Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dipole-Dipole Interactions > Page 25

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