Dissolution of Zinc Oxide Using Choline Chloride-Based Deep Eutectic Solvents with Various Hydrogen Bond Donors

Metal oxides (MO) are essential in various industrial applications, yet their limited solubility in common
solvents often hinders their utility. This study investigates the dissolution of Zinc Oxide (ZnO) using
green and cost-effective deep eutectic solvents (DESs) composed of Choline Chloride (ChCl) as a
hydrogen bond acceptor (HBA) paired with three different hydrogen bond donors (HBDs): Ascorbic
Acid (AA), Maleic Acid (MA), and Urea. The synthesized DESs were characterized using Fourier-
transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), and differential scanning
calorimetry (DSC) to confirm hydrogen bond formation and thermal stability. Dissolution experiments
were conducted to evaluate the effects of HBD type, ZnO mass, heating time, and temperature, with
zinc concentrations measured via atomic absorption spectroscopy (AAS). The results indicate that
ChCl:AA (2:1) exhibits the highest dissolution ability, achieving a dissolution efficiency of 80.6% at 5
mg of ZnO, significantly outperforming ChCl:MA (2:1) at 71.4% and ChCl:Urea (1:2) at 77.4% under
identical conditions. While both ChCl:AA (2:1) and ChCl:MA (2:1) were capable of dissolving a
maximum mass of 20 mg of ZnO, the superior efficiency of ChCl:AA is attributed to its unique chemical
properties as a reducing agent and its capacity to form stable, soluble zinc ascorbate complexes. The
optimum dissolution parameters were identified as 70 °C for 48 hours for ChCl:AA (2:1), 60 °C for 18
hours for ChCl:MA (2:1), and 80 °C for 48 hours for ChCl:Urea (1:2). These findings highlight the
potential of acid-based DESs, particularly those with complexing capabilities like ChCl:AA, as
effective and sustainable solvents for metal oxide dissolution.