which is derived from the Bikermann model considering finite ion size, 𝜙 OHP is the potential at the OHP, 𝜎 free is the free surface charge density defined as negative of the net ionic charge in the diffuse layer, 𝜆 D = √𝜖 𝑆 𝑅𝑇/2𝑐 0 𝐹 2 is the Debye length, 𝜖 S is the dielectric permittivity of the bulk solution, 𝑐 0 is the bulk ion concentration, 𝑅 is the gas constant, 𝐹 is the Faraday constant, 𝑇 is temperature, 𝛾 = 2𝑐 0 /𝑐 max with 𝑐 max being the maximal concentration. Equation ( ) was obtained earlier by Kornyshev. Size asymmetry of cations and anions is not considered here but was studied elsewhere. The potential difference in the space between the IHP and the OHP is,