Gels : Defination, Types, Properties

– In this topic, we will discuss The Gels : Defination, Types, and Properties.

What are Gels?

– A gel is a jelly-like colloidal system in which a liquid is dispersed in a solid medium.

– For example, when a warm sol of gelatin is cooled, it sets to a semisolid mass which is a gel.

– The process of a gel formation is known as Gelation.


– Gelation may be thought of as partial coagulation of a sol.

– The coagulating sol particles first unite to form long thread-like chains.

– These chains are then interlocked to form a solid framework.

– The liquid dispersion medium gets trapped in the cavities of this framework.

– The resulting semisolid porous mass has a gel structure.

– A sponge soaked in water is an illustration of gel structure.

Types of Gels

– Gels may be classified into two types :

(a) Elastic gels

– They are those which posses the property of elasticity.

– They change their shape on applying force and return to original shape when the force is removed.

– Gelatin, starch and soaps are examples of substances which form elastic gels.

– Elastic gels are obtained by cooling fairly concentrated lyophilic sols.

– The linkages between the molecules (particles) are due to electrical attraction and are not rigid.

(b) Non-elastic gels

– They are those which are rigid e.g., silica gel.

– These are prepared by appropriate chemical action.

– Thus silica gel is produced by adding concentrated hydrochloric acid to sodium silicate solution of the correct concentration.

– The resulting molecules of silicic acid polymerise to form silica gel.

– It has a network linked by covalent bonds which give a strong and rigid structure.

Properties of Gels

(1) Hydration

– A completely dehydrated elastic gel can be regenerated by addition of water.

– But once a nonelastic gel is freed from moisture, addition of water will not bring about gelation.

(2) Swelling

– Partially dehydrate elastic gels imbibe water when immersed in the solvent.

– This causes increase in the volume of the gel and process is called Swelling.

(3) Syneresis

– Many inorganic gels on standing undergo shrinkage which is accompanied by exudation of solvent. This process is termed Syneresis.

(4) Thixotropy

– Some gels are semisolid when at rest but revert to liquid sol on agitation.

– This reversible sol-gel transformation is referred to as Thixotropy.

– Iron oxide and silver oxide gels exhibit this property.

– The modern thixotropic paints are also an example.

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