Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dispersion Forces > Page 20

Molecular Forces

All intermolecular forces are electrical in nature. The three different types are termed dispersion forces, polar forces and ionic forces. All interactions between molecules are composites of these three forces.

Dispersion Forces

Dispersion forces were first described by London (3), and for this reason were originally called 'London's dispersion forces'. London's name has now 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 (4) suggested that dispersion forces could be best described as follows,

 

"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 they would offer electrical interactions with another molecule resulting in interactive forces".

 

Dispersion forces are typically those that occur between hydrocarbons and because of them, hexane is a liquid boiling at 68.7oC and not a gas. In the biotechnology field, dispersive interactions are often referred to as 'hydrophobic' or 'lyophobic' interactions, probably because dispersive substance (e.g., aliphatic hydrocarbons) do not dissolve readily in water. For academic interest only, to a first approximation the interaction energy, (UD), involved with dispersive forces has been