Ion Chromatography - Ionic Interactions
Polar compounds, although possessing dipoles or induced 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 but clearly cannot play a part in GC. Again it must be emphasized that ionic interactions are always accompanied by dispersive interactions and usually, also with polar interactions. Polar interactions are often exploited in ion chromatography where many ionizable substances in biological molecules have very weak dissociation constants and, thus, polar interactions are used in conjunction with the ionic interactions (and perhaps also with dispersive interactions) to achieve the necessary retention and selectivity.
Interacting Phase Molecule with a Permanent
Dipole and a Permanent Positive Charge
Figure 5. Ionic, Polar and Dispersive Interactions
A molecular interaction that involves all three interactive forces, dispersive, polar and ionic is shown in figure 5. It is seen that the exact factors that control retention can be quite difficult to diagnose. The cation with polarizable characteristics might be benzylamine and the anion with a permanent dipole might be glycolic acid. All three types of interaction are present and in order to adjust retention, the dispersive interactions could be increased by adding acetonitrile to the mobile phase, the polar interactions could be increased by adding methanol or water to the mobile phase and the ionic interactions could be modified by changing the pH or the concentration of the counter ion which will be discussed in due course.
A molecule can have many interactive sites comprised of the three basic types, dispersive, polar and ionic. In particular, high molecular weight substances can have an enormous number of 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 biotechnologist would 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 biotechnologist call "hydrophilic" or "lyophylic". If acidic or basic groups dominate then the substances are accepted as 'acidic' or 'basic'. However, the terms hydrophobic, lyophobic, hydrophilic and lyophilic are terms that 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 are not so frequently used in general analytical chromatography and, in particular ion chromatography. The terms are, in fact, misnomers.