A general expression for the kT-coefficient of the Gibbs equation is initially obtained without introducing any model of double layer. In order to calculate n Cl − n Na + in this expression, the Helmholtz, Goüy, Stern, and penetration double-layer models have been used successively. The counter-ion binding in the surface and bulk phases for the adsorption of the polyelectrolytes and charged micelles is also considered in detail.