Density functional study of the phase behavior of asymmetric binary dipolar mixtures.

Using density functional theory in the modified mean-field (MMF) approximation we study the phase behavior of asymmetric binary mixtures of equisized dipolar hard spheres with different dipole moments in the fluid phase regime. We focus on "dipole-dominated" systems where isotropic attractive interactions are absent. Despite these restrictions our results reveal complex fluid-fluid phase behavior involving demixing and first- and second-order isotropic-to-ferroelectric phase transitions the relative importance of which depends on two "tuning" parameters, that is, the parameter Gamma measuring the ratio of the dipolar coupling strengths, and the chemical potential difference Deltamu controlling the composition. The interplay of these effects then yields three different types of phase behavior differing in the degree to which demixing dominates the system. A generic feature of the resulting diagrams is that the isotropic-to-ferroelectric transition is shifted towards significantly higher densities compared to the one-component case, and is therefore destabilized. Furthermore, demixing in the MMF approach turns out to be always accompanied by spontaneous ferroelectricity, which is in contrast to recent integral equation and simulation results for the limiting case of a mixture of dipolar and pure hard spheres (Gamma=0).