Geometry and energy effects on the structure of hard sphere fluids

The density profile of molecules is an interesting property in the theoretical study of inhomogeneous fluids. In this work, the density profile of a hard sphere fluid around various hard and soft spheres is studied using a well-established version of the density functional theory called “modified fundamental measure theory”. The results obtained from this study show that an increase in the size of the central molecule leads to both an enhanced density profile at the contact point and an amplified layering structure. Similar effects can be observed when attraction between original and bulk molecules increases. Another finding of the present study is that increasing the size of the central molecule has a quasi-attractive role of an entropic origin called ‘depletion potential’. Since the molecule is not very large, the depletion potential can be mapped in a hard core with an attractive Yukawa tail. Increasing molecule softness, however, has an opposite effect and causes the height of the first peak of the density profile to diminish and the inhomogeneity of the structure to lighten.