The ouzo effect ( ), also known as the louche effect ( ) and spontaneous emulsification, is the phenomenon of formation of a milky oil-in-water

emulsion An emulsion is a mixture of two or more liquids that are normally Miscibility, 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 colloi ...

when water is added to

ouzo Ouzo (, ) is a dry anise-flavored aperitif that is widely consumed in Cyprus and 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, sam ...

and other anise-flavored liqueurs and spirits, such as

pastis Pastis (, , ; ) 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 in 1932 and enj ...

rakı Rakı, Türk Rakısı or Turkish Raki (, Turkish pronunciation: ) is an alcoholic beverage made of twice-distilled grape pomace and flavored with aniseed. It is a national drink of Turkey, although fewer than 17% of Turks drink alcohol. Among ...

, arak,

sambuca Sambuca () is an Italian anise-flavoured liqueur. Its most common variety is often referred to as "white sambuca" to differentiate it from other varieties that are deep blue ("black sambuca") or bright red ("red sambuca"). Like other anise-fla ...

and

absinthe Absinthe (, ) is an anise-flavored Liquor, 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. His ...

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

Observation and explanation

First a strongly

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

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 ...

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). Such substances are said to be miscible (etymologically ...

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

, such as

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

, and the ethanol itself forms a

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solu ...

(a ''homogeneous

mixture In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...

'') with water. If then the concentration of ethanol is lowered by addition of more water the hydrophobic substance precipitates from the solution and forms an

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-water

s are not normally stable. Oil droplets coalesce until complete

phase separation Phase separation is the creation of two distinct Phase (matter), phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. This type of phase separation is kn ...

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 phenome ...

levels. Addition of a small amount of

surfactant Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word ''surfactant'' is a Blend word, blend of "surface-active agent", coined in ...

or the application of high

shear rate In physics, mechanics and other areas of science, shear rate is the rate at which a progressive shear strain is applied to some material, causing shearing to the material. Shear rate is a measure of how the velocity changes with distance. Simple ...

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 variat ...

by liquid–liquid

nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

. The size of the droplets when 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 ...

was found to be on the order of a

micron The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...

. 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 and liquid sols that involves the change of an inhomogeneous structure over time, in that small crystals or sol particles first dissolve and then redeposit onto larger crystals or s ...

, 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 In thermodynamics, the binodal, also known as the coexistence curve or binodal curve, denotes the state of a multi-component system at which the system's distinct phases straddle between coexistence or miscibility. Equivalently, it is the bound ...

and

spinodal In thermodynamics, the limit of local stability against phase separation 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 inflecti ...

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

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

detergent A detergent is a surfactant or a mixture of surfactants with Cleanliness, cleansing properties when in Concentration, dilute Solution (chemistry), solutions. There are a large variety of detergents. A common family is the alkylbenzene sulfonate ...

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.

Nanoparticle A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...

s formed using the ouzo effect are thought to be kinetically stabilized as opposed to thermodynamically stabilized

micelles A micelle () or micella () ( or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated colloidal system). ...

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