Phase transitions in dipolar fluids: An integral equation study

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Abstract
We consider several dipolar fluids in their isotropic phase by integral equations in the reference hypernetted chain approximation. The systems differ in the form of the nondipolar potential. At low temperatures all of these fluids exhibit a spinodal line. Approaching this line from above, several fluctuations strongly increase. We interpret these fluctuations as indications for the low temperature states of the systems. In agreement with simulation results we find that the fluctuations at low densities strongly depend on the form of the nondipolar potential. In the dipolar hard sphere fluid we see a strong tendency that the particles associate into chainlike structures. Considering the same system with an additional Lennard-Jones attraction (Stockmayer fluid), we find that fluctuations of the number density which point to a usual condensation, clearly dominate. By switching off this isotropic attraction gradually, there is a smooth changeover from condensation to dipole clustering. At higher densities the behavior of the fluctuations becomes independent of the nondipolar potential: now the fluctuations indicate a transition into a fluid state with long-ranged ferroelectric order. By minimizing a density functional which has direct correlation functions of the isotropic phase as an input, we find the corresponding coexistence lines. Only at very high densities do the fluctuations point to crystallization.