microemulsion

An ordinary emulsion is milky and slowly separates; a microemulsion looks like clear water and never separates. The difference is droplet size: by making the oil or water domains exceptionally tiny — usually below about 100 nanometres — light passes straight through and the system becomes transparent or faintly bluish.

What truly sets a microemulsion apart is that it forms spontaneously and is thermodynamically stable, not just kinetically slowed. Gentle mixing of oil, water, surfactant, and usually a co-surfactant (such as a short-chain alcohol) is enough; the large amount of surfactant drives the interfacial tension down to nearly zero, so the enormous interface created by the tiny droplets actually lowers the system's free energy rather than raising it. As a result, a properly composed microemulsion will re-form by itself even after dilution or temperature change.

In pharmacy microemulsions are prized for boosting the solubility and absorption of poorly water-soluble drugs, because the drug can sit in whichever domain it prefers and is presented in a very fine, easily absorbed state. The main practical cost is the high surfactant and co-surfactant load needed, which can limit the dose volume and raise tolerability concerns. Self-emulsifying drug delivery systems exploit the same principle to form fine emulsions in the gut.