Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 13
j i
E d− id =−
r
αμ
2
( 4πε)
2 6
[2.1.5]
where:
μ i dipole moment
αj polarizability
r intermolecular distance
In a similar way, the dissolution of an ionic substance in a nonpolar solvent also will
occur with the induction of the dipoles in the molecules of the solvent by the solute ions.
These equations make reference to the interactions between two molecules. Because
the polarization energy (of the solute or of the solvent) is not pairwise additive magnitude,
the consideration of a third molecule should be carried out simultaneously, it being impossible
to decompose the interaction of the three bodies in a sum of the interactions of two bodies.
The interactions between molecules in solution are different from those which take
place between isolated molecules. For this reason, the dipolar moment of a molecule may
vary considerably from the gas phase to the solution, and will depend in a complicated fashion
on the interactions which may take place between the molecule of solute and its specific
surroundings of molecules of solvent.
2.1.2.3 Dispersion
Even when solvent and solute are constituted by nonpolar molecules, there is interaction between
them. It was F. London who was first to face up to this problem, for which reason
these forces are known as London’s forces, but also as dispersion forces, charge-fluctuations
forces or electrodynamic forces. Their origin is as follows: when we say that a substance
is nonpolar we are indicating that the distribution of the charges of its molecules is
symmetrical throughout a wide average time span. But, without doubt, in an interval of time
sufficiently restricted the molecular movements generate displacements of their charges
which break that symmetry giving birth to instantaneous dipoles. Since the orientation of
the dipolar moment vector is varying constantly due to the molecular movement, the average
dipolar moment is zero, which does not prevent the existence of these interactions between
momentary dipoles. Starting with two instantaneous dipoles, these will be oriented to
reach a disposition which will favor them energetically. The energy of this dispersion interaction
can be given, to a first approximation, by:
E
disp
3II
i j αα
=−
2 6
24 ( πε)
( I + I ) r
i j
i j
[2.1.6]
where:
Ii,Ij ionization potentials
αi, αj polarizabilities
r intermolecular distance
From equation [2.1.6] it becomes evident that dispersion is an interaction which is
more noticeable the greater the volume of molecules involved. The dispersion forces are often
more intense than the electrostatic forces and, in any case, are universal for all the atoms
and molecules, given that they are not seen to be subjected to the requirement that permanent
dipoles should exist beforehand. These forces are responsible for the aggregation of
the substances which possess neither free charges nor permanent dipoles, and are also the