Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 154

The attraction force between two particles is proportional to the distance of separa-
tion and a Hamaker constant (specific to the system). The magnitude of H is of the
order of 10 −12 erg (Adamson and Gast, 1997; Birdi, 2002).
The DLVO theory is thus found to be useful to predict and estimate colloidal sta-
bility behavior. Of course, in such systems with many variables, a simplified theory
is to be expected to fit all kinds of systems.
In the past decade, much development has taken place in regard to measuring the
forces involved in these colloidal systems. In one method, the procedure used is to
measure the force present between two solid surfaces at very low distances (less than
micrometer). The system can operate under water, and thus the effect of addictives
has been investigated. These data have provided verification of many aspects of the
DLVO theory. Recently, the atomic force microscope (AFM) has been used to mea-
sure these colloidal forces directly (Birdi, 2002). Two particles are brought closer,
and the force (nanoNewton) is measured. In fact, commercially available apparatus
are designed to perform such analyses. The measurements can be carried out in flu-
ids and under various experimental conditions (such as added electrolytes, pH, etc.).
7.3.1 f l of c c u l a T I of n a n d c of a g u l a T I of n of f c o l l o I d a l S u S p e n S I o n
It is known from common experience that a colloidal dispersion with smaller par-
ticles is more stable than one with larger particles. The phenomenon of smaller par-
ticles forming aggregates with larger particles is called flocculation or coagulation .
For example, to remove insoluble and colloidal metal that is precipitated, floccula-
tion is used. This is generally achieved by reducing the surface charges that give rise
to weaker charge-charge repulsion forces. As soon as the attraction forces (vdW)
become larger than the electrostatic forces, coagulation takes place. It is initiated by
particle charge neutralization (by changing pH or other methods [such as charged
polyelectrolytes]), which leads to aggregation of particles to form the larger size. This
approach is based on to change an initially charged particle to a neutral particle:
Initial state: Charge-charge repulsion
Final state: Neutral-neutral (attraction)
Coagulation can also be brought about by adding suitable substances (coagulants)
that are particular to a given system. This method reduces the effective radius of the
colloid particle and leads to coagulation. Flocculation is the secondary process after
coagulation, and this leads to very large particle (floccs) formation. Experiments show
that coagulation takes place when the zeta potential is around ± 0.5 mV. Coagulants
such as iron and aluminum inorganic salts are effective in most cases. In wastewater
treatment plants, the zeta potential is used to determine coagulation and flocculation
phenomena. Most of the solid material in wastewater is negatively charged.
7.4 dISPerSIonS oF SolId PartIcleS In FluIdS
As described earlier, when a solid particle or a liquid drop is broken down in size
the free energy of the system increases (because the magnitude of surface area per