Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dipole-Induced-Dipole Interactions > Page 26

Dipole-Induced-Dipole Interactions

Certain compounds, such as those containing the aromatic nucleus and thus (p) electrons, are polarizable. When such molecules come into close proximity with a molecule having a permanent dipole, the electric field from the dipole induces a counter dipole in the polarizable molecule. This induced dipole acts 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, a diagrammatic impression of a dipole-induced dipole interaction is shown in figure 9.

 

Induced dipole interactions are always accompanied by dispersive interactions just as dipole interactions take place coincidentally with dispersive interactions. Thus, compounds such as aromatic hydrocarbons can be retained and separated purely on the basis of dispersive interactions, for example in GC using an hydrocarbon stationary phase. In addition, they can be retained and separated by combined induced-polar and dispersive interactions in LC using silica gel as a stationary phase and a dispersive mobile phase such as n-heptane.

Molecules need not exhibit one type of interaction only.

Figure 9 Polar Interactions: Dipole-Induced Dipole Interactions

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.