Thermodynamic perturbative approach for simple fluids: Structure of a confined square-well fluid

An approximation, which is based on both the weighted-density approximation (WDA) of Tarazona [Phys. Rev. A 31, 2672 (1985)] for the repulsive part of the model potential, and the density-functional approximation (CNPR) of Calleja et al. [Mel. Phys. 59, 973 (1991)] for the attractive part of the model potential, has been proposed to study the structural properties of simple fluids and is abbreviated as WDA-CNPR. The WDA-CNPR approximation can be considered as an extended Choudhury-Ghosh approximation, which is based on the density-functional expansion of a one-particle direct correlation function corresponding to the attractive part of the model potential. It has been applied to predict the density profiles of a confined square-well fluid, and compared with the computer simulation. The calculated results show that for the square-well fluid confined in planar slits the WDA-CNPR approximation is a significant improvement upon those of the Choudhury-Ghosh approximation, and compares well with the computer simulation. For the weak fluid-wall interaction a very good agreement between the results of theory and computer simulation are obtained, but the agreement deteriorates with the increase of the fluid-wall interaction.