Colloid Destabilization

Colloid stability may further be investigated by the use Figure 12.1b. This figure portrays the competition between two forces at the surface of the colloid particle: the van der Waal's force of attraction, represented by the lower dashed curve, and the force of repulsion, represented by the upper dashed curve. The solid curve represents the resultant of these two forces. As shown, this resultant becomes zero at a - a' and becomes fully an attractive force to the left of the line. When the resultant force becomes fully attractive, two colloid particles can bind themselves together.

FIGURE 12.3 Charged double layer around a negatively charged colloid particle (left) and variation of electrostatic potential with distance from particle surface (right).

The force of repulsion, as we have seen, is due to the charges on the surface. Inherent in any body is a natural force that tends to bind particles together. This force is exactly the same as the force that causes adsorption of particles to an adsorbing surface. This is caused by the imbalance of atomic forces on the surface.

Whereas atoms below the surface of a particle are balanced with respect to forces of neighboring atoms, those at the surface are not. Thus, the unbalanced force at the surface becomes the van der Waal's force of attraction. By the presence of the primary charges that exert the repulsive force, however, the van der Waal's force of attraction is nullified until a certain distance designated by a - a' is reached. The distance can be shortened by destabilizing the colloid particle.

The use of chemicals to reduce the distance to a - a' from the surface of the colloid is portrayed in Figure 12.4. The zeta potential is the measure of the stability of colloids. To destabilize a colloid, its zeta potential must be reduced; this reduction is equivalent to the shortening of the distance to a - a' and can be accomplished through the addition of chemicals.

The chemicals to be added should be the counterions of the primary charges. Upon addition, these counterions will neutralize the primary charges reducing the zeta potential. This process of reduction is indicated in Figures 12.4a and 12.4b; the potential is reduced in going from Figure 12.4a to 12.4b. Note that destabilization is simply the neutralization of the primary charges, thus reducing the force of repulsion between particles. The process is not yet the coagulation of the colloid.

(b) After addition of counter ions

FIGURE 12.4 Reduction of zeta potential to cause destabilization of colloids.

(b) After addition of counter ions

FIGURE 12.4 Reduction of zeta potential to cause destabilization of colloids.

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