Abstract: The model proposed in previous paper was applied to metal/(mixed-solvents) solution system. According to this model, the solvation monolayer at the charged metal surface was formed by competitive adsorption of composed solvent molecules, and the dipole orientation of every solvent molecules was considered to be present in three probable states, namely, A(r)-negative end towards the metal, B(r)-positive end towards the metal, and C(r)-the dipole vector oriented at an angle Ψ(r) to the direction of electric field. Assuming the dipole orientation distribution for this adsorption layer obeys Bragg-Williams approximation and three distinctive oriented states may be converted to each other, then the mixed-adsorption isotherm could be derived by using the statistical mechanical method together with the thermodynamic equilibrium conditions.
Using the equilibrium relations mentioned above and the expression for potential drop acrossed the monolayer regions, the inner-layer differential capacity (C_1) as a function of electrode charge density (σ) was estimated with curve fitting. Figure 1 shows the results of this treatment for Hg/(H_2O+CH_3OH) solution interface. In Figure 2 the corresponding mixed-adsorption isotherm curve is plotted as example, it is seen that the monolayer at Hg electrode in (H_2O+CH_3OH) solution is almost fully occupied by methanol molecules.

Key words: Electrode/Solution interface, Hg/(H₂O+CH₃OH) Mixed-solvents solution system, Electrical double layer, Inner layer differential capacity