Nanostructure of Deep Eutectic Solvents at Graphite Electrode Interfaces as a Function of Potential

Atomic force microscopy (AFM), density functional theory (DFT) calculations, and contact angle measurements have been used to investigate the liquid-highly ordered pyrolytic graphite (HOPG) electrode interface for three deep eutectic solvents (DESs) as a function of applied potential. The DESs examined are 1:2 mixtures of choline chloride and urea (ChCl:urea), choline chloride and ethylene glycol (ChCI:ethylene glycol), and choline chloride and glycerol (ChCl:glycerol). DFT calculations reveal that in all cases the molecular component is excluded from the graphite interface at all potentials, while chloride and choline are attracted into the Stern layer at positive and negative potentials, respectively. AFM force curves confirm these trends and also show that the first near surface liquid layer in contact with the Stern layer is rich in the molecular component. The extent of near surface layering increases with potential and the hydrogen bonding capacity of the molecular component. The variation in the macroscopic contact angle with potential is consistent with changes in the Stern layer composition.