The reported linear dependence of standard ion-transfer potentials of heteropolyanions, e.g. [SiW12O40](4-), at the nitrobenzenewater interface on the surface charge densities of polyanions cannot be explained completely in terms of the ionic properties and electrostatic energy. A model leading to better understanding is proposed here, in which the ion is solvated with an intermolecular force between the ion and oil or water molecules under electrochemical equilibrium conditions. Assuming that the solvation energy is proportional to the surface area of the polyanion, the partition function of the charged polyanion in the electrostatic potential was derived. The potential detected electrochemically is not a local potential in the vicinity of the ion, but is the potential averaged over each phase. The partition function leads to a linear relation of the potential difference with the interaction energy density and an inverse linear relation with the charge. The potential difference contains no Born-type electrostatic energy. The linear dependence agrees with the experimental result of polyanions, tetralkylammonium ions and simple inorganic anions for various values of radii and charge numbers of the ions. The slope of the linear plot gives the surface energy density or the surface tension at the interface between the ion and the solvent in the first solvation shell. The surface energy of the polyanions is larger than that of the conventional liquidliquid interface, suggesting a hard solvated shell. The discussion is directed to the energy forming the cavity, the effect of the second solvation shell, the Kelvin effect and the electrostatic contribution except the electrode potential.