Van der Waals interactions
There are 3 types of Van der Waals forces:
- Orientation forces (or Keesom forces)
- Induction forces (or Debye forces)
- Dispersion forces (or London forces)
This interaction takes place between two polar molecules. Thus, a polar solvent (permanent dipole moment µ1) will interact with a polar solute (dipole moment µ2): the solute molecule tends to orient itself so that the two dipole moments are aligned.
ε0: permittivity of the medium
r: distance between the 2 dipoles
This interaction takes place between a polar molecule and a polarizable molecule. Thus, a polar solvent (permanent dipole moment µ1) will interact with a polarizable solute (polarizability α): the solute molecule acquires a dipole moment (µinduced = α.E) under the effect of the electric field E created by the polar molecule, and orients itself so that the two dipole moments are aligned.
Electrons are in continuous motion; atoms or molecules without a dipole therefore have, at any instant, a dipole moment resulting from the permanent deformation of their electronic cloud. This dipole moment acts on the electronic system of the neighboring molecule as a fluctuating polarizing dipole. This results in a mutual attraction, of the same kind as the permanent dipole-induced dipole interaction.