These are noncrystalline, nonglassy, thermoreversible (thermoplastic) solid materials composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid may be an organic solvent, mineral oil or vegetable oil. These systems are based on self-assembly of the structurant molecules.

Organogels find potential applications in pharmaceuticals, cosmetics, art conservation and food. An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.

Gel-forming Substances or Gelling Agents

Polymers are used to give the three-dimensional structural network, which is highly essential for the preparation of gels. Gel-forming polymers are classified as follows:

1. Natural polymers
   • Proteins such as collagen and gelatin
   • Polysaccharides such as agar, alginate acid, sodium or potassium carrageenan, tragacanth, pectin, guar gum, cassia tora, xanthan and gellum gum
2. Semisynthetic polymers: Cellulose derivatives such as carboxymethyl cellulose, methylcellulose, hydroxypropyl cellulose, hydroxypropyl methylcellulose and hydroxyethyl cellulose.
3. Synthetic polymers
   • Carbomers such as Carbopol 940 and Carbopol 934
   • Poloxamer
   • Polyacrylamide
   • Polyvinyl alcohol
   • Polyethylene and its copolymers
4. Inorganic substances
   • Aluminum hydroxide
   • Bentonite
5. Surfactants
   • Cebrotearyl alcohol
   • Brij-96

Methods of Preparation of Gels

The following are the methods used for preparation of gels:

1. Fusion method
2. Cold method
3. Dispersion method