Solute-solvent interactions from admittance measurements: Potential induced and water structure-enforced ion-pair formation

Admittance measurements of aqueous 0.010, 0.10 and 1.0 M sodium chloride solutions and 0.010 M solutions of other sodium halides were investigated using a mercury working electrode. The measurements indicated a decrease or increase in admittance with decreasing frequency, depending on the concentration of sodium chloride and the range of the scanning potential. As the potential changes from negative to less negative, to zero and finally to positive, the admittance increased and passed through a maximum. There was also a slight anodic shift in the maximum with decreasing frequencies, suggesting the role of solute-water interactions and orientation effects of water near the double layer changeover potential. To explain the admittance data of NaF, NaCl, NaBr, and NaI, we have proposed a new model, "potential induced and water structure-enforced ion-pair formation", at or near the double layer. A comparison of the concentration dependence of the admittance data for NaCl suggested that this type of ion-pair formation was also favored even in very dilute solutions at or near the double layer, contrary to the normal electrolyte behavior in bulk. The present model also points out the need to modify the current models employed to explain the differential capacitance and the surface excess entropy data.