Energy of interaction between solid surfaces and liquids

We consider the wetting transition on a planar surface in contact with a semi-infinite fluid. In the classical approach, the surface is assumed to be solid, and when interaction between solid and fluid is sufficiently short-range, the contribution of the fluid can be represented by a surface free energy with a density of the form Φ(ρS), where ρS is the limiting density of the fluid at the surface. In the present paper we propose a more precise representation of the surface energy that takes into account not only the value of ρS but also the contribution from the whole density profile ρ(z) of the fluid, where z is coordinate normal to the surface. The specific value of the functional of ρS at the surface is expressed in meanfield approximation through the potentials of intermolecular interaction and some other parameters of the fluid and the solid wall. An extension to the case of fluid mixtures in contact with a solid surface is proposed.