The ouzo effect occurs when a strongly hydrophobicessential oil is dissolved in a water-miscible solvent, and the concentration of ethanol is lowered either by addition of small amounts of water or by evaporation of ethanol. Oil-in-water emulsions are not normally 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 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. Smaller droplets, in order of 100 nanometers, were found in Limoncello. 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.
Applications
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. Emulsions have many commercial uses. A large range of prepared food products, detergents, and body-care products take the form of emulsions that are required to be stable over a long period of time. 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.
