with water and ice]]
The ouzo effect
(also louche effect
and spontaneous emulsification
) is a milky (
) oil-in-water emulsion
that is formed when water is added to ouzo
and other , such as pastis
. Such microemulsions occur with only minimal mixing and are highly stable.
Observation and explanation
and essential oils from grapefruit
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.
s 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 rate
s (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
|author2=Joseph L. Katz
|title=Liquid Droplet Dispersions Formed by Homogeneous Liquid-Liquid Nucleation: The Ouzo Effect
|publisher= American Chemical Society
}} The size of the droplets has been measured by small-angle neutron scattering
to be on the order of a micron
|title=Small-angle neutron scattering study of a world-wide known emulsion: Le Pastis |journal=Colloids and Surfaces A, Physicochemical and Engineering Aspects
Using dynamic light scattering
, Sitnikova et al. showed that the droplets of oil in the emulsion grow by Ostwald ripening
, and that droplets do not coalesce. The Ostwald ripening rate is observed to diminish with increasing ethanol concentrations until the droplets stabilize in size with an average diameter of .
Based on thermodynamic considerations of the multi-component mixture, the emulsion derives its stability from trapping between the binodal and spinodal
curves in the phase diagram
. However, the microscopic mechanisms responsible for the observed slowing of Ostwald ripening rates at increasing ethanol concentrations appear not fully understood.
Emulsions have many commercial uses. A large range of prepared food products, detergent
s, and body-care products take the form of emulsions that are required to be 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. The creation of a variety of dispersions such as pseudolatexes, silicone emulsions, and biodegradable polymeric nanocapsules, have been synthesized using the ouzo effect, though as stated previously, the exact mechanism of this effect remains unclear.