Bulk and Surface Properties of Dipolar Fluids

Abstract

Based on density-functional theory we analyze the full phase diagram, the occurrence of long-ranged orientational order, and the structural properties of dipolar fluids. As a model system we consider the Stockmayer fluid that consists of spherical particles interacting via a Lennard-Jones potential plus dipolar forces. For sufficiently strong dipole moments one finds a region where a fluid phase with long-ranged orientational order is stable. For all sample shapes with the exception of a long thin needle this phase exhibits a spatially inhomogeneous magnetization which depends on the actual shape. We determine the details of the magnetization structure in a cubic sample in the absence and in the presence of an external magnetic field. One obtains a vortexlike structure with an escape of the magnetization into the axis direction near the vortex axis and two point defects where the absolute value of the magnetization is strongly reduced. If the spherical cores of the particles are replaced by elongated or oblate shapes a nematic phase without spontaneous magnetization is also possible due to the anisotropic steric interactions. We study the interplay of this nematic ordering with ferromagnetism in fluids of dipolar hard ellipsoids. Orientational order arises locally in the isotropic fluid phases near the liquid-gas interface of the Stockmayer fluid. Density-functional theory allows us to determine density and orientational order profiles as well as the surface tension of this interface.