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Supersaturation is a state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound.
Very small particles (seeds) can trigger the separation of the dissolved material from the solvent or condensation of the vapor. Seeds triggeing the conversion of vapor are referred to as condensation nuclei, as in the case of water vapor. In the solid form these seeds can lead to the formation of crystallites or even large single crystals.
Supersaturated solutions are prepared or result when some condition of a saturated solution is changed, for example decreasing (or, rarely, increasing) temperature, decreasing volume of the saturated solvent (as by liquid evaporation), or increasing pressure.
Carbonated water is a supersaturated solution of carbon dioxide gas in water. At the elevated pressure in the bottle, more carbon dioxide can dissolve in water than at atmospheric pressure. At atmospheric pressure, the carbon dioxide gas escapes very slowly from the supersaturated liquid. This process may be accelerated by the presence of nucleation sites within the solution, such as small bubbles, caused by shaking the bottle, or another solute, such as sugar powder or a widget. A Diet Coke and Mentos eruption is a rather extreme example. Some beverage products such as ales and stouts, e.g. Guinness, rely on this effect to produce the 'head' on the surface of the poured product. This has led to the invention of the widget, a device developed to produce enhanced bubble seeding in liquids, especially with dual supersaturated gas phases (carbon dioxide and nitrogen) (see patents by Nicholas Fitzpatrick and Kuzniarski).
Scuba divers' tissues become saturated with breathing gases during a dive. Supersaturation is a theoretical term describing a state in which the tension of a dissolved gas is greater than its inspired partial pressure when the diver ascends, in contrast to Henry's law. If the diver ascends too fast, these gases form bubbles, resulting in decompression sickness. The term was popularized by J.S. Haldane.
Supersaturated solutions of sugar and water are commonly used to make rock candy.
Supersaturation has also been gaining momentum as a formulation concept in pharmaceutical sciences. So-called supersaturating drug delivery systems (SSDS) are a promising to obtain adequate oral bioavailability of poorly water- soluble drugs. Contemporary pharmaceutical pipelines are often highly populated with poorly water-soluble drug candidates necessitating novel formulation technologies to provide dosage forms with appropriate biopharmaceutical properties. SDDS contain the drug in a high energy or otherwise rapidly dissolving form such that intraluminal concentrations above the saturation solubility of the drug are generated. For the strategy to be useful, the formed supersaturated solution must then be stabilized to allow for significant absorption and eventually sufficient bioavailability. The stabilization of a supersaturated solution can be accomplished by adding precipitation inhibitors which may act through a variety of mechanisms.
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