Polar Forces

Polar interactions arise from electrical forces between localized charges that result from permanent or induced dipoles. Polar interactions cannot occur in isolation, but must be accompanied by dispersive interactions and under some circumstances may also be combined with ionic interactions.

Polar interactions can be very strong and result in molecular associations that approach, in energy, that of a weak chemical bond. Examples of such instances are 'hydrogen bonding' and in particular the association of water with itself.

Dipole-Dipole Interactions

The interaction energy (U_P) between two dipolar molecules is given, to a first approximation, by

where (a) is the polarizability of the molecule (m) is the dipole moment of the molecule,

and (r) is the distance between the molecules.

The interaction energy depends on the square of the dipole moment, which can vary in strength over a wide range of values. Unfortunately, the numerical value of the dipole moment, taken from bulk measurements of dielectric constant, does not always give an indication of the strength of any polar interactions that it might have with other molecules. Water, for example, is an extremely polar molecule but has a dipole moment of only 1.76 debyes. In a similar manner the dipole moment of methanol, another very polar substance , has a dipole moment of only 2.9 debyes. Unusually low values for dipole moments of strongly polar substances could result from electric field compensation due to molecular association and/or from internal field compensation when more than one dipole is present in the molecule. For example, water associates strongly with itself by very strong polar forces or 'hydrogen bonding' that reduces the net dipole character of the associated molecules when determined from bulk measurements of the substance. Methanol also associates strongly with itself in a similar manner. Possible associates of water and methanol are shown in figure 30.

Figure 30. Possible Self Associates of Water and Methanol

It is seen that 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. Consequently, bulk property measurement may not necessarily give a true value for the dipole moment of the individual dipoles. Another molecule, however, approaching a water or methanol molecule would experience the uncompensated field of the single dipole and interact accordingly.