The ouzo effect (also louche effect and spontaneous emulsification) is a milky ('' louche'') oil-in-water

emulsion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Alth ...

that is formed when water is added to

ouzo Ouzo ( el, ούζο, ) is a dry anise-flavored aperitif that is widely consumed in Greece. It is made from rectified spirits that have undergone a process of distillation and flavoring. Its taste is similar to other anise liquors like pastis, ...

and other anise-flavored liqueurs and spirits, such as

pastis Pastis (; oc, Pastís, ; or ) is an anise-flavoured spirit and apéritif traditionally from France, typically containing less than 100 g/L sugar and 40–45% ABV (alcohol by volume). Origins Pastis was first commercialized by Paul Ricard i ...

rakı Rakı or raki (, Turkish pronunciation: ) is an alcoholic drink made of twice-distilled grapes. It is the national drink of Turkey. It is also popular in other Balkan countries as an apéritif as well as in Kazakhstan. It is often served with ...

, arak, sambuca and

absinthe Absinthe (, ) is an anise-flavoured spirit derived from several plants, including the flowers and leaves of '' Artemisia absinthium'' ("grand wormwood"), together with green anise, sweet fennel, and other medicinal and culinary herbs. Historica ...

. Such emulsions occur with only minimal mixing and are highly stable.

Observation and explanation

The ouzo effect occurs when a strongly

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...

such as ''trans''-anethole is dissolved in a water-

miscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

, such as

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

, and the concentration of ethanol is lowered by addition of small amounts of water. Oil-in-water

s are not normally stable. Oil droplets

coalesce Coalesce may refer to: *Coalesce (band), a metalcore band from Kansas City, Missouri, active from 1994 to 1999, 2005– ** Coalesce discography, a list of Coalesce's albums and songs * COALESCE, an SQL function *Null coalescing operator, a binary ...

until complete phase separation is achieved at

macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenomena a ...

levels. Addition of a small amount of

surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...

or the application of high

shear rate In physics, shear rate is the rate at which a progressive shearing deformation is applied to some material. Simple shear The shear rate for a fluid flowing between two parallel plates, one moving at a constant speed and the other one stationary ...

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 Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

by liquid–liquid

nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...

. The size of the droplets has been measured by

small-angle neutron scattering Small-angle neutron scattering (SANS) is an experimental technique that uses elastic neutron scattering at small scattering angles to investigate the structure of various substances at a mesoscopic scale of about 1–100 nm. Small angle n ...

to be on the order of a

micron The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Un ...

. Using

dynamic light scattering Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution. In the scope of DLS, temporal fluctuations are usually analyzed using ...

, Sitnikova et al. showed that the droplets of oil in the emulsion grow by

Ostwald ripening Ostwald ripening is a phenomenon observed in solid solutions or liquid sols that describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles ...

, 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 In thermodynamics, the limit of local stability with respect to small fluctuations is clearly defined by the condition that the second derivative of Gibbs free energy is zero. : 0 The locus of these points (the inflection point within a G-x or G- ...

curves in the

phase diagram A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseous ...

. However, the microscopic mechanisms responsible for the observed slowing of Ostwald ripening rates at increasing ethanol concentrations appear not fully understood.

Applications

Emulsions have many commercial uses. A large range of prepared food products,

detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are m ...

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

-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. Nanoparticles formed using the ouzo effect are thought to be kinetically stabilized as opposed to thermodynamically stabilized micelles formed using a surfactant due to the fast solidification of the polymer during the preparation process.

References

External links

{{commons category-inline, Ouzo effect Colloidal chemistry Chemical mixtures Soft matter Absinthe Articles containing video clips

Observation and explanation

Applications

See also

References

External links