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

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, H2S and PH3. 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.