Ouzo effect

The ouzo effect (also louche effect and spontaneous emulsification) is a milky (louche) oil-in-water microemulsion that is formed when water is added to ouzo and other anise-flavored liqueurs and spirits, such as pastis, raki, arak, sambuca and absinthe. Because such microemulsions occur with only minimal mixing and are highly stable, the ouzo effect may have commercial applications.

Observation and explanation

The ouzo effect occurs when a strongly hydrophobic essential oil such as trans-anethole is dissolved in a water-miscible solvent, such as ethanol, and the concentration of ethanol is lowered by addition of small amounts of water.

Oil-in-water emulsions are not stable. Oil droplets coalesce until complete phase separation is achieved at macroscopic levels. Addition of a small amount of surfactant or the application of high shear rates (strong stirring) can stabilize the oil droplets.

In a water-rich ouzo mixture the droplet coalescence is dramatically slowed without mechanical agitation, dispersing agents, or surfactants. It forms a stable homogeneous fluid dispersion by liquid-liquid nucleation. The size of the droplets has been measured by small-angle neutron scattering to be on the order of a micron.