Wednesday, August 21, 2013

Protective Colloids

Lyophobic sols like those of metals (Au, Ag etc) are unstable and are easily coagulated by addition of electrolyte. However, it is observed that when certain lyophilic colloids such as gum, geletin, Agar – Agar etc. are added to a lyophobic sol, the stability of the lyophobic colloids is markedly increased i.e. the addition of small amount of electrolytes does not cause coagulation of lyophobic colloids. This action of lyophilic colloids to prevent the coagulation of lyophobic colloid by addition of electrolyte is called Protection of Colloidal Sol and the lyophilic colloid is called Protective Colloid.
Explanation
The Particles of protective colloid is believed to be adsorbed by lyophobic colloidal particles and thus forms a covering over the surface of lyophobic colloidal particles. The lyophobic colloid thus behaves as lyophilic colloid and is precipitated less easily by electrolytes.
It may be noted that protective colloid not only increases stability of lyophobic colloids but it also makes them reversible.

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