Microemulsions

Microemulsions are clear, thermodynamically stable mixtures of oil, water, a surfactant, and often a cosurfactant. They form spontaneously at suitable compositions and temperatures to yield isotropic liquids with droplet sizes typically in the range of 10 to 100 nanometers. Unlike conventional emulsions, microemulsions are thermodynamically stable and do not require continuous energy input to persist.

Structures of microemulsions can be oil-in-water (O/W), water-in-oil (W/O), or bicontinuous, depending on the balance of oil, water, surfactant, and cosurfactant. The internal morphology is influenced by composition and temperature, and many microemulsions are transparent or translucent due to their small droplet size.

Formation and properties are governed by spontaneous self-assembly of amphiphilic molecules at the oil–water interface. The presence of a cosurfactant lowers interfacial tension to near zero, enabling rapid emulsification and the creation of a large interfacial area with low curvature energy. Droplet sizes are typically 5–100 nm with narrow size distributions, contributing to their optical clarity and stability.

Applications include solubilization and delivery of hydrophobic compounds, enhanced bioavailability for pharmaceuticals, and formulation components for topical, cosmetic, and food products. Microemulsions are also used in chemical synthesis, materials science, and some processes in enhanced oil recovery due to their unique transport and mixing properties.

Limitations involve sensitivity to temperature and composition, potential toxicity or irritation from surfactants or cosurfactants, and challenges in scaling up and reproducibly selecting formulations. Phase diagrams are commonly used to map usable regions for a given system.