Theouzo effect (/ˈuːzoʊ/OO-zoh), also known as thelouche effect (/luːʃ/LOOSH) andspontaneous emulsification, is the phenomenon of formation of a milky oil-in-wateremulsion when water is added toouzo and otheranise-flavored liqueurs and spirits, such aspastis,rakı,arak,sambuca andabsinthe. Such emulsions occur with only minimal mixing and are highly stable.[1]
First a stronglyhydrophobicessential oil such astrans-anethole is dissolved in a water-misciblesolvent, such asethanol, and the ethanol itself forms asolution (ahomogeneousmixture) with water.
If then the concentration of ethanol is lowered by addition of more water the hydrophobic substance precipitates from the solution and forms anemulsion with the remaining ethanol-water-mixture. The tiny droplets of the substance in the emulsion scatter light and thus make the mixture appear white.
Oil-in-wateremulsions are not normally stable. Oil dropletscoalesce until completephase separation is achieved atmacroscopic levels. Addition of a small amount ofsurfactant or the application of highshear 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 fluiddispersion by liquid–liquidnucleation.[2] The size of the droplets when measured bysmall-angle neutron scattering was found to be on the order of amicron.[3]
Usingdynamic light scattering, Sitnikovaet al.[1] showed that the droplets of oil in the emulsion grow byOstwald 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 of3microns.
Based on thermodynamic considerations of the multi-component mixture, the emulsion derives its stability from trapping between thebinodal andspinodal curves in thephase diagram.[3] 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,detergents, 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 generatingsurfactant-free emulsions 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.[4]Nanoparticles formed using the ouzo effect are thought to be kinetically stabilized as opposed to thermodynamically stabilizedmicelles formed using a surfactant due to the fast solidification of the polymer during the preparation process.[5]
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