Influence of molecular shape on the dielectric constant of polar liquids

Abstract

The Onsager theory of the static dielectri c constant of polar liquids has bee n modified to allow for t he influence of molecular shape. Equations have been derived for bot h prolate and oblate spheroids in which the permanent dipole is directed along t he axis of symmetry . These equations have beep applied to a number of pure polar liquids, exclusive of t he hydrogen bonded liquids, where reliable data were available over extended ranges of temperat ure. A satisfactory interpretation of the variation of dielectric constant with temperature is obtained provided the volume of t he molecular cavity is maintained fixed and independent of temperat ure rather than allowed to expand in proportion to t he molar volume a s assumed by Onsager. The required molecular shapes are in general accord with the actual molecular geom etries. The behavior of dilute solutions and mixt ures with a nonpolar solve nt are also satisfactorily correlated using the same shape parameters. It appears to he unnecessary to consider specifi c interactions betwee n neighboring molecules in order to accou nt for the static dielectri c behavior of t hese systems.

Dielectric constant of hydrogen-bonded liquids

“…The calcu1ation of the dielectric polarization neglects shor trange forces of interaction between neighbors, and asswnes that the influence of long-range forces i fully accounted for by the r eaction field produced by the polarization of the continuous dielectric medillm smrounding the cavity. 1 Figures ill brackets ind icate the lit,'rature references at the end of this paper.…”

“…The influence of molecular sh ape on the dielectric con stant of polar liquids has been consider ed in a n earlier paper [1). 1 The phenomenological theory of Onsage r was shown to give a generally satisfactory descrip tion of th e dielec tric constant (1) of pme liquids as a fun ction of the temperatUTe, and (2) of a large class of binary mixtures as a fun ction of the composition.…”

“…1 The phenomenological theory of Onsage r was shown to give a generally satisfactory descrip tion of th e dielec tric constant (1) of pme liquids as a fun ction of the temperatUTe, and (2) of a large class of binary mixtures as a fun ction of the composition. :For thc restricted class of compounds studied it was found unnecessary to assume specific in teractions beLween near est neighbors.…”

The Static Dielectric Constants of the Liquified Fluoromethanes

“…Using Biickley and Maryott's data (2), that is using the "sodium D" valiies of the polarizations w11icl1 shoiild be a lower limit, the spheroidal Onsager eqilation fits CH,CI and CH,Br quite well with G = I, assuming the gas phase geometries. CH,I, however, requires a polarization which is 22% smaller than this lower limit (Table 3).…”

“…, C o l~~m n s 1 and 2 of Table I show that the spherical Onsager equation cannot describe these systems well, although the G values were closer to unity when atomic polarization was included. The data for the prolate spheroid, CH,F, is particularly interesting, since its shape should cause the Onsager value of G to be less than unity (2). The axial ratios, b/a were determined from Fisher-Hirschfelder-Taylor atomic models to be 0.9 for CH,F, 1.2 for CH2F2, and 1.1 for CHF,.…”