The Structure of Inhomogeneous Molecular Fluids - A Restricted Orientation Model

It is suggested that the structural properties of an inhomogeneous fluid of very anisotropic molecules can be studied through a theoretical model in which the orientations of the molecules are restricted to certain discrete values. The general basis of the model is explained and then for illustrative purposes, it is applied to a fluid of cylindrical molecule confined between parallel planar hard structureless walls. Density functional theory and the analogue of the PY equation are used to obtain equations for the density profiles of the fluid which depend on the direct correlation function of the bulk fluid. As this function is unknown, it is approximated by a linear combination of geometrical functions. The equations are solved and the results presented for various values of the packing fraction of the fluid, the separation of the walls and the length to diameter ratio (l/d) of the molecules. A comparison with a recent Monte Carlo simulation of the fluid indicates that the approximations provide as good results as similar approximations for a simple fluid.