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

Abstract The nanostructure of a series of choline chloride/urea/water deep eutectic solvent mixtures was characterized across a wide hydration range by neutron total scattering and empirical potential structure refinement (EPSR). As the structure is significantly altered, even at low hydration levels, reporting the DES water content is important. However, the DES nanostructure is retained to a remarkably high level of water (ca. 42 wt % H2O) because of solvophobic sequestration of water into nanostructured domains around cholinium cations. At 51 wt %/83 mol % H2O, this segregation becomes unfavorable, and the DES structure is disrupted; instead, water–water and DES–water interactions dominate. At and above this hydration level, the DES–water mixture is best described as an aqueous solution of DES components.


Empirical Potential Structure Refinement (EPSR) modelling
Parameters for the water oxygen 'O1' and hydrogen 'H1' atoms were derived from the TIP3P model, [5] and are given in Table 2. The composition of each simulation box is shown in Table 3. The DES components of the hydrated EPSR models were parameterised and labelled in exactly the same way that was reported previously for the pure DES; [6] the atom labels are shown again for reference in Figure 2. . DES molecules used to create the EPSR reference potential. The shown atom type labels will be referred to in the text. Reprinted with permission from the Royal Society of Chemistry. [6] Otherwise, the experimental procedure for the EPSR modelling was functionally identical to previous work on the pure reline DES. [6] The reference potential for each system was allowed to equilibrate to the experimental density described in Table 3, and the empirical potential was then allowed to equilibrate. Finally, ensemble information was interrogated from the model to gain information about the structure and bonding within these mixtures.

SDF plots
SDF plots are a way to aid with the visualisation of the 3D structure of a disordered liquid, with the surfaces showing the most likely places that a solvating species can be found around a certain molecule. Some additional spatial density function (SDF) plots are shown below, demonstrating some of the interesting and subtle changes occurring throughout the regime change. Firstly, the solvation of chloride by water is shown in supporting Figure 3, demonstrating the gradual increase in water-chloride interactions as the hydration level is increased. The appearance of the second solvation shell of chloride about water at 15w shows that water-chloride interactions are dominant over choline-chloride, and urea-chloride interactions. Secondly, the hydration of choline at 10w, the point at which this interaction is maximised, is shown in supporting Figure 4.
Here, the water forms a radial solvation band around the choline, much like is seen in the pure solvent, for the choline-urea and choline-chloride interactions, but at closer length scale.

Calculated Intermolecular Coordination Numbers
The tabulated intermolecular coordination numbers are shown in Tables 4 -10, and these were calculated by integrating partial radial distribution functions (pRDFs) up to a radius Rmax corresponding with their first minima, where Rmax is accurate to a maximum of one data bin, ie. Rmax ± 0.02 A -1 . For these intermolecular coordination numbers the polyatomic molecular centres were defined as the C2N atom of choline, the CU atom of urea, and the O1 atom of water.

Integrated partial (site-site) coordination numbers
As with the intermolecular coordination numbers, the tabulated site-site radial distribution function analysis is shown in Tables 11 -17. These values were calculated by integrating a selection of partial radial distribution functions (pRDFs) best describing the specific intermolecular bonding, up to a radius Rmax corresponding with their first minima, where Rmax is accurate to a maximum of one data bin, ie. Rmax ± 0.02 A -1 . For these coordination numbers, the 'error' reflects the disorder present in the liquid, rather than a lack of general confidence in the data. In this case, 'important' coordination numbers are those which have a variance significantly smaller than the coordination number itself, signifying a persistent correlation.