The Mechanism of Chromatographic Retention - Chromatographic Interactions > Polar Interactions > Dipole-Dipole Interactions. > Page 8

Dipole-Dipole Interactions.

The strength of the intermolecular force involved in polar interaction is determined by the strength of the dipoles and can vary widely. In the extreme (e.g., the association of water with methanol) the dipole-dipole interaction energy can approach that of a chemical bond; such energies are involved in hydrogen bonding.

To a first approximation, the energy (UP) that arises during the interaction between two dipolar molecules is given by,



where(ap) is the polarizability of the molecule,
(mp) is the dipole moment of the molecule,
and (l) is the distance between the molecules.

It is seen that the energy involved varies inversely as the sixth power of the distance between the molecules and as the square of the dipole moment. Dipole moments are normally determined from bulkmeasurementsof dielectric constant taken over a range of temperatures. However, data obtained in this manner does not necessarily relate to the strength of any polar interactions that might occur with other molecules. As an example, an extremely polar solvent, water, has a dipole moment of only 1.76 Debyes. In addition, the dipole moment of another extremely polar substance, methanol, is only 2.9 Debyes. Strongly polar substances that exhibit unexpectedly low dipole moments are often interacting strongly with themselves, e.g., by molecularassociation and/or by internal electric field compensation. Internal field compensation often happens when more than one dipole is present in a single molecule.