Adsorption phenomena at the interface between mercury and solutions containing choline chloride, ethylene glycol and water

Abstract

Adsorption of the components of deep eutectic solvent ethaline (ethylene glycol and choline chloride) on mercury electrode is investigated by electrocapillary measurements. It is determined that choline cations are mainly adsorbed on the negatively charged surface of mercury, while chloride anions are mainly adsorbed on the positively charged surface. The corresponding values of free energies of adsorption and interactions of adsorbate and solvent with metal are calculated and analyzed. An anomalous increase in both the apparent value of the adsorption at limiting coverage and the free energy of the interaction of the choline cation with mercury is observed in the transition from aqueous to ethylene glycol solutions, which is explained by the formation of complexes in a surface layer that exist in deep eutectic solvents and are capable of adsorbing on the electrode surface. The free energy of interaction with the mercury surface is higher than the energy of squeezing out from the volume of the solution onto its surface, which indicates the specific interaction of the adsorbate with mercury. A marked decrease in interfacial tension on both branches of the electrocapillary curve is observed when water is added to ethaline.