Microemulsion

A microemulsion is a thermodynamically stable, isotropic liquid dispersion of oil and water stabilized by surfactants and often cosurfactants. Microemulsions form spontaneously under appropriate composition and mild conditions, and their droplet sizes typically range from 10 to 100 nanometres. They may be oil-in-water (O/W), water-in-oil (W/O), or bicontinuous, where oil and water phases form interwoven networks within a single phase.

Formation requires surfactants that reduce interfacial tension to very low values and, frequently, cosurfactants to broaden the single-phase region. The resulting systems can be represented on phase diagrams and are commonly categorized as O/W, W/O, or bicontinuous (sometimes referred to as Winsor type I, II, or III microemulsions).

Thermodynamically stable means that, once formed, they persist without phase separation, unlike ordinary emulsions that require ongoing energy input. Microemulsions exhibit low interfacial tension (often orders of magnitude lower than conventional emulsions), near-zero interfacial free energy, and often transparent or translucent appearance. They typically display low to moderate viscosity and near-Newtonian flow.

Their high solubilization capacity for hydrophobic compounds makes them attractive in pharmaceutical and cosmetic applications, including topical formulations and oral drug delivery, as well as in chemical synthesis, material science, and enhanced oil recovery.

Preparation is usually achieved by simple mixing of oil, water, surfactant, and cosurfactant at ambient temperature; the process can be aided by gentle stirring or low-energy methods. Phase behavior diagrams guide formulation to stable regions.