Anisometric molecules in dense fluids

Abstract

A new theory for the static permittivity of liquids comprised of anisometric polar and polarizable molecules is proposed. The Onsager model, i.e. a reference molecule embedded in a dielectric continuum is used, the embedding cavity being ellipsoidal. Our approach consists in calculating the source dipoles of all the dipolar fields which are produced by the thought experiment of replacing an ellipsoidal region of the continuum by a cavity containing the molecule. The sum of these sources, which we call the total dipole increment is calculated, and is averaged under the influence of an applied Maxwell field. Closed expressions for the permittivity are obtained for both dilute solutions and neat polar liquids. They contain as parameters the permanent dipole moment of the molecule, its three principal polarizabilities, and the shape of the cavity. Formulae are also derived for the special case that the polarizability tensor is equal to that of a homogeneous ellipsoidal body having an arbitrary internal permittivity which can be taken to be equal to the refractivity of the solution or the neat liquid. All our final expressions differ from those proposed in the literature for the same model. However, the Onsager result reduces to our formula in the spherical case, because then a cavity-shape dependent term factors out from our equations.