What are Gels ? Explain their Types and Properties.
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. Boot polish, cheese, jelly, curd, silica, etc. are some common examples of gel.
Explanation: 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.
Two types of Gels
Gels may be classified into two types: Elastic gets and Non-elastic gels
1. Elastic gels 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 but flexible.
2. Non-elastic gels 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. It is not reversible like elastic gels.
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.
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.
Source: Essential of Physical Chemistry by B.S.Bahl
