Liquid structure of the choline chloride-urea deep eutectic solvent (reline) from neutron diffraction and atomistic modelling

The liquid structure of the archetypal Deep Eutectic Solvent (DES) reline, a 1 : 2 molar mixture of choline chloride and urea, has been determined at 303 K. This is the first reported liquid-phase neutron diffraction experiment on a cholinium DES. H/D isotopic substitution is used to obtain differential neutron scattering cross sections, and an Empirical Potential Structure Refinement (EPSR) model is fitted to the experimental data. Radial distribution functions (RDFs) derived from EPSR reveal the presence of the anticipated hydrogen bonding network within the liquid, with significant ordering interactions not only between urea and chloride, but between all DES components. Spatial density functions (SDFs) are used to map the 3D structure of the solvent. Interestingly, choline is found to contribute strongly to this bonding network via the hydroxyl group, giving rise to a radially layered structure with ordering between all species. The void size distribution function calculated for reline suggests that the holes present within DESs are far smaller than previously suggested by hole theory. These observations have important implications in the future development of these 'designer solvents'.