The Mechanism of Chromatographic Retention - Chromatographic Interactions > Polar Interactions > Dipole-Induced Dipole Interactions > Page 11
Dipole-Induced Dipole Interactions
Polar interactions can also take place between substances that do not possess permanent dipoles. Certain compounds, for example, those that contain an aromatic nucleus (and thus p electrons), are termed polarizable. On close proximity of such compounds with a molecule having a permanent dipole, the electric field from the permanent dipole induces a counter dipole in the polarizable molecule.
Induced counter-dipole will behave in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) can result in strong polar interactions. Typical examples of polarizable compounds are the aromatic hydrocarbons. However, just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic hydrocarbons can be retained and separated by purely dispersive interactions as in GC. This again is demonstrated in the upper chromatogram in figure 1. Alternatively, a polyethylene glycol stationary phase will separate aromatic hydrocarbons largely by dipole-induced dipole interactions combined with some dispersive interactions. This effect is also demonstrated in the lower chromatogram in figure 2.
Molecules can exhibit multiple interactive properties. For example, phenyl ethanol possesses both a dipole as a result of the hydroxyl group and is polarizable due to the aromatic ring. Phenyl acetic acid contain groups that will provide dispersive interactions(the methylene group and the aromatic ring), induced dipole interactions (the aromatic ring) polar interactions (the carbonyl group and also ion interaction (the acid group). Ionic interactions are discussed below. Complex molecules such as biopolymers can contain many different interactive groups. Dipole-induced dipole interactions are depicted in figure 4.