Specific Adsorption of Ions and Effect on Double-Layer

7 Specific Adsorption of Ions and Effect on Double-Layer 

The specific adsorptions of ions can affect the Helmholtz layer. For example, if the total charge of anions adsorbed on the electrode surface is more than the 

Equation (2.21a) suggests that the capacitance of the Helmholtz layer is mainly determined by the IHP s capacitance in the presence of a specific ion adsorption. For deeper understanding about this model, please refer to the related electrochemistry books [7]. 

Equation (2.21) suggests that the capacitance of the Helmholtz layer can be changed when the dielectric constant or the thickness of the Helmholtz layer is changed by a specific ion adsorption. The ion adsorption is dependent on the electrode potential, and the capacitance of the Helmholtz layer changes with the electrode potenhal. In this case, the Helmholtz capacitor may not be considered a linear capacitor. 

It is often observed that in practical systems, even using high concentration of electrolytes, the measured differential capacitances that are supposed to be those of the Helmholtz layers show some degree of electrode potenhal dependency. This may be explained by ion adsorption at high electrolyte concentrations. 

---