ENERGY AND ENTROPY RELATIONS IN HIGHLY POLAR LIQUIDS - ELECTROSTATIC APPROXIMATIONS USING THE ONSAGER MODEL

The simple model for a polar liquid proposed by Onsager consists of an electric moment located in a molecular sized cavity surrounded by a dielectric medium. The free energy, entropy, and internal energy changes within this system are examined when the interaction between the central electric moment and its reaction field from the surrounding medium is permitted to occur reversibly. Also of interest is the reverse procedure in which, while keeping the cavity intact, the central moment is reversibly uncoupled from interacting with the medium. The entropy change is initially obtained from the free energy by differentiation with respect to temperature. In polar liquids of large dielectric constant, the model entropy change for the uncoupling approaches the interestingly small limiting value of R/2, about 1 eu. This single theoretical process is thermodynamically equivalent to the sum of two real transfer processes. Except for processes involving water, the sum of entropy changes for each of several appropriate pairs of transfers is found to be small in absolute value and positive. The appropriate entropy sums in water are small and negative. A second method with direct application of the first and second laws is used to derive the theoretical entropy change. The same limiting expression and small value for the entropy change is obtained. In this second method, the absolutely small entropy is shown to be the sum of two larger entropy changes which have opposing signs. One of these components of entropy change is positive for the uncoupling and is derived from the change in interaction between the electric moment within the cavity and the electric moments within the medium; the other component of entropy change is negative for the uncoupling and is strictly attributable to the change in mutual interaction among the electric moments located within the dielectric medium.