To study the interface between a solid surface and nitrogen vapor, the theo
ry of Cahn was applied. The Peng-Robinson equation of state (PREOS) was inc
orporated in the theory of Cahn to model the thermodynamic functions of the
fluid. In this study the wetting transition, as earlier reported by Cahn,
was shown. In addition, wetting transitions occurring at a liquid-vapor int
erface of the three-phase LLV equilibrium of the binary mixtures hexane/wat
er and benzene/water were examined. It was found that by making use of the
gradient theory of van der Waals, in which the PREOS was incorporated, the
transitions seem to be of third order. For the case where the PREOS was rep
laced by the Associated-Perturbed-Anisotropic-Chain-Theory, no wetting regi
on was found. II was argued that, in principle, it should he possible to mo
del first-order welting transitions with the square gradient theory.