The polarizability of a substance containing no dipoles will give an indication of the strength of any the dispersive interactions that might take place with another molecule. In contrast, due to internal compensation, the dipole moment of a substance, determined from bulk dielectric constant measurements, will not always give an indication of the strength of any polar interaction that might take place. A diagrammatic impression of a dipole-dipole interaction is shown in figure 8.
It is seen that the dipoles interact directly, but it is important to realize that with the dipole-dipole interaction is the dispersive interactions from the charge fluctuations on both molecules. The net interactive force will, therefore, be a combination of both. Dispersive interactions are the only interactions that can occur in the absence of any other. All other types of interaction, polar and/or ionic, will occur in conjunction with dispersive interactions. Examples of some substances that have permanent dipoles and exhibit polar interaction with other molecules are alcohols, esters, ethers, amines, amides, nitriles, etc.
Figure 8. Polar Interactions: Dipole-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.