Adsorption complexes of copper and copper oxide in the deep eutectic solvent 2:1 urea–choline chloride

Adsorption of metal oxide particulates by soft surface solvent action using the deep eutectic solvent (DES) mixture of choline chloride and urea is examined at the molecular level. Quantum chemical calculations are employed to determine the binding energy of neutral and anionic urea to copper oxide and elemental copper, the main complexes believed to participate in surface cleaning of this DES. The possibility of the existence of urea anions in this deep eutectic solvent is studied in terms of the possibility of proton transfer taking place. Results show that the anion formation is stabilized in a dielectric and that the relative binding of neutral and anionic urea to copper oxide is similar whereas elemental Cu significantly favors binding with the urea anion. This work also shows that the hydrogen bond interactions of the urea with the chloride ion maintains an open cluster structure as suggested by experiment and that proton transfer can occur with increasing temperature or with metallic complex formation leading to more aggressive solvent action.