Ionic Forces

Polar compounds, although possessing dipoles, have no net charge on the molecule. 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 and liquid chromatography and, thus, do not play a part in GC. For completion, it should be said that ionic interactions are always accompanied by dispersive interactions and often, also with polar interactions. However, in ion exchange chromatography the dominant forces controlling retention usually result from ionic interactions. Ionic interaction is depicted diagrammatically in figure 33.

Figure 33. Ionic and Dispersive Interactions

A molecule can have many interactive sites comprised of the three basic types, dispersive, polar and ionic. Larger molecules can have many 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 would be considered dispersive which the biotechnologists call "hydrophobic" or lyophobic". If dipoles and polarizable groups dominate in the molecule, then the overall property of the molecule would be considered polar, which the biotechnologists call "hydrophilic" or lyophylic". However, these terms are not based on physical chemical argument but have evolved in the discipline of biology and, consequently, although they have important significance to biologists and biochemists they do not have the same consequence in Chiral GC.