The Mechanism of Chromatographic Retention - Chromatographic Interactions > Dispersive Interactions > Page 3

 

Dispersive Interactions

Dispersive interactions are the most difficult to describe and visualize. as they are caused by charge fluctuations that occur throughout a molecule that arise from electron/nuclei vibrations. The fluctuations are random in nature and are basically a statistical effect. Some years ago Glasstone (2) proffered a simple description of dispersion forces which is still probably the best description that has been rendered, so far, and is given 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".

London (3) was the first to describe dispersion forces, which were consequently termed 'London's dispersion forces'. Unfortunately, over the years, London's name has been dropped and the simpler term 'dispersion' forces is now used.

Dispersion forces are ubiquitous and must arise in all molecular interactions. They can, themselves, occur in isolation, but are always present even when other types of interaction dominate. An example of interactions that are exclusively dispersive are those between hydrocarbons. The lower molecular weight hydrocarbons (hexane, heptane, octane etc.) are liquids and not gasses due entirely to the dispersion forces that act between the hydrocarbon molecules. Dispersive interactions are sometimes referred to as 'hydrophobic' or 'lyophobic' interactions, particularly in the fields of biotechnology and biochemistry. These terms appear to have arisen because dispersive substances, e.g., the aliphatic hydrocarbons, do not dissolve readily in water. Thesetermsformolecularinteractions in relation to the physical chemical terms will be considered later.