Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 145

Electrostatic

Repulsion

Attraction

vdw

FIGure 7.3 Variation of repulsion and attraction forces versus distance between two par-
ticles (schematic).

(Adamson and Gast, 1997). DLVO theory has been modified in later years, and dif-
ferent versions are found in the current literature.
The electrostatic forces will give rise to repulsion at large distances (Figure 7.3).
This arises from the fact that the electrical charge-charge interactions take place at
a large distance of separation. The resultant curve is shown (schematic) in Figure 7.3.
The barrier height determines the stability with respect to the quantity k T , the kinetic
energy. DLVO theory predicts, in most simple terms, that if the repulsion potential
(Figure 7.4) exceeds the attraction potential by a value

W >> k T (7.4)

then the suspension will be stable. On the other hand, if

W <= k T (7. 5)

then the suspension will be unstable, and it will coagulate. It must be stressed that
DLVO theory does not provide a comprehensive analysis. It is basically a very useful
tool for analysis of complicated systems. Especially, it is a useful guidance theory in
any new application or industrial development.

Surface Potential

FIGure 7.4 Variation of EDL between two charged particles with different ion concentra-
tions (low; high).