AbstractDeep Eutectic Solvents (DES) are a recently-discovered category of potentially more sustainable alternative solvents. They are liquid eutectics formed upon the combination of various precursors, normally organic halide salts and neutral species. The most common DES is a 1:2 mixture of choline chloride:urea. DES are beginning to be used as non-aqueous alternative solvents for a variety of processes, such as synthesis of small molecules and materials, and electrodeposition.
For the successful development and implementation of DES as a drop-in green solvent, it is important to build a strong fundamental understanding of the structure and properties. This will aid in the realisation of DES as ‘task-specific’ solvents which can be rationally tuned to fit the application of interest. There are several fundamental issues presently impeding the progression of the field of DES. Firstly, due to their similar properties and designer nature, DES are often presented as a sub-category of ionic liquids (ILs) though the combination of ionic and molecular species will yield a more structurally complex system, with contributions from electrostatic forces as well as H-bonding. In this thesis the structure of various DES has been explored primarily using neutron diffraction and atomistic modelling studies. These works showed evidence for a disordered and extensive H-bond network in the liquid rather than extensive ion complexation, which has important consequences for the design of chemical processes using DES.
Another issue impeding the progress of research is that pure DES are often very viscous which causes handling issues and often imposes a diffusion limitation upon processes. Moreover, being charge-dense liquids, they are hygroscopic and quickly absorb large quantities of water, and it is not known what structurally occurs to DES upon hydration. To reconcile this issue, we have studied the solvation of water by DES, and what happens to the interactions between DES components when hydrated, at known water contents, using neutron diffraction studies as well as study of the DES/solid interface using AFM. It was found that low-level water, such as that absorbed during preparation and handling, does not significantly perturb the DES structure but alters the intermolecular interaction strength. Up to a threshold concentration, the DES structure resists hydration and strong choline-water interactions are seen, but the system becomes an aqueous solution when the water volume fraction dominates. The same behaviour was observed at a Pt electrode interface, with unusually strong structure induced when water was added. These findings show the potential of using hydrated DES as replacement green solvents.
Finally, it was attempted to apply DES and hydrated DES in the synthesis of nanostructured iron and cerium oxide and make use of the insights gained of the solvent structure. It was found that pure DES formed small nanoparticles whereas hydrated DES formed highly extended 1D morphologies which were active catalysts. Initially, neutron diffraction was used to understand the solvation of metal ions in the pure DES, which showed unusual structuring between reactive components. Later studies of the hydrated system revealed that this structure is not retained on addition of water, as DES ligands are substituted by water. Time-resolved studies using EXAFS and SANS respectively gave evidence for the solvent breakdown and structural rearrangement around metal ions, and the nanoparticle self-assembly process.
Overall, this thesis is a coherent body of independent systematic investigations into the solvent structure and solvation behaviour of DES, and synthesis of nanoparticles with wide-reaching environmental applications. We have built further upon the fundamental understanding of DES and have drawn comparison between systematic structural observations and the performance of DES in relevant applications. It is hoped that these findings will help with the onward development of DES as alternative solvents for efficient and sustainable future industrial technologies, which can make the world a cleaner place.
|Date of Award||19 Jun 2019|
|Sponsors||Rutherford Appleton Laboratory|
|Supervisor||Salvador Eslava Fernandez (Supervisor) & Karen Edler (Supervisor)|
- Deep eutectic solvents
- Neutron scattering
- Liquid structure