Dispersion Forces

Dispersion forces were first described by London (14), and for this reason were originally called 'London's dispersion forces'. However, over the years London's name has been dropped and they are now simply referred to as 'dispersion' forces. They arise from charge fluctuations throughout a molecule resulting from electron/nuclei vibrations. Glasstone (15) suggested that "although the physical significance probably cannot be clearly defined, it may be imagined that an instantaneous picture of a molecule would show various arrangements of nuclei and electrons having dipole moments. These rapidly varying dipoles when averaged over a large number of configurations would give a resultant of zero. However, at any instant and at any point they would offer electrical interactions with another, adjacent molecule, resulting in interactive forces".

Although this description of dispersive interactions was put forward by Glastone over fifty years ago it still conveys a good impression of the contemporary interpretation of the mechanism of dispersive interactions. Dispersion forces are typically those that occur between hydrocarbons that have no dipole moment and are not polarizable.

Some confusion has arisen with regard to the terms polarisable and polarization or polarizability.

If a substance is placed in an electric field and, as a result, dipoles are induced in its molecules the substance is said to be polarizable.

The term polarizability, however, is a term that has been given to a physical chemical property which will be defined in due course,

In biotechnology and biochemistry, dispersive interactions are often referred to as 'hydrophobic' or 'lyophobic' interactions, apparently because dispersive substances such as the aliphatic hydrocarbons do not dissolve readily in water. The interpretation of the biochemical terms for molecular interactions will be discussed later.

The theoretical treatment of molecular interactions is complicated and in the following discussion, certain simplifying assumptions are made, and, consequently the expressions given below (for both dispersive forces and polar forces) will not be exact. They will, however, be sufficiently precise to allow the parameters that control the different types of interaction to be identified.