flocculation and deflocculation

  1. Flocculation

    Flocculation is the formation of light, fluffy groups of particles held together by weak Van der Waal's forces.

    The basic concern in developing a suitable suspension is to adequately control the rate of settling and ease of redispersion, as well as, the prevention of caking the particles as a dense mass at the bottom of the container. The best approach is to achieve a controlled flocculation of the particles, where they appear as floccules or like tufts of wool with a loose fibrous structure. These particles settle rapidly, forming a loose adhering system with a large sediment height. In contrast to deflocculated particles, flocculated suspensions can always be resuspended with gentle agitation.

  2. Deflocculation

    Deflocculation is the absence of association which occurs when repulsive forces between particles predominate. Particles repel each other and remain as discrete, single particles.

    If repulsion forces prevail, the particles separate or deflocculate. Particles in these systems settle very slowly in stages, but ultimately form a dense sediment which is considerably more compact than the corresponding sediment of a flocculated system and more difficult to resuspend.

    Particle motion in the suspension is due to Brownian motion, convection currents, and sedimentation (gravity). When the particles settle, a dense mass is formed since there is no association between deflocculated particles. Downward movement due to gravity and the lateral motion due to Brownian movement facilitates tight packing of larger particles with the smaller particles filling the void spaces. Particles at the bottom of the cake are gradually pressed together by the weight of the ones above. Ostwald Ripening and Temperature Cycling effects cause particles to fuse together into a solid mass.

    In order to stabilize deflocculated systems, it is necessary to add a suspending and/or a gelling agent to retard settling and agglomeration of the particles by functioning as an energy barrier.

    Good pharmaceutical suspensions are best achieved through the formation of a stable floc which resists the tendency toward either deflocculation or agglomeration.

  3. Relative Properties of Flocculated and Deflocculated Particles in Suspension

    Deflocculated Flocculated
    Particles exist in suspension as separate entities. Particles form loose aggregates.
    Rate of sedimentation is slow, since each particle settles separately and particle size is minimal. Rate of sedimentation is high, since particles settle as a floc, which is a collection of particles.
    A sediment is formed slowly. A sediment is formed rapidly.
    The sediment eventually becomes very closely packed, due to weight of upper layers of sedimenting material. Repulsive forces between particles are overcome and a hard cake is formed which is difficult, if not impossible, to redisperse. The sediment is loosely packed and possesses a scaffold-like structure. Particles do not bond tightly to each other and a hard, dense cake does not form. The sediment is easy to redisperse, so as to reform the original suspension.
    The suspension has a pleasing appearance, since the suspended material remains suspended for a relatively long time. The supernatant also remains cloudy, even when settling is apparent. The suspension is somewhat unslightly, due to rapid sedimentation and the presence of an obvious, clear supernatant region. This can be minimized if the volume of sediment is made large. Ideally, volume of sediment should encompass the volume of the suspension.