Free-energy functional approximation for hard-sphere fluid

Abstract

A free-energy functional approximation (FEFA) is extended up to the third-order term in the density expansion. The extended free-energy functional approximation (EFEFA) is applied to predict the density profiles of a hard-sphere fluid restricted by hard and permeable walls. For a hard-sphere fluid confined in hard walls and spherical cages, the EFEFA gives better results than the FEFA of Tarazona, and is comparable with computer simulations at even higher density. However, for a hard-sphere fluid restricted by permeable walls the FEFA of Tarazona shows better agreement than the EFEFA when compared with computer simulations. These results suggest that the structural properties of a hard-sphere fluid depend on the geometrical features of walls and wall potentials as well as the model approximations.