Liquid structure of a choline chloride-water natural deep eutectic solvent: A molecular dynamics characterization

The liquid structure of a representative of the first water-in-salt (WiS) Natural Deep Eutectic Solvents (NADES), hereinafter indicated as aquoline, a mixture of choline chloride (ChCl) and water with molar ratio 1:3.33, is explored at ambient conditions. Using Molecular Dynamics (MD) simulation tools, we extract structural information at atomistic level on the nature of inter-correlations between the different moieties. Despite being a very fluid liquid, with much lower viscosity than other common ChCl-based DES, aquoline turns out to be very structured. Computed X-ray and neutron weighted scattering patterns (the latter also on selectively deuterated mixtures) highlight the existence of mesoscopic organization that is rationalised in terms of choline vs. water/chloride structural alternation. The study shows that choline cations are highly coordinating the surrounding environment: strong hydrogen bonding mediated correlations between the hydroxyl group and water or chloride are detected. In addition, the ammonium group drives the formation of a complex solvating environment, with water, chloride and hydroxyl moieties approaching it, between the hindering methyl groups. Strong hydrogen-bonding interactions between water molecules and between water and anions are detected and, while water cannot create a bulk water-like environment around itself, its network with neighbour water or anions develops long chains across the bulk phase. This is a first study that will be extended based on complementary experimental work as a function of water content and temperature/pressure, to explore structural and dynamic properties of this class of materials.