  Liquids in contact with solids are submitted to intermolecular forces making
liquids heterogeneous and, in a mechanical model, the stress tensor is not any
more spherical as in homogeneous bulks. The aim of this article is to show that
a square-gradient functional taking into account the volume liquid free energy
corrected with two surface liquid density functionals is a mean field
approximation allowing to study structures of very thin liquid nanofilms near
plane solid walls. The model determines analytically the concept of disjoining
pressure for liquid films of thicknesses of a very few number of nanometers and
yields a behavior in good agreement with the shapes of experimental curves
carried out by Derjaguin and his successors.
