Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?

We have performed small-angle neutron scattering in a momentum transfer range (0.05 < Q < 0.5 Å^-1) to study long-range order and concentration fluctuations in deep eutectic solvents (DESs) and their aqueous solutions. Ethaline (choline chloride/ethylene glycol), glycerol/lactic acid, and menthol/decanoic acid mixtures were selected to illustrate individually the case of ionic, nonionic, and hydrophobic mixtures. Carefully designed isotopic labeling was used to emphasize selectively the spatial correlations between the different solvent components. For ethaline DESs and their aqueous solutions, a weak low-Q peak observed only for certain compositions and some partial structure factors revealed the mesoscopic segregation of ethylene glycol molecules that do not participate in the solvation of ionic units, either because they are in excess with respect to the eutectic stoichiometry (1:4 neat ethaline) or substituted by water (4w-ethaline and higher aqueous dilutions). For the nonionic hydrophilic solutions, such a mesoscopic segregation was not observed. This indicates that the better balanced interactions between the three nonionic H-bonded components (water, lactic acid, and glycerol) favor homogeneous mixing. For the hydrophobic DESs, we observed an excess of coherent scattering intensity centered at Q = 0, which could be reproduced by a model of noninteracting spherical domains. Local concentration fluctuations are not excluded either. However, unlike liquid mixtures with a tendency to demix, we have found no evidence of expansion of domains with different compositions to a large scale.