Chemistry Reference
In-Depth Information
ζ = μ η/ε
o
D
(7.29)
where η is the viscosity of the solution, ε
o
is the permittivity of the free space, and
D
is the dielectric constant. The velocity of a particle in a unit electric field is related
to its electric mobility.
In another application, the magnitude of the zeta potential is measured as a func-
tion of added counterions. The variation in zeta potential is found to be related to the
stability of the colloidal suspension. The results of a gold colloidal suspension (gold
solute) are reported as follows:
Stability
(Flocculation character)
counterion
velocity
0 Al + 3
3(−)
Very high stability
20 10
−6
mol
2(−)
Flocculates (4 h)
30 10
−6
mol
0(zero)
Flocculates fast
40 10
−6
mol
0.2(+)
Flocculates (4 h)
70 10
−6
mol
1(+)
Flocculates slowly
These data show that the charge on the colloidal particles goes from negative to
zero (when the particles do not show any movement) to positive at high counterion
concentration. This is a very general picture. Therefore, in wastewater treatment
plants, counterions are added until the movement is almost zero, and fast floc-
culation of pollutant particles can be achieved. The variation of ζ of silica par-
ticles has been investigated as a function of pH. The dissociation of the surface
groups −Si(OH) is also involved. In this process, the zeta potential is constantly
monitored by using a suitable instrument. In some plants, this is carried out under
continuous measurement.
In some biological systems with the charge-charge interactions between larger
protein molecules, such as hemoglobin (mol. wt. 68,000), the aggregation becomes
critical if surface charges change.
7.5.2
S
T e r I c
S
T a b I l I z a T I of n
o f
S
of l I d
o r
l
I q u I d
c
of l l of I d S
The stability of solid particles or liquid drops can be also controlled by using large
molecules (
polymers
). The addition of a polymer will result in adsorption in a solid
or penetration in a liquid (Figure 7.9).
The mechanism of polymer stabilization is many-faceted. It introduces following
new parameters:
The colloid is imparted a different charge, depending on the polymer. In fact,
if the polymer is neutral, then the colloid may even become neutral.
The size of the polymer imparts special stability.
The polymer affects the viscosity of the media.
The main advantages in using polymers for colloidal stability are
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