Investigating the effect of mono di carboxylic acids as hydrogen bond donor on solvation of copper in choline chloride-based deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) are green alternatives to highly acidic solvents or ionic liquids (ILs). The flexible complexation properties of eutectic solvents open their scope as a prospective dissolution medium for metal extraction. The present study investigates the range of monocarboxylic acids-choline chloride (Monocarboxy A-ChCl) and dicarboxylic acid-choline chloride (Dicarboxy A-ChCl) DESs as solvent media for copper oxide dissolution with a discussion on physical properties and their effect on dissolution. Thermal stability, freezing point, Kamlet Taft parameter, density, and viscosity of Carboxy A-ChCl DESs were experimentally determined.The density of Carboxy A-ChCl DESs follows the trend: FA-ChCl < Acetic A-ChCl < OA-ChCl < MA-ChCl < CA-ChCl. The viscosity of Monocarboxy A-ChCl was observed to be lesser than Dicaboxy A-ChCl DESs. The polarity of Monocarboxy A-ChCl was observed to be in the range of 1.2–0.8, while Dicarboxy A-ChCl, polarity was greater than 1. Further, the effect of hydrogen bond donors (HBD), hydrogen bond acceptors (HBA), KFT parameters, viscosity, fukui function of HBD, reaction time and temperature affecting the dissolution of Cu are elaborated in detail. Most dependant to least dependent parameters with respect to dissolution is recognized and some crucial conclusions were put together. Formic acid: Choline chloride (FA-ChCl) DES indicated the highest copper dissolution in the range > 106 ppm. Copper dissolution from ionic CuO to low polar Monocarboxy A-ChCl DESs was higher than in high polar Dicarboxy A-ChCl DESs. DFT calculation reveals that Formic Acid (FA) has the highest fukui functions compared to other carboxylic acids, which is directly reflected in the higher solubility of copper oxides. Additionally, the effect of reaction time and temperature on CuO dissolution in best-performing FA-ChCl DES was studied, 75 °C, 27 h, 100 °C, 18 h were chosen as the optimum time and temperature for maximum dissolution of Cu. The results cover important parameters of Carboxy A-ChCl DES, which could be helpful in metal extraction applications.