INTERFACE POTENTIALS AND PHYSICAL CONSTRAINTS

We derive a wall-interface potential V(Gamma) for wetting phenomena, t hrough minimization of the surface free energy under the constraint of fixed adsorption Gamma. We show that the upper bound V-BILB(Gamma), p reviously proposed by Bukman et al. [Phys. Rev. B 47 (1993) 1577], is not the minimum. The actual minimum apparently coincides with the equi librium value of the free energy. Thus, the constraint is too weak and we impose an additional physical constraint. This ensures that the de cay length, associated with the approach of the order parameter toward s the bulk value, does not exceed the bulk correlation length xi. The resulting free energy minimum is a boundary minimum and coincides with V-BILB(Gamma)