STRUCTURE OF A DIPOLAR HARD-SPHERE FLUID NEAR A NEUTRAL HARD-WALL

The position-orientation distribution of a dipolar hard sphere fluid in contact with a neutral hard wall is studied using a density functional theory. Effects of hard sphere exclusions are incorporated in the excess hard sphere free energy density functional. Orientational correlations between the dipoles are taken into account through approximating the pair correlation function of the inhomogeneous dipolar fluid by that of the homogeneous one of the same chemical potential and dipole moment. The latter is obtained by solving reference quadratic hypernetted chain equation. Theoretical results agree well with those of computer simulations which are available at high density. The theory predicts a qualitatively different interfacial structure at low density and high dipole moments from that at high density; the former is very similar to the liquid/vapor interface of dipolar fluids with regard to both the density distribution and orientational order. The theory also demonstrates that different density distributions correspond closely to different orientational orderings of the dipoles at the interface.