Microemulsion is defined as a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution. ‘‘This definition should be widened, however, to include metastable states, spontaneous emulsions of long-lived kinetic stability.’’ The term microemulsion may be a misnomer, because microemulsions consist of large or ‘‘swollen’’ micelles containing the internal phase, much like that found in a solubilized solution.

Microemulsions contain oil droplets in a water phase or water droplets in oil with diameters of about 10 to 200 nm. Therefore they appear as isotropic, optically clear liquid or gel-like systems. Unlike micellar solubilized systems, microemulsions may not be thermodynamically stable; nevertheless, they are more stable than ordinary emulsions. They are a type of ternary system composed from water, lipid, and surfactant mixture in a distinct ratio

Microemulsions may be used to incorporate or dissolve active substances and have been found to improve skin penetration and permeation.
The disadvantage of microemulsions is their rather high concentration of surfactants, which is a risk for increased skin irritation and sensitization. Nevertheless, modern microemulsion formulation is based on alkyl polyglycosides which are regarded to be milder than conventional nonionic surfactants with polyoxyethylene chains.

Hydrogels are hydrophilic, consisting mainly (85–95%) of water or an aqueous-alcoholic mixture and the gelling agent.

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