Molecular association in liquids. III. A theory of cohesion of polar liquids

Abstract

This paper is an extension to polar liquids of the cell theory of liquid structure originally developed by Lennard-Jones & Devonshire. The free energy due to dipole interaction is evaluated using a model consisting of point dipoles fixed on the sites of a lattice but free to rotate in the electrical field of all the others. It is shown that, to a first approximation, the total free energy can be expressed as the sum of the free energy of this model and the free energy of a similar liquid without the dipole-dipole forces. The latter can be evaluated by the method of Lennard-Jones & Devonshire. The theory is applied to the polar liquids HCl, H 2 S and PH 3 . It is found that dipole interaction by itself is not sufficient to explain the difference between the cohesion of these liquids and similar non-polar liquids. The importance of other intermolecular forces between polar molecules is discussed.