DIELECTRIC-CONSTANT AND DENSITY VARIATIONS IN PURE LIQUIDS - THEORETICAL RELATIONS, COMPARISON WITH EXPERIMENT

Theoretical expressions of the density fluctuation of the dielectric constant (depsilon/dd)P or DFDC have been deduced from Clausius-Mossotti, Onsager relations and from a more general expression of the molecular polarization. The dielectric constant at each point of the liquid has been considered as a function of the number N of molecules per volume unit, of the temperature T and of the pressure P. In the general case the calculation of DFDC leads to three terms : an isothermal one (partial derivative epsilon/partial derivative d)P,T and two others from (partial derivative epsilon/partial derivative T)P,N/(dd/dT)P. By testing a hundred and eighteen organic solvents the results have been compared to the experimental values deduced from (depsilon/dT)P and (dd/dT)P measurements. The general DFDC expression gives numerical results in better agreement with experimental data than the expressions from Onsager and Clausius Mossotti relations. In about thirty solvents strong discrepancies observed mainly arise from specific temperature dependent effects i.e. association or conformation equilibria.