1. Gels

2. Gels are defined - as semisolid systems consisting of dispersions made up of either small inorganic particles or large organic molecules enclosing and interpenetrated by a liquid - A high degree of physical or chemical cross- linking may be involved. - The increased viscosity caused by the internal friction is responsible for the semisolid state.

3. -- Some gel systems are as clear as water, and others are turbid because the ingredients may not be completely molecularly dispersed (soluble or insoluble). - The concentration of the gelling agents is mostly less than 10%, usually in 0.5% to 2.0% range, with some exceptions.

4. Classification and Types of Gels General classification of gels listing two classification schemes. - The first scheme divides gels into inorganic and organic. - Most inorganic hydrogels are two-phase systems, such as aluminum hydroxide gel and bentonite magma. - Most organic gels are single-phase systems and may include such gelling agents as carbomer and tragacanth.

5. The second classification scheme divides gels into hydrogels and organogels. Hydrogels include ingredients that are dispersible as colloidals or soluble in water. - They include organic hydrogels, natural and synthetic gums, and inorganic hydrogels. - Examples include hydrophilic colloids such as silica, bentonite, tragacanth, pectin, sodium alginate, methylcellulose, sodium CMC. - Sodium alginate has been used to produce gels that can be employed as ointment bases.

6. Organogels -Include the hydrocarbons, animal and vegetable fats, soap base greases, and the hydrophilic organogels. -The hydrophilic organogels, or polar organogels, include the polyethylene glycols of high molecular weight, the carbowaxes. -They are soluble to about 75% in water and are completely washable. They are nonionic and stable.

7. Preparation of Gels - Are prepared by freshly precipitating the disperse phase to achieve a fine degree of subdivision of the particles and a gelatinous character to those particles. - The desired gelatinous precipitate results when solutions of inorganic agents react to form an insoluble chemical having a high attraction for water. - As the microcrystalline particles of the precipitate develop, they strongly attract water to yield gelatinous particles, which combine to form the desired gelatinous precipitate. - Other gels may be prepared by directly hydrating the inorganic chemical, which produces the disperse phase of the dispersion. - In addition to the water vehicle, other agents as propylene glycol, propyl gallate, and hydroxypropyl cellulose may be used to enhance gel formation. - To ensure uniform dosage, gels should be shaken before use

8. Examples of Gelling Agents - Gelling agents include - acacia, - alginic acid, - bentonite, - carbomer, - CMC sodium, - ethylcellulose, - gelatin, - xanthan gum.

9. - guar gum, - hydroxy ethylcellulose, - hydroxypropyl cellulose, - Hydroxypropyl methylcellulose, - methylcellulose, - polyvinyl alcohol (PVA), - povidone, - sodium alginate - Starch - tragacanth, - xanthan gum

10. Thank you..!!