Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 149

ψ = ψ o exp − (κ x ) (7.12)

where x is the distance from the charged colloid. The value of ψ o is found to be 100
mV (in the case of monovalent ions) (= 4 kB T T/z e). Experimental data and theory
show that the variation of ψ is dependent on the concentration and the charge of the
ions (Figure 7.7).
These data show that the surface potential drops to zero at a faster rate if the ion
concentration (C) increases, and that the surface potential drops faster if the value
of z goes from 1 to 2.

7.2.2 e l e c T r o K I n e T I c p r o c e S S e S
In the following text, let us consider what happens if the charged particle or surface
is under dynamic motion of some kind. Further, there are different systems under
which electrokinetic phenomena are investigated. These systems are

1. Electrophoresis : This system refers to the movement of the colloidal par-
ticle under an applied electric field.

2. Electroosmosis : This system is one where a fluid passes next to a charged
material. This is actually the complement of electrophoresis. The pressure
needed to make the fluid flow is called the electroosmotic pressure .

3. Streaming potent : If fluid is made to flow past a charged surface, then an
electric field is created, which is called streaming potential . This system is
thus the opposite of electroosmosis.

4. Sedimentation potential : A potential is created when charged particles
settle out of a suspension. This gives rise to sedimentation potential,

Z =

1

2

3

Distance

FIGure 7.7 Variation of surface potential (schematic) in the diffuse double layer as a func-
tion of charge on the ions (z).