The Structure of Dielectric Fluids. III. Interaction between Impurities and Dielectric Saturation in Polar Fluids

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Abstract
The potential of mean force between two impurities in a sample of a fluid of rigid dipoles is studied. Each impurity may be a charge, an electric dipole or an electric quadrupole. It is found that the longest‐range part of the potential of mean force obtained from a molecular theory is the same as the interaction energy obtained from macroscopic electrostatics, when the polar fluid is treated as a dielectric continuum, except that the impurity dipole and quadrupole have to be replaced by effective dipole moments. These effective dipole moments arise because of the local polarization of the polar molecules in the surrounding of the impurities, due to the local interactions. Next a pure polar fluid in the presence of an arbitrary external electric field is considered. The one‐particle distribution function in the presence of the field is studied to all orders in the field. It is found that both this one‐particle distribution function and the dielectric polarization are shape‐independent functions of the local average macroscopic field.