Statistical mechanics of fluids in a step potential

The paper deals with the problem of surface effects at a fluid boundary produced by a step force field. A classical simple fluid with a locally placed field simulating a solid is considered. The specific surface Omega-potential gamma, the surface number density, and the Henry adsorption constant are determined. A surface cluster expansion is obtained. It is similar to the cluster expansion for the pressure in which the integrals of the Ursell factors are replaced by sums of the first-order moments of the Ursell factors. The contact theorem is extended to the case of a finite step field. It is found that the surface number density (its invariant part) is determined by the first-order moment of the pair Ursell function taken over the entire space. The high-and low-temperature limits are analyzed and are shown to be consistent with the previously obtained general results.