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Coagulation of colloids

Dr. Dmitri Kopeliovich

Coagulation is the destabilization of Colloids by neutralizing the electric charge of the dispersed phase particles, which results in aggregation of the colloidal particles.

Aggregation is a formation of groups of particles (aggregates) bonded to each other by van der Waals or other intermolecular forces.
Aggregation usually refers to solid particles.
If a coagulation of a liquid or gaseous dispersed phase occurs the term coalescence instead of aggregation is used. Coalescence is characterized by disappearance of the boundary between the particles resulting in the reduction of the interfacial area.
In a stable colloid system the dispersed particles are electrically (commonly negatively) charged. Two charged particles repeal each other preventing collision and aggregation.
When the charges are neutralized the particles may collide and bond to each other.

A coagulation process may be subdivided in two stages: electric neutralization of the dispersed phase and aggregation of the destabilized particles.

The following methods are used for the destabilization of the colloidal particles by neutralization:

  • Addition of an electrolyte to the colloid. The colloidal particles are neutralized by the oppositely charged electrolyte ions. The destabilization of an lyophobic colloid occurs at the electrolyte concentrations exceeding the value of the critical coagulation concentration. The critical coagulation concentration is strongly dependent on the valence of the electrolyte ions. The higher the valence the lower the critical concentration of the electrolyte required for the coagulation of the colloid.
  • Addition of another colloid, particles of which are charged oppositely to the particles of the first colloid. The oppositely charged particles of the colloids attract each other and neutralize the electric charge. The best results of the destabilization by this method are achieved when the second colloid is added at a concentration precisely required for full neutralization. Too low and too high concentrations of the second colloid do not result in complete destabilization.
  • Introduction of electrodes connected to a DC power supply. The electric circuit provides the charges for neutralizing the colloidal particles.

Aggregation of the dispersed particles of a destabilized colloidal system is determined by the frequency of particle-particle collisions and the collision energy, which is required for the bond formation.

There are two types of aggregation according to the force driving the process:

  • Perikinetic aggregatiion caused by Brownian motion of the dispersed particles
  • Orthokinetic aggregation caused by either convection of the colloid or selective sedimentation of the dispersed particles (heavier particles sedimentate faster).

The term coagulation is often interchangeably used with the term flocculation. However the coagulation and flocculation processes are similar but not the same.

Flocculation is the aggregation of a colloid by polymer molecules (flocculant) bridging between the colloidal particles and forming large fragile network structures (flocs), which may be easily removed by sedimentation/flotation and filtering.

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coagulation_of_colloids.txt · Last modified: 2013/06/01 by dmitri_kopeliovich
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