Physical properties of biomass-derived novel natural deep eutectic solvents based on choline chloride and (R)-3-hydroxyacids

We report the synthesis and physiochemical characterization of a series of Natural Deep Eutectic Solvents (NADESs), a subclass of ionic liquids that have demonstrated their potential as green solvents for widespread commercial application, including bio-safe chemical engineering technologies. The novel NADESs comprise of hydrogen bond donating (HBD) (R)-3-hydroxyalkanoic acids (R3HAs), biodegradable monomers of renewable polymer polyhydroxyalkanoate (PHA), and an appropriate conjugate hydrogen bond acceptor (HBA) species; namely choline chloride (ChCl). We combined theoretical and experimental methods to characterize a series of NADESs with increasing content of R3Has. Gas-phase theoretical modelling provided insight on potential hydrogen bond interactions between the components of the novel NADES and facilitated interpretation of their IR and H NMR spectra.The experiments revealed that the viscosity of R3HA:ChCl NADESs increased with decreasing temperature and increasing fraction of ChCI. The viscosity-temperature dependence was best described by a polynomial relationship; the Vogel-Tamman-Fulcher (VFF) model was also used but did not represent the empirical data, suggesting R3HA:ChCl NADES's viscosity-temperature behaviour is of non-Arrhenius type. Similarly, density was shown to increase with decreasing temperature and increasing ChCl fraction. In contrast to viscosity, the bonding ratio change was found to have little effect on density at a fixed temperature, other than the proportional increase associated with a higher fraction of the denser component. Solubility and wettability investigations concluded that R3HA:ChCl NADESs are compatible with mildly polar and hydrophobic solvents and as a result have a polarity index in the range 0-4.1. (C) 2020 Elsevier B.V. All rights reserved.