This tipping point is expected to be a feature of other DESs, albeit at a different water level.

The samples in this research were examined using the SANDALS and NIMROD total scattering instruments. According to Dr Daniel Bowron, leader of the ISIS Disordered Materials Group, neutrons are the perfect investigative tool for DESs, due to the light elements and hydrogen bonding they involve. Neutrons allow researchers to see the hydrogen atoms, so you see the critical solvent molecule interactions.

The next steps in this research are to study more DESs - reactions which work in one do not necessarily work in others - and to try more syntheses using these media. Tailoring the solvent to the reaction will give liquid state chemists new routes to the production of useful compounds and materials, as well as the delivering the possibility of new products to explore. All using solvents that are easy to make, non-toxic and easy to recover.

The interdisciplinary nature of this work is supported by funding from both STFC and EPSRC (via the Centre for Doctoral Training in Sustainable Chemical Technologies at the University of Bath).

Emma Cooper

Research date: May 2017

Further Information

Hammond OS et al. Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution. Angewandte Chemie (2017). DOI: 10.1002/anie.201702486.

Hammond OS et al. Deep eutectic-solvothermal synthesis of nanostructured ceria. Nature Communications 8 (2017). DOI: 10.1038/ncomms14150.

Hammond OS et al. Liquid structure of the choline chloride-urea deep eutectic solvent (reline) from neutron diffraction and atomistic modelling. Green Chemistry 18.9 (2016). DOI: 10.1039/C5GC02914G .

Abbott AP et al. Preparation of novel, moisture-stable, Lewis-acidic ionic liquids containing quaternary ammonium salts with functional side chains. Chemical Communications 19, 2010-2011 (2001). DOI: 10.1039/B106357J.

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