STATIC TRANSVERSE DIELECTRIC FUNCTION OF MODEL MOLECULAR FLUIDS

It is customary in the physicochemical literature to define the static transverse dielectric function of a polar fluid according to the relation epsilon(T)(k) = 1 + 4pibetaS(T)(k), where beta = (k(B)T)-1 and ST(k) is a measure of the static fluctuations of the Fourier components of the transverse part of the electric polarization field in the fluid. In this work we evaluate epsilon(T)(k) for models of two classes. ( 1 ) In a DS model each molecule is a hard sphere with a point dipole at its center. It is the simplest representative of models in which the long range intermolecular interactions are generated by a set of point multipoles located at a single point in the molecule. In this case S(T)(k) is S(M,T)(k), the static correlation function of the transverse part of the dipole polarization density M(k). (2) In a zetaDS model fluid, comprising nonideal dipolar hard spheres, the long range interactions are accounted for by two opposite partial charges on either side of the sphere's center and separated by a distance l. The zetaDS model is the simplest interaction site model (ISM) of a polar fluid. For an ISM the relevant structure factor S(T)(k) is S(Pmu,T)(k), the static correlation of the transverse part of the full electric polarization density vector P(mu)(k). Here we compare epsilon(T)(k) for DS and zetaDS models with the help of the mean spherical approximation for the required structure functions. The zetaDS-DS difference in epsilon(T)(k) at large k is found to parallel the behavior of the static longitudinal dielectric function epsilon(L)(k). However, epsilon(T)(k), unlike epsilon(L)(k), is a damped oscillatory function of k with no poles on the real k axis.