Learning Objective

• Differences between flocculated and deflocculated suspensions

The suspensions are said to be flocculated when the individual particles are in contact with each other to form loose aggregates or a network-like structure. They are easily redispersible, but the rate of sedimentation is fast, the product will look inelegant and there is also a danger of an inaccurate dose being administered.

Nonflocculated or deflocculated suspensions are those in which the dispersed particles exist as separate entities. They have a slow rate of sedimentation, thereby enabling a uniform dose to be taken from the container. However, when settling occurs, the sediment forms a highly compacted cake, which is difficult to redisperse (Table 6.5).

Controlled flocculation: Correct degree of flocculation must be induced. Underflocculation will result in all the undesirable properties associated with deflocculated suspensions. An overflocculated product will look inelegant, and to minimize settling, the viscosity should be high.

Controlled flocculation can be achieved by the combination of particle size control, addition of polymers to enable cross-linking between particles and the use of electrolytes to control zeta potential.

 

Table 6.5 Differences between Flocculated and Deflocculated Suspensions

Sl. No.	Flocculated Suspension	Deflocculated Suspension
1.	Particles form loose aggregates or network-like structure called floccules.	Particles exist as separate entities.
2.	Rate of sedimentation is high.	Rate of sedimentation is less.
3.	Sediment is rapidly formed.	Sediment is slowly formed.
4.	Sediment is loosely packed and does not form a hard cake.	Sediment is closely packed and forms a hard cake.
5.	Sediment is easy to redisperse.	Sediment is difficult to redisperse.
6.	The supernatant liquid becomes clear very quickly.	Supernatant liquid will remain cloudy for a long time due to slow settling of particles.
7.	The floccules stick to the sides of the bottle.	The particles do not stick to the sides of the bottle.
8.	Product will be unpleasant in appearance.	Product will be pleasant in appearance.

(Note: Zeta potential [ζ]: is defined as the difference in the potential between the surface of a tightly bound layer (shear plane) and the electro neutral region of the solution. The potential located at the shear plane is known as the electrokinetic or zeta potential. It has practical application in the stability of disperse systems since this potential governs the degree of repulsion between adjacent, similarly charged, dispersed particles. If the ζ is reduced below a certain value, the attractive forces exceed the repulsive forces and the particles come together, leading to flocculation. If the ζ value is increased, repulsion occurs.

Electrolytes act as flocculating agents by decreasing the electric barrier between the particles due to decrease in the ζ and the formation of a bridge between adjacent particles so as to link them together in a loosely arranged structure.)