The colloidal particles of a given system carry same type of charge. Hence, they repel each other and remain in suspended state. It is obvious that if the charge on the colloidal particles is removed by some method, they will aggregate and form a precipitate. The precipitation of colloid is called coagulation (or flocculation, if the particles after removal of charge float on the surface of liquid, instead of settling down.)
The coagulation can be brought about by various ways.
1. Mutual coagulation: If two oppositely charged colloids are mixed, they neutralize each other and thus get coagulated.
2. Heating: Some colloids like egg albumin coagulate merely on heating.
3. Electrophoresis: The movement of colloidal particles under the influence of an electrical field results in coagulation. The Colloidal particles move towards one of the electrodes during electrophoresis and get discharged at the electrode.
4. Addition of an electrolyte: When an electrolyte is added to the colloidal solution, the ions of the electrolyte neutralize the charge of the colloidal particles and thus the colloidal particles get coagulated.
Protection of Colloids:
The colloidal nature of particles vanishes on coagulation. The Lyophobic sols coagulate immediately after the removal of their charges. However, in case of many lyophilic colloids, the removal of their charges may decrease the stability but does not necessarily lead to coagulation. The reason is that lyophilic particles are heavily hydrated.
The layer of solvent covering the particle does not allow it to come in contact with the precipitating reagent. Thus, the presence of charge is the only reason for the stability of lyophobic sols but in case of lyophilic sols both factors; charge and hydration, are responsible for stability. The fact, that lyophilic colloids have relatively more stability, is utilized to increase the stability of lyophobic sols.
A small amount of lyophilic sol like gelatin, gum, and albumin is added to the solution of lyophobic colloid. The added lyophilic colloid forms a protective layer around lyophobic particle. This layer does not allow the lyophobic colloidal particle to come in contact with the Precipitating reagent.
This process of protecting the colloidal Particles from coagulation by the addition of lyophilic colloid is called protective action the lyophilic colloid which is used for this purpose is called protective colloid.
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