Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Ionic Forces > Page 27

Ionic Forces

Polar compounds possessing dipoles, have no net charge. In contrast, ions possess a net charge and consequently can interact strongly with ions having an opposite charge. Ionic interactions are exploited in ion exchange chromatography where the counter ions to the ions being separated are situated in the stationary phase.

In a similar manner to polar interactions, ionic interactions are always accompanied by dispersive interactions and usually, also with polar interactions. Nevertheless, in ion exchange chromatography, the dominant forces controlling retention usually result from ionic interactions. Ionic interaction is depicted diagramatically in figure 10.

Figure 10 Ionic and Dispersive Interactions

A molecule can have many interactive sites comprised of the three basic types, dispersive, polar and ionic. Large molecules (for example biopolymers) may have hundreds of different interactive sites throughout the molecule and the interactive character of the molecule as a whole will be determined by the net effect of all the sites. If the dispersive sites dominate, the overall property of the molecule will be dispersive which the biotechnologists call "hydrophobic" or "lyophobic". If dipoles and polarizable groups dominate in the molecule, then the overall property of the molecule will be polar, which the biotechnologist call "hydrophilic" or lyophilic". These terms are not based on physical chemical argument but have evolved largely in the discipline of biology.