View source for Ouzo effect

[[Image:Yeni Raki.jpg|thumb|The ouzo effect, here after the dilution of Turkish [[Raki (alcoholic beverage)|raki]].]]

The '''ouzo effect''' (also '''louche effect''' and '''spontaneous emulsification''') is a phenomenon observed when water is added to [[ouzo]] and other [[:Category:Anise liqueurs and spirits|anise-flavored liqueurs and spirits]], such as [[pastis]], [[Raki (alcoholic beverage)|raki]], [[Arak (distilled beverage)|arak]] and [[absinthe]], forming a milky (''[[wikt:louche#French|louche]]'') oil-in-water [[microemulsion]]. Because such microemulsions occur with only minimal mixing and are highly stable,<ref name="Sitnikova+2005"/> the ouzo effect may have commercial applications.

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

In water-immiscible solvents,  oil-in-water [[emulsion]]s are not stable as the oil droplets [[Coalescence (chemistry)|coalesce]] until complete 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 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 (materials science)|dispersion]] by liquid-liquid [[nucleation]].<ref>{{cite journal
|last=Vitale
|first=Stephen A.
|coauthors=Joseph L. Katz
|month=May|year=2003
|title=Liquid Droplet Dispersions Formed by Homogeneous Liquid-Liquid Nucleation: The Ouzo Effect
|journal=Langmuir|volume=19|issue=10|pages=4105–4110
|publisher=American Chemical Society
|doi=10.1021/la026842o
}}</ref> The size of the droplets has been measured by [[small-angle neutron scattering]] to be on the order of a [[micrometre]].<ref name=grillo>{{cite journal
|last=Grillo
|first=Isabelle
|month=September
|year=2003
|title=Small-angle neutron scattering study of a world-wide known emulsion: Le Pastis |journal=Colloids and Surfaces A, Physicochemical and Engineering Aspects
|volume=225|issue=1-3|pages=153–160
|doi=10.1016/S0927-7757(03)00331-5
|url=http://www.crmcn.univ-mrs.fr/pics/linked-files/LePastis_IGrillo.pdf
|format=PDF
|accessdate=2007-01-29
}}</ref>

Using [[dynamic light scattering]], Sitnikova et al.<ref name="Sitnikova+2005">{{cite journal |last=Sitnikova |first=Natalia L. |coauthors=Rudolf Sprik, Gerard Wegdam and Erika Eiser |month= |year=2005 |title=Spontaneously Formed trans-Anethol/Water/Alcohol Emulsions: Mechanism of Formation and Stability |journal=Langmuir |volume=21 |issue=16 |pages=7083–7089 |doi=10.1021/la046816l |url=http://www.science.uva.nl/~sprik/personal/pdf/05/la046816l.pdf |format=PDF|accessdate=2007-01-29 |pmid=16042427}}</ref> 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 {{gaps|3|micrometre}}.

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]].<ref name=grillo/> However, the microscopic mechanisms responsible for the observed slowing of Ostwald ripening rates at increasing ethanol concentrations appears not fully understood.

==Applications==
Microemulsions 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. It has been conjectured<ref>{{cite journal |last=Ganachaud |first=François |coauthors=Joseph L. Katz |month= |year=2005 |title=Nanoparticles and Nanocapsules Created Using the Ouzo Effect: Spontaneous Emulsification as an Alternative to Ultrasonic and High-Shear Devices |journal=ChemPhysChem |volume=6 |issue=2 |pages=209–216 |doi=10.1002/cphc.200400527 |pmid=15751338}}</ref> that the synthesis of a variety of dispersions such as pseudolatexes, silicone emulsions, and biodegradable polymeric [[nanocapsule]]s, have been synthesized as a result of the ouzo effect.

==See also==
* [[Interface and colloid science]]
* [[Miniemulsion]]
* [[List of liqueurs#Anise-flavored liqueurs|Anise-flavored liqueurs on the list of liqueurs]]

==References==
{{reflist}}

==External links==
{{commonscat-inline|Ouzo effect}}

[[Category:Colloidal chemistry]]
[[Category:Chemical mixtures]]
[[Category:Condensed matter physics]]
[[Category:Soft matter]]

[[cs:Ouzo efekt]]
[[de:Louche-Effekt]]
[[ru:Эффект анисового ликёра]]
[[sv:Opalisering]]
[[Category:2Fix]]