Associating Behaviour of Pure Polar Liquids: Dielectric Properties of Pelargonie Acid

The theory of dielectrics brought in by Onsager [1], which takes into account the interaction between molecules and their environment, though applicable to pure polar liquids, is inadequate to justify the behaviour of some liquids whose molecules exhibit associating properties, such as acids, alcohols, amides and cyanides. This inadequacy led Kirk­ wood [2] to hypothesize an orientational correlation of neighbouring molecules as the result of shortrange specific interactions. Kirkwood’s theory, extended by Fröhlich [3, 4] to the case of polarisable molecules with permanent dipole moment, affords, through the introduction of a correlation factor, a useful tool for obtaining, from static permittivity measurements, information about the interactions among molecules in dependence on the liquid struc­ ture. The results are, as yet, not confirmable through comparison with otherwise gained determinations. In the present work the static permittivity of pelargonic acid has been measured as a function of temperature, and the results are discussed on the basis of the Kirkwood-Fröhlich theory.