Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 146

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A S B

(a)

(b)

FIGure 7.5 Variation of (a) surface potential, ψ o , and (b) ions (sodium/chloride) versus
distance from the solid.

7.2.1 c h a r g e d c o l l o I d S (e l e c T r I c a l c h a r g e d I S T r I b u T I o n a T I n T e r f a c e S )
The interactions between two charged bodies will be dependent on various param-
eters (e.g., surface charge, electrolyte in the medium, charge distribution; see
Figure 7.4). The distribution of ions in an aqueous medium needs to be investigated
in such charged colloidal systems. This observation indicates that the presence of
charges on surfaces means that there exists a potential that needs to be investigated.
On the other hand, in the case of neutral surfaces, one has only the van der Waals
forces to be considered. This was clearly seen in the case of micelles, where the addi-
tion of small amounts of NaCl to the solution showed

• A large decrease in CMC in the case of ionic surfactant

• Almost no effect in nonionic micelles (since in these micelles there are no

charges or EDL)

Electrostatic and EDL forces are found to play a very important role in a variety of
systems in science and engineering. It would be useful to consider a specific example
in order to understand these phenomena. Let us take a surface with positive charge
that is suspended in a solution containing positive and negative ions. There will be
a definite surface potential, ψ o , which decreases to a value zero as one moves away
into solution (Figure 7.5).
It is obvious that the concentration of positive ions will decrease as one approaches
the surface of the positively charged surface (charge-charge repulsion). On the other