Gels are dispersed systems, originally liquids (solutions) that have a certain consistency useful and practical for topical application. In contrast to emulsions, gels generally do not comprise two immiscible phases of opposite lyophilicity. Therefore, the polarity and solubility characteristics of the incorporated substances are either hydrophilic—in hydrogels—or lipophilic—in lipogels (or oleogels). The consistency of gels is caused by gelling (thickening) agents, usually polymers, building a three dimensional network. Intermolecular forces bind the solvent molecules to the polymeric network, and thus the reduced mobility of these molecules results in a structured system with increased viscosity.

Pure gels are transparent and clear or at least opalescent. Transparency is only achieved if all ingredients are dissolved or occur at least in colloidal form, i.e., the size of particles is in the submicron range. Transparency in particular is an attractive property of gels. Gel products have positive aesthetic characteristics and are thus becoming more and more popular in cosmetic care products today. Gels can also serve as the basis for more complex formulations:

• Solid particles can be incorporated, resulting in stabilized suspensions
• Incorporation of oily lipids results in so-called hydrolipid dispersions or quasiemulsions

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach

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