Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dipole-Dipole Interactions > Page 24

The energy is seen to depend on the square of the dipole moment, the magnitude of which can vary widely. Unfortunately, values of the dipole moment, taken from bulk measurements over a range of temperatures, does not always give a correct indication of the strength of any polar interactions that it might have with other molecules. For example, water, an extremely polar molecule, has a dipole moment of only 1.76 debyes. Similarly, the dipole moment of methanol, another extremely polar substance, is only 2.9 debyes. Unusually low values of dipole moments for strongly polar substances is often due to internal electric field compensation when more than one dipole is present in the molecule.(e.g., water associates strongly with itself by very strong polar forces or 'hydrogen bonding' . Methanol also associates strongly with itself in a similar manner. Examples of possible associates of water and methanol are shown in figure 5.

Figure 7 Possible Self Associates of Water and Methanol

Thus, with such associates (should they exist) the electric field from each dipole would oppose that from the other, resulting in a reduction in the net field as measured externally. It follows, bulk properties may not reflect the true value for the dipole moment of the individual dipoles. A molecule, however, approaching a water or methanol molecule would experience the uncompensated field of the single dipole and interact accordingly.