Choline chloride-water mixtures as new generation of green solvents: A comprehensive physico-chemical study

Natural deep eutectic solvents (NADES) are an increasingly appreciated class of mixtures composed by eco-sustainable and environmentally responsible components with hydrogen bonding (HB) donor and acceptor capabilities that enable the establishment of an extended HB network. The latter eventually leads to a substantial drop in the mixture melting point with respect to the one of the ideal mixture. Water-based NADES represent an exciting system that responds to the above mentioned criteria and contain water as a major component, rather than as an impurity or a deliberately added minor ingredient. Choline Chloride (ChCl, a well known HB acceptor in DES systems) forms NADES when mixed with water, which plays the role of HB donor. Several applications have already been identified for this system, but chemical physical properties of choline-rich mixtures, including the DES concentration, are not well established, yet. In order to overcame this limitation and introduce a subsequent, more systematic investigation of atomistic organization in these mixtures, here we report results from a series of chemical physical characterizations of water:ChCl with ratio ranging from 2 to 10 between ca. 280 and 330 K. Calorimetric measurements, density, viscosity, electric conductivity, refractive index, X-ray diffraction patterns and several 1H and 35Cl NMR spectroscopy observables are reported and compared with literature (when available). The deep eutectic nature of the water: ChCl 4:1 mixture is assessed, leading to the individuation of aquoline, as the corresponding DES. The other chemical physical properties are observed to vary, upon water content, in a monotone way, without abrupt changes across the eutectic concentration, thus providing support for the exploitation of the ChCl/water class of solvents, with tunable chemical-physical properties across their wide liquid range.