Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 144

7.2 attractIve ForceS-rePulSIve ForceS
If the attractive forces are larger than the repulsive forces, then the two particles will
merge together. However, if the repulsion forces are larger than the attractive forces,
then the particles will remain separated. The medium in which these particles are
present is also a factor. pH and ionic strength (i.e., concentration of ions) are espe-
cially found to exhibit very specific effects.
Different forces of interest can be

• Van der Waals

• Electrostatic

• Steric

• Hydration

• Polymer-polymer interactions (if polymers are involved in the system)

In many systems, one may add large molecules (polymers), which, when adsorbed on
the solid particles, will impart a special kind of stability criteria.
It is well known that neutral molecules such as alkanes attract one another, mainly
through van der Waals forces. Van der Waals forces arise from the rapidly fluctuat-
ing dipoles moment (10 15 S −1 ) of a neutral atom, which leads to polarization and con-
sequently to attraction. This is also called the London potential between two atoms
in a vacuum, and is given as

V vdw = − (L 11 / R 6 ) (7.3)

where L 11 is a constant that depends on the polarizability and the energy related to
the dispersion frequency, and R is the distance between the two atoms. Since the
London interactions with other atoms may be negated as an approximation, the total
interaction for any macroscopic bodies may be estimated by a simple integration.
When two similarly charged colloid particles, under the influence of the EDL,
come close to each other, they will begin to interact. The potentials will detect one
another, and this will lead to various consequences. The charged molecules or par-
ticles will be under both van der Waals and electrostatic interaction forces. The van
der Waals forces, which operate at a short distance between particles, will give rise
to strong attraction forces. The potential of the mean force between colloid particle
in an electrolyte solution plays a central role in the phase behavior and the kinetics
of agglomeration in colloidal dispersions. This kind of investigation is important in
these various industries:

• Inorganic materials (ceramics, cements)

• Foods (milk)

• Biomacromolecular systems (proteins and DNA)

The DLVO ( Derjaguin-Landau-Verwey-Overbeek ) theory describes the stability
of a colloidal suspension as mainly dependent on the distance between the particles