Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Molecular Forces > Dipole-Dipole Interactions > Page 23

Polar Forces

Polar interactions arise from electrical forces between localized charges resulting from permanent or induced dipoles. They 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 (UP) 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.