Admittance as a useful tool for investigation of solute–solvent interactions at the double layer

The application of admittance measurements, an often neglected topic in impedance spectroscopy, to further our understanding of solute–solvent interactions at the double layer is presented by comparing aqueous solutions of sodium acetate and protonated lidocaine acetate. Unlike impedance, the overlap of concentration-dependent hydration cospheres and the potential- and frequency-dependent orientation effects of water molecules around the ions at or near the double layer could be manifested in admittance data. Admittance, but not impedance, also allows identification of interactions between two dipoles, such as amino and carbonyl groups, by changing frequencies and applied potentials. The effects manifest themselves by exhibiting cathodic and anodic shifts with frequency changes. All the admittance data could be reasonably explained by utilizing the concept of “potential induced and water structure-enforced ion-pair formation”.