Difference between revisions of "Emulsion" - New World Encyclopedia
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[[Image:Emulsions.png|frame|right|A. Two immisicble liquids, not emulsified; B. An emulsion of Phase B dispersed in Phase A; C. The unstable emulsion progressively separates; D. The surfactant (purple outline) positions itself on the interfaces between Phase A and Phase B, stabilizing the emulsion]] | [[Image:Emulsions.png|frame|right|A. Two immisicble liquids, not emulsified; B. An emulsion of Phase B dispersed in Phase A; C. The unstable emulsion progressively separates; D. The surfactant (purple outline) positions itself on the interfaces between Phase A and Phase B, stabilizing the emulsion]] | ||
| − | An '''emulsion''' is a mixture of two [[immiscible]]* (unblendable) substances. One substance | + | An '''emulsion''' is a mixture of two [[immiscible]]* (unblendable) substances. One substance, called the "dispersed phase," is [[dispersion|dispersed]] in the other, known as the "continuous phase." Examples of emulsions include [[butter]], [[margarine]], [[espresso]], [[mayonnaise]], the photosensitive side of [[photographic film]], and [[cutting fluid]] for [[metalworking]]. In the case of butter and margarine, a continuous lipid phase surrounds droplets of water, and they are examples of "water-in-oil" emulsions. The process of preparing emulsions is called '''emulsification'''. |
| − | Emulsions | + | Emulsions are part of a more general class of two-phase systems of matter called '''[[colloid]]'''s. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that the dispersed and continuous phases are both [[liquid]]s. |
| − | Emulsions | + | == Characteristics == |
| + | |||
| + | Emulsions tend to have a cloudy appearance, as a result of the scattering of light. As [[light]] passes through the emulsion, it is scattered by the many interfaces (boundaries) between the different phases (such as water and oil). | ||
| + | |||
| + | In addition, emulsions are unstable and do not form spontaneously. An example of an unstable emulsion is homemade salad dressing made by mixing [[cooking oil|oil]] and [[vinegar]]. The two phases separate quickly unless shaken repeatedly. The phases separate when smaller droplets recombine to form larger ones—a phenomenon called "[[coalescence (meteorology)|coalescence]]. Fluid emulsions may also suffer from [[#Types of emulsion instability|creaming]]—the migration of one of the substances to the top of the emulsion under the influence of [[buoyancy]] or [[centripetal force]] when a [[centrifuge]] is used. | ||
| + | |||
| + | == Forming and stabilizing emulsions == | ||
| + | |||
| + | To form an emulsion, the mixture of substances needs energy input through processes such as shaking, stirring, spraying, or homogenizing. To stabilize an emulsion, certain substances called '''[[surfactant]]s''' ("surface active substances") are added. A good surfactant increases the kinetic stability of the emulsion so greatly that the emulsion, once formed, does not change significantly over years of storage. | ||
| + | |||
| + | == Types of emulsion instability == | ||
There are three types of emulsion instability: | There are three types of emulsion instability: | ||
| − | * | + | * Breaking: The dispersed particles (droplets) coalesce and form a layer of liquid. |
| − | * | + | * Flocculation: The particles of the dispersed phase form clumps. |
| − | + | * Creaming: The particles of the dispersed phase concentrate near the surface of the mixture while staying separated. | |
| − | == | + | ==Emulsifiers and their uses== |
[[Image:propofol.jpg|left|thumb|An ampule containing 20 milliliters of 1% propofol emulsion suitable for intravenous injection. Manufacturers emulsify the lipid-soluble propofol in a mixture of water, soy oil, and egg lecithin.]] | [[Image:propofol.jpg|left|thumb|An ampule containing 20 milliliters of 1% propofol emulsion suitable for intravenous injection. Manufacturers emulsify the lipid-soluble propofol in a mixture of water, soy oil, and egg lecithin.]] | ||
| − | An '''emulsifier''' (also known as an '''emulgent''' or '''[[surfactant]]''') is a substance | + | An '''emulsifier''' (also known as an '''emulgent''' or '''[[surfactant]]''') is a substance that stabilizes an emulsion. Examples of food emulsifiers are [[egg yolk]] (where the main emulsifying chemical is the [[phospholipid]] [[lecithin]]), and [[mustard]], where a variety of chemicals in the [[mucilage]] surrounding the seed hull act as emulsifiers. [[Protein]]s and low-molecular-weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called [[Pickering stabilization]]. Both [[mayonnaise]] and [[Hollandaise sauce]] are oil-in-water emulsions stabilized with egg yolk lecithin. |
| + | |||
| + | [[Detergent]]s are another class of surfactants. They chemically interact with both [[Cooking oil|oil]] and [[water]], thus stabilising the interface between oil or water droplets in suspension. This principle is exploited in [[soap]] to remove [[Yellow grease|grease]] for the purpose of [[cleaning]]. A wide variety of emulsifiers are used in [[pharmacy]] to prepare emulsions such as [[cream (pharmaceutical)|creams]] and [[lotion]]s. | ||
Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the "[[Bancroft rule]]*" applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well; for example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water). | Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the "[[Bancroft rule]]*" applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well; for example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water). | ||
Revision as of 20:00, 13 February 2007
An emulsion is a mixture of two immiscible (unblendable) substances. One substance, called the "dispersed phase," is dispersed in the other, known as the "continuous phase." Examples of emulsions include butter, margarine, espresso, mayonnaise, the photosensitive side of photographic film, and cutting fluid for metalworking. In the case of butter and margarine, a continuous lipid phase surrounds droplets of water, and they are examples of "water-in-oil" emulsions. The process of preparing emulsions is called emulsification.
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 tends to imply that the dispersed and continuous phases are both liquids.
Characteristics
Emulsions tend to have a cloudy appearance, as a result of the scattering of light. As light passes through the emulsion, it is scattered by the many interfaces (boundaries) between the different phases (such as water and oil).
In addition, emulsions are unstable and do not form spontaneously. An example of an unstable emulsion is homemade salad dressing made by mixing oil and vinegar. The two phases separate quickly unless shaken repeatedly. The phases separate when smaller droplets recombine to form larger ones—a phenomenon called "coalescence. Fluid emulsions may also suffer from creaming—the migration of one of the substances to the top of the emulsion under the influence of buoyancy or centripetal force when a centrifuge is used.
Forming and stabilizing emulsions
To form an emulsion, the mixture of substances needs energy input through processes such as shaking, stirring, spraying, or homogenizing. To stabilize an emulsion, certain substances called surfactants ("surface active substances") are added. A good surfactant increases the kinetic stability of the emulsion so greatly that the emulsion, once formed, does not change significantly over years of storage.
Types of emulsion instability
There are three types of emulsion instability:
- Breaking: The dispersed particles (droplets) coalesce and form a layer of liquid.
- Flocculation: The particles of the dispersed phase form clumps.
- Creaming: The particles of the dispersed phase concentrate near the surface of the mixture while staying separated.
Emulsifiers and their uses
An emulsifier (also known as an emulgent or surfactant) is a substance that stabilizes an emulsion. Examples of food emulsifiers are egg yolk (where the main emulsifying chemical is the phospholipid lecithin), and mustard, where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers. Proteins and low-molecular-weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called Pickering stabilization. Both mayonnaise and Hollandaise sauce are oil-in-water emulsions stabilized with egg yolk lecithin.
Detergents are another class of surfactants. They chemically interact with both oil and water, thus stabilising the interface between oil or water droplets in suspension. This principle is exploited in soap to remove grease for the purpose of cleaning. A wide variety of emulsifiers are used in pharmacy to prepare emulsions such as creams and lotions.
Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the "Bancroft rule" applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well; for example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water).
See also
- Liquid
- Materials science
- Phase (matter)
ReferencesISBN links support NWE through referral fees
- Johan Sjoblom (editor), Emulsions and Emulsion Stability (second edition), Surfactant Science Series Vol. 61, CRC (2005). ISBN 0824726952.
- Johan Sjoblom (editor), Encyclopedic Handbook of Emulsion Technology, CRC (2001). ISBN 0824704541.
- Paul Becher, Emulsions: Theory and Practice (third edition), American Chemical Society (2001). ISBN 0841234965.
External links
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