Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 142

Criteria of Stability

Metastable Unstable Stable

FIGure 7.1 Stability criteria of any colloidal system: metastable-unstable-stable states.

A colloidal suspension may be unstable and exhibit separation of particles within
a very short time, or it may be stable for a very long time, such as for over a year. In
between, a metastable state can be found. This is an oversimplified example, but it
shows that any colloidal system should be analyzed following these three criteria.
As an example, consider a wastewater treatment process. The wastewater with
colloidal particles is a stable suspension. However, by treating it with pH control,
electrolyte concentration, etc., the stability of the system can be altered as shown in
Figure 7.1.

7.1.1 V a n d e r W a a l S f o r c e S
In colloidal systems, van der Waals forces play a prominent role. When any two
particles (neutral or with charges) come very close to each other, the van der Waals
forces will be strongly dependent on the surrounding medium. In a vacuum, two
identical particles will always exhibit an attractive force. On the other hand, if two
different particles are present in a medium (in water), then there may be repulsion
forces. This can be due to one particle adsorbing with the medium more strongly
than with the other particle. One example will be silica particles in water medium
and plastics (as in wastewater treatment). It is critical to understand under what con-
ditions it is possible that colloidal particles remain suspended. For example, if paint
aggregates in the container, then it is obviously useless for its intended purpose.
When solid (inorganic) particles are dispersed in an aqueous medium, ions are
released in the medium. The ions released from the surface of the solid are of oppo-
site charge. This can be easily shown when glass powder is mixed in water, and con-
ductivity is seen to increase with time. The presence of the same charge on particles
in close proximity gives repulsion, which keeps the particles apart (Figure 7.2).
The positive-positive particles will show repulsion. On the other hand, the posi-
tive-negative particles will attract each other. The ion distribution will also depend
on the concentration of any counterions or coions in the solution. Even glass, when
dipped in water, exchanges ions with its surroundings. Such phenomena can be eas-
ily investigated by measuring the change in the conductivity of the water.
The force F 12 , acting between these oppositely charges, is given by Coulomb's
law, with charges q 1 and q 2 separated at a distance R 12 in a dielectric medium D e :

F 12 = (q 1 q 2 )/(4 π D e ε o R 122 ) (7.1)