  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 Phi(rho_s), where rho_s 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 rho_s but also the contribution from the whole density profile
rho(z) of the fluid, where z is the coordinate normal to the surface. The
specific value of the functional of rho_s at the surface is expressed in
mean-field 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.
