The ouzo effect (also louche effect and spontaneous emulsification) is a phenomenon seen when water is added to ouzo and other anise-flavored liqueurs and spirits: a cloudy (louche) oil-in-water microemulsion forms. Because such microemulsions occur with only minimal mixing and are highly stable, the ouzo effect may have valuable commercial applications.
The ouzo effect is due to a strongly hydrophobic essential oil of trans-anethole dissolved in a water-miscible solvent (ethanol). In the absence of such a water-miscible solvent, oil-in-water emulsions are not stable as the oil droplets will continue to grow via droplet coalescence until phase separation is achieved at macroscopic levels. It is well-known that the addition of a small amount of surfactant or the application of high shear rates (strong stirring) can stabilise the oil droplets.
The surprising fact that, without the use of such means, the oil droplets in a water-rich ouzo mixture slow down their growth and form a stable liquid dispersion, was first studied by Stephen A. Vitale and Joseph L. Katz, who termed it the "ouzo effect". Using small-angle neutron scattering techniques, the size of the droplets has been measured: the droplet radii typically are of the order of a micrometre.
Using dynamic light scattering, Sitnikova et al. showed that the droplets of oil in the emulsion grow via Ostwald ripening, and that droplets do not coalesce. The Ostwald ripening rate is observed to reduce at higher ethanol concentrations and eventually to reduce such that the droplets stabilize in size. The average diameter of the droplets stabilized at a value of typically 3 micrometre.
Although it is clear that stable emulsification occurs because the trans-anethole is soluble in ethanol but not in water, so that the trans-anethole molecules aggregate together within the emulsion droplets, the exact physical mechanisms responsible for the slowing down of the Ostwald ripening rate at high concentrations of ethanol are not fully understood.
Microemulsions have many commercially important uses. A large range of prepared food products, detergents, and body-care products take the form of emulsions that need to stay stable over a long period of time. The Ouzo effect is seen as a potential mechanism for generating surfactant-free microemulsions without the need for high-shear stabilisation techniques that are costly in large-scale production processes. It has been conjectured that the synthesis of a variety of dispersions such as pseudolatexes, silicone emulsions, and biodegradable polymeric nanocapsules, have actually been synthesized using the ouzo effect.