Ion Chromatography - Dipole-Induced-Dipole Interactions
Particular compounds, (e.g. those containing the aromatic nucleus and thus (p) electrons) are polarizable.
Interacting Phase Molecule with Permanent Dipole
Figure 4. Polar Interactions: Dipole-Induced Dipole Interactions
When a polarizable molecule comes in close contact with another molecule that has a permanent dipole, the electric field from the permanent dipole induces a counter dipole in the polarizable molecule. The induced dipole interacts in the same manner as a permanent dipole and the polar forces between the two dipoles result in interaction between the molecules. Aromatic hydrocarbons are typically polarizable compounds and an example of their separation using induced dipole interactions to affect retention and selectivity will be given later. A diagrammatic impression of a dipole-induced dipole interaction is shown in figure 4.
In the same manner that dipole interactions must always occur coincidentally with dispersive interactions so must induced dipole interactions also always be accompanied by dispersive interactions. Thus, aromatic hydrocarbons can be retained and separated purely by dispersive interactions, for example in gas chromatography by using a hydrocarbon stationary phase. However, they can also be retained and separated by combined induced-polar and dispersive interactions using, for example, the polar stationary phase polyethylene glycol. It should be pointed out that molecules need not exhibit one type of polarity only. Phenyl ethanol, for example, will possess both a dipole as a result of the hydroxyl group and be polarizable due to the aromatic ring. More complex molecules can have many different interactive groups.
Ionic forces are those that usually dominate (but not necessarily exclusively) In ion chromatography and are, thus, the most pertinent to the subject of this book. It must be emphasized, however, that although ionic forces may well dominate in the separation process, dispersive forces will always be present and may well be essential to achieve particularly difficult separations and often for the separation of substances of biological origin.