Equations that are developed describe enthalpic effects of ion adsorption, which is predicted by the four-layer model assuming that the anions and cations of the inert electrolyte (1:1) are adsorbed in two distinct layers located different distances from the surface. The theoretical development has been based on the 2-pK charging mechanism. Furthermore, the expressions that are developed for the four-layer model fake into account the energetic heterogeneity of the surface, assuming small correlations exist between adsorption energies of different surface complexes.