Ouzo effect
The ouzo effect is a phenomenon where the addition of water to some alcoholic beverages, such as ouzo, causes the formation of a cloudy oil-in-water microemulsion that appears stable. When the effect is observed in absinthe, it is specifically called louche (French for "shady", hence "cloudy"). The fact that ouzo contains a strongly hydrophobic oil (trans-anethole) solved in a water-miscible solvent (ethanol) is key to the effect. In the absence of such a water-miscible solvent, oil-in-water emulsions are not stable as the oil droplets will continue to grow via droplet coalescence until 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 rates (strong stirring) can stabilise the oil droplets. The surprising fact that, without the use of such means, the oil droplets in a water-rich ouzo mixture slow down their growth and form a stable liquid dispersion, was first studied by Stephen A. Vitale and Joseph L. Katz,[1] who termed it the "ouzo effect". This effect is also referred to as "spontaneous emulsification".
Recent research
In recent years, the ouzo effect has been subject of active research. One study[2] has determined the size of the droplets using small-angle neutron scattering techniques. The droplet radii typically are of the order of a micrometre.
Using dynamic light scattering, another study[3] showed that the droplets of oil in the emulsion grow via Ostwald ripening, and that no droplet coalescence takes place. The Ostwald ripening rate is observed to reduce at higher ethanol concentrations and eventually to reduce such that the droplets stabilize in size. In this experiment, the average diameter of the droplets stabilized at a value of typically 3 micrometre.
Although it is clear that stable emulsification occurs because the trans-anethole is soluble in ethanol but not in water, so that the trans-anethole molecules aggregate together within the emulsion droplets, the exact physical mechanisms responsible for the slowing down of the Ostwald ripening rate at high concentrations of ethanol are not fully understood.
Applications
Microemulsions have many commercially important uses. A large range of prepared food products, detergents, and body-care products take the form of emulsions that need to stay 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[4] that the synthesis of a variety of dispersions such as pseudolatexes, silicone emulsions, and biodegradable polymeric nanocapsules, have actually been synthesized using the ouzo effect.
See also
References
- ^ Vitale, Stephen A. (2003). "Liquid Droplet Dispersions Formed by Homogeneous Liquid-Liquid Nucleation: The Ouzo Effect". Langmuir. 19 (10): 4105–4110. doi:10.1021/la026842o.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ Grillo, Isabelle (2003). "Small-angle neutron scattering study of a world-wide known emulsion: Le Pastis" (PDF). Colloids and Surfaces A: Physicochemical and Engineering Aspects. 223 (1–3): 153–160. doi:10.1016/S0927-7757(03)00331-5. Retrieved 2007-01-29.
{{cite journal}}: Cite has empty unknown parameter:|coauthors=(help); Unknown parameter|month=ignored (help) - ^ Sitnikova, Natalia L. (2005). "Spontaneously Formed trans-Anethol/Water/Alcohol Emulsions: Mechanism of Formation and Stability" (PDF). Langmuir. 21 (16): 7083–7089. doi:10.1021/la046816l. Retrieved 2007-01-29.
{{cite journal}}: Cite has empty unknown parameter:|month=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Ganachaud, François (2005). "Nanoparticles and Nanocapsules Created Using the Ouzo Effect: Spontaneous Emulsification as an Alternative to Ultrasonic and High-Shear Devices". ChemPhysChem. 6 (2): 209–216. doi:10.1002/cphc.200400527.
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