Ouzo effect
Encyclopedia
Ouzo
Ouzo is an anise-flavored aperitif that is widely consumed in Greece and Cyprus, and a symbol of Greek culture.-History:Traditionally, tsipouro is said to have been the pet project of a group of 14th century monks living in a monastery on holy Mount Athos. One version of it is flavored with anise...
and other anise-flavored liqueurs and spirits, such as pastis
Pastis
Pastis is an anise-flavored liqueur and apéritif from France, typically containing 40–45% alcohol by volume, although alcohol-free varieties exist.-Origins:...
, raki
Raki (alcoholic beverage)
Raki is a Turkish unsweetened, anise-flavored hard alcoholic drink that is popular in Turkey, Greece, Serbia, and other Balkan countries as an apéritif. It is often served with seafood or meze...
, arak
Arak (distilled beverage)
Arak or Araq , is a highly alcoholic spirit from the anis drinks family. It is a clear, colorless, unsweetened anise-flavoured distilled alcoholic drink...
and absinthe
Absinthe
Absinthe is historically described as a distilled, highly alcoholic beverage. It is an anise-flavoured spirit derived from herbs, including the flowers and leaves of the herb Artemisia absinthium, commonly referred to as "grande wormwood", together with green anise and sweet fennel...
, forming a milky (louche) oil-in-water microemulsion
Microemulsion
Microemulsions are clear, thermodynamically stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins...
. Because such microemulsions occur with only minimal mixing and are highly stable, the ouzo effect may have commercial applications.
Observation and explanation
The ouzo effect occurs when a strongly hydrophobic essential oilEssential oil
An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...
of trans-anethole
Anethole
Anethole is a phenylpropene, a type of aromatic compound that occurs widely in nature, in essential oils...
is dissolved in a water-miscible
Miscibility
Miscibility is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases , but the main focus is usually on the solubility of one liquid in another...
solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, such as ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
, and the concentration of ethanol is lowered by addition of small amounts of water.
In water-immiscible solvents, oil-in-water emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
s are not stable as the oil droplets coalesce
Coalescence (chemistry)
In chemistry, coalescence is a process in which two phase domains of the same composition come together and form a larger phase domain.-References:*...
until complete phase separation is achieved at macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
levels. It is well-known that the addition of a small amount of surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
or the application of high shear 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 fluid dispersion
Dispersion (materials science)
In materials science, dispersion is the fraction of atoms of a material exposed to the surface. In general:where D is the dispersion, NS is the number of surface atoms and NT is the total number of atoms of the material...
by liquid-liquid nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
. The size of the droplets has been measured by small-angle neutron scattering
Small-angle neutron scattering
Small angle neutron scattering is a laboratory technique, similar to the often complementary techniques of small angle X-ray scattering and light scattering, used for investigations of structure of various substances, with spatial sensitivity of about 1 - 1000 nm...
to be on the order of a micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
.
Using dynamic light scattering
Dynamic light scattering
thumb|right|350px|Hypothetical Dynamic light scattering of two samples: Larger particles on the top and smaller particle on the bottomDynamic light scattering is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers...
, Sitnikova et al. showed that the droplets of oil in the emulsion grow by Ostwald ripening
Ostwald ripening
right|thumb|300px|Basic schematic of the Ostwald ripening process Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time...
, 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
Spinodal
In thermodynamics, the spinodal is the limit of stability of a solution, denoting the boundary of absolute instability of a solution to decomposition into multiple phases. Within this curve, infinitesimally small fluctuations in composition and density will lead to phase separation via spinodal...
curves in the phase diagram
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
. 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, detergentDetergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
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
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
-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 been synthesized as a result of the ouzo effect.
See also
- Interface and colloid scienceInterface and colloid scienceInterface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous...
- MiniemulsionMiniemulsionA miniemulsion is a special case of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases, one surfactant and one co-surfactant ....
- Anise-flavored liqueurs on the list of liqueurs
- SpinodalSpinodalIn thermodynamics, the spinodal is the limit of stability of a solution, denoting the boundary of absolute instability of a solution to decomposition into multiple phases. Within this curve, infinitesimally small fluctuations in composition and density will lead to phase separation via spinodal...