W. Richard Bowen and Nidal Hilal 4

Interaction potential, w(r)
0
where � 01 and � 02 are the electronic polarisabilities of the two molecules.
Both the Keesom and the Debye interactions involve polar molecules
and are thus not always present, depending upon the molecules involved
in the interaction. However, the dispersive, or London, component of
the interaction described by London during the 1930s is always present.
As two molecules come into close proximity, the repulsion between
the negative charges in the electron shells causes the induction of temporary
dipoles. As the molecules do not have to be polar and can be
electrically neutral, this interaction can and does occur between any molecules
within sufficient range of each other. The interaction free energy
for the dispersion interaction between two molecules can be described as
follows [6, 7]:
w
( r)
2.3 INTERACTION FORCES 37
3 �01�02 hv1v2 C
� �
� �
2 6 2 ( 4πε
) r ( v � v ) r
0
1 2
w (r)
Attractive
Repulsive
0.2 0.4 0.6 0.8 1
Distance (nm)
FIGuRE 2.3 Sketch of the Lennard-Jones potential for an interacting molecular pair
described by equation (2.5). Also shown are the individual attractive and repulsive components.
As the two molecules approach, attractive forces increase until repulsive forces due
to the proximity of the electron shells of the molecules overcome the attraction.
L
6
(2.8)
where v 1 and v 2 are the orbiting frequencies of electrons, and h is Planck’s
constant (6.626 � 10 � 34 m 2 kg s �1 ).