Editing Emulsion

{{Short description|Mixture of two or more immiscible liquids}}
{{About|mixtures of liquids|the light-sensitive mixture used in photography|Photographic emulsion}}
[[File:Emulsions.svg|frame|right| {{ordered list |list_style_type=upper-alpha
    |1=<!--A-->Two immiscible liquids, not yet emulsified
    |2=<!--B-->An emulsion of Phase II dispersed in Phase I
    |3=<!--C-->The unstable emulsion progressively separates
    |4=<!--D-->The [[surfactant]] (outline around particles) positions itself on the interfaces between Phase II and Phase I, stabilizing the emulsion
}}]]

An '''emulsion''' is a [[mixture]] of two or more [[liquid]]s 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 [[colloid]]s. Although the terms ''colloid'' and ''emulsion'' are sometimes used interchangeably, ''emulsion'' should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid (the dispersed [[phase (matter)|phase]]) is [[dispersion (chemistry)|dispersed]] in the other (the continuous phase). Examples of emulsions include [[vinaigrette]]s, homogenized [[milk]], liquid [[biomolecular condensate]]s, and some [[cutting fluid]]s for [[metal working]].

Two liquids can form different types of emulsions. As an example, oil and water can form, first, an oil-in-water emulsion, in which the oil is the dispersed phase, and water is the continuous phase. Second, they can form a water-in-oil emulsion, in which water is the dispersed phase and oil is the continuous phase. Multiple emulsions are also possible, including a "water-in-oil-in-water" emulsion and an "oil-in-water-in-oil" emulsion.<ref>{{cite journal|pmid=17076645 |year=2006 |last1=Khan |first1=A. Y. |title=Multiple emulsions: An overview |journal=Current Drug Delivery |volume=3 |issue=4 |pages=429–43 |last2=Talegaonkar |first2=S |last3=Iqbal |first3=Z |last4=Ahmed |first4=F. J. |last5=Khar |first5=R. K. |doi=10.2174/156720106778559056}}</ref>

Emulsions, being liquids, do not exhibit a static internal structure. The droplets dispersed in the continuous phase (sometimes referred to as the "dispersion medium") are usually assumed to be [[Probability distribution|statistically distributed]] to produce roughly spherical droplets.

The term "emulsion" is also used to refer to the photo-sensitive side of [[photographic film]]. Such a [[photographic emulsion]] consists of [[silver halide]] colloidal particles dispersed in a [[gelatin]] matrix. [[Nuclear emulsion]]s are similar to photographic emulsions, except that they are used in particle physics to detect high-energy [[elementary particle]]s.

{{Quote box
 |title =[[International Union of Pure and Applied Chemistry|IUPAC]] 
 |quote = A fluid system in which liquid droplets are dispersed in a liquid.

''Note 1'': The definition is based on the definition in ref.<ref>{{cite book|title=Compendium of Chemical Terminology (The "Gold Book")|year=1997|publisher=[[Blackwell Scientific Publications]]|location=Oxford|author=IUPAC|chapter-url=http://goldbook.iupac.org/E02065.html|url-status=dead |archive-url= https://web.archive.org/web/20120310221658/http://goldbook.iupac.org/E02065.html|archive-date=2012-03-10 |doi=10.1351/goldbook.E02065 |chapter=Emulsion|isbn=978-0-9678550-9-7}}</ref>

''Note 2'': The droplets may be amorphous, liquid-crystalline, or any<br/>mixture thereof.

''Note 3'': The diameters of the droplets constituting the ''[[Dispersion (chemistry)|dispersed phase]]''<br/>usually range from approximately 10&nbsp;nm to 100&nbsp;μm; i.e., the droplets<br/>may exceed the usual size limits for [[colloid]]al particles.

''Note 4'': An emulsion is termed an oil/water (o/w) emulsion if the<br/>dispersed phase is an organic material and the ''continuous phase'' is<br/>water or an aqueous solution and is termed water/oil (w/o) if the dispersed<br/>phase is water or an aqueous solution and the continuous phase is an<br/>organic liquid (an "oil").

''Note 5'': A w/o emulsion is sometimes called an inverse emulsion.<br/>The term "inverse emulsion" is misleading, suggesting incorrectly that<br/>the emulsion has properties that are the opposite of those of an emulsion.<br/>Its use is, therefore, not recommended.<ref>{{cite journal|title=Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)|journal=[[Pure and Applied Chemistry]]|year=2011|volume=83|issue=12|pages=2229–2259|doi=10.1351/PAC-REC-10-06-03 |last1=Slomkowski |first1=Stanislaw |last2=Alemán|first2=José V.|last3=Gilbert|first3=Robert G. |last4=Hess |first4=Michael |last5=Horie |first5=Kazuyuki |last6=Jones |first6=Richard G. |last7=Kubisa |first7=Przemyslaw |last8=Meisel |first8=Ingrid |last9=Mormann |first9=Werner |last10=Penczek |first10=Stanisław |last11=Stepto|first11=Robert F. T.|s2cid=96812603|url=https://espace.library.uq.edu.au/view/UQ:266979/UQ266979_OA.pdf}}</ref>
}}

==Etymology==
The word "emulsion" comes from the Latin ''emulgere'' "to milk out", from ''ex'' "out" + ''mulgere'' "to milk", as milk is an emulsion of fat and water, along with other components, including [[colloid]]al [[casein]] micelles (a type of secreted [[biomolecular condensate]]).<ref name="OnlineEtymol">{{cite web |last1=Harper |first1=Douglas |title=Online Etymology Dictionary |url=https://www.etymonline.com/search?q=emulsion |website=www..etymonline.com |publisher=Etymonline |access-date=2 November 2019}}</ref>

==Appearance and properties==
Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface".<ref name=":2">{{Citation|last1=Loi|first1=Chia Chun|title=Protein-Stabilised Emulsions|date=2018|work=Reference Module in Food Science |publisher=Elsevier |doi=10.1016/b978-0-08-100596-5.22490-6|isbn=9780081005965|last2=Eyres|first2=Graham T.|last3=Birch|first3=E. John}}</ref> Emulsions tend to have a cloudy appearance because the many [[phase boundary|phase interfaces]] [[scattering|scatter]] light as it passes through the emulsion. Emulsions appear [[white]] when all light is scattered equally. If the emulsion is dilute enough, higher-frequency (shorter-wavelength) light will be scattered more, and the emulsion will appear [[blue]]r&nbsp;– this is called the "[[Tyndall effect]]".<ref>{{Cite book|last=Joseph Price Remington|title=Remington's Pharmaceutical Sciences|editor-last=Alfonso R. Gennaro|publisher=Mack Publishing Company (Original from Northwestern University) (Digitized 2010)|year=1990|isbn=9780912734040|pages=281}}</ref> If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear more [[yellow]]. This phenomenon is easily observable when comparing [[skimmed milk]], which contains little fat, to [[cream]], which contains a much higher concentration of milk fat. One example would be a mixture of water and oil.<ref>{{Cite web |title=Emulsion - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/emulsion |access-date=2022-03-01 |website=www.sciencedirect.com}}</ref>

Two special classes of emulsions&nbsp;– [[microemulsion]]s and nanoemulsions, with droplet sizes below 100&nbsp;nm&nbsp;– appear translucent.<ref name="Mason">{{cite journal|vauthors=Mason TG, Wilking JN, Meleson K, Chang CB, Graves SM|title=Nanoemulsions: Formation, structure, and physical properties|journal=Journal of Physics: Condensed Matter|volume=18|issue=41|pages=R635–R666 |doi=10.1088/0953-8984/18/41/R01 |url=http://www.firp.ula.ve/archivos/pdf/06_JPCM_Mason.pdf |year=2006 |bibcode=2006JPCM...18R.635M |s2cid=11570614 |access-date=2016-10-26|archive-url= https://web.archive.org/web/20170112080749/http://www.firp.ula.ve/archivos/pdf/06_JPCM_Mason.pdf|archive-date=2017-01-12|url-status=dead}}</ref> This property is due to the fact that light waves are scattered by the droplets only if their sizes exceed about one-quarter of the wavelength of the incident light. Since the [[visible spectrum]] of light is composed of wavelengths between 390 and 750 [[nanometer]]s (nm), if the droplet sizes in the emulsion are below about 100&nbsp;nm, the light can penetrate through the emulsion without being scattered.<ref>{{cite journal|vauthors=Leong TS, Wooster TJ, Kentish SE, Ashokkumar M |title=Minimising oil droplet size using ultrasonic emulsification|journal=Ultrasonics Sonochemistry|volume=16|issue=6|pages=721–7 |pmid=19321375 |year=2009 |doi=10.1016/j.ultsonch.2009.02.008|hdl=11343/129835|url=http://minerva-access.unimelb.edu.au/bitstream/11343/129835/1/Minerva.pdf|doi-access=free}}</ref> Due to their similarity in appearance, translucent nanoemulsions and microemulsions are frequently confused. Unlike translucent nanoemulsions, which require specialized equipment to be produced, microemulsions are spontaneously formed by "solubilizing" oil molecules with a mixture of [[surfactant]]s, co-surfactants, and co-[[solvent]]s.<ref name="Mason" /> The required surfactant concentration in a microemulsion is, however, several times higher than that in a translucent nanoemulsion, and significantly exceeds the concentration of the dispersed phase. Because of many undesirable side-effects caused by surfactants, their presence is disadvantageous or prohibitive in many applications. In addition, the stability of a microemulsion is often easily compromised by dilution, by heating, or by changing pH levels.{{Citation needed|date= February 2018}}

Common emulsions are inherently unstable and, thus, do not tend to form spontaneously. Energy input&nbsp;– through shaking, stirring, [[Homogenization (chemistry)|homogenizing]], or exposure to power [[ultrasound]]<ref>{{cite journal| doi=10.1016/j.ifset.2007.07.005 | volume=9 | issue=2 | title=The use of ultrasonics for nanoemulsion preparation | year=2008 | journal=Innovative Food Science & Emerging Technologies | pages=170–175 | last1 = Kentish | first1 = S. | last2 = Wooster | first2 = T.J. | last3 = Ashokkumar | first3 = M. | last4 = Balachandran | first4 = S. | last5 = Mawson | first5 = R. | last6 = Simons | first6 = L.| hdl=11343/55431 | hdl-access = free }}</ref>&nbsp;– is needed to form an emulsion. Over time, emulsions tend to revert to the stable state of the phases comprising the emulsion. An example of this is seen in the separation of the oil and vinegar components of [[vinaigrette (food)|vinaigrette]], an unstable emulsion that will quickly separate unless shaken almost continuously. There are important exceptions to this rule&nbsp;– microemulsions are [[thermodynamics|thermodynamically]] stable, while translucent nanoemulsions are [[Kinetics (physics)|kinetically]] stable.<ref name="Mason" />

Whether an emulsion of oil and water turns into a "water-in-oil" emulsion or an "oil-in-water" emulsion depends on the volume fraction of both phases and the type of emulsifier (surfactant) (see ''Emulsifier'', below) present.<ref>{{Cite web|url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/emulsion|title = Emulsion - an overview &#124; ScienceDirect Topics}}</ref>

===Instability===
Emulsion stability refers to the ability of an emulsion to resist change in its properties over time.<ref name=":0">{{cite book|author=McClements, David Julian |title=Food Emulsions: Principles, Practices, and Techniques, Second Edition|url=https://books.google.com/books?id=wTrzBPbf_WQC&pg=PA269|date=16 December 2004|publisher=[[Taylor & Francis]]|isbn=978-0-8493-2023-1|pages=269–}}</ref><ref>{{cite journal|doi=10.1016/S0268-005X(99)00027-2|title=Influence of copper on the stability of whey protein stabilized emulsions|journal=Food Hydrocolloids |volume=13 |issue=5 |pages=419 |year=1999 |last1=Silvestre |first1=M.P.C. |last2=Decker |first2=E.A.|last3=McClements|first3=D.J.}}</ref> There are four types of instability in emulsions: [[flocculation]], [[Coalescence (physics)|coalescence]], [[creaming (chemistry)|creaming]]/[[sedimentation]], and [[Ostwald ripening]]. Flocculation occurs when there is an attractive force between the droplets, so they form flocs, like bunches of grapes. This process can be desired, if controlled in its extent, to tune physical properties of emulsions such as their flow behaviour.<ref>{{Cite journal|last1=Fuhrmann|first1=Philipp L.|last2=Sala|first2=Guido|last3=Stieger|first3=Markus|last4=Scholten|first4=Elke|date=2019-08-01|title=Clustering of oil droplets in o/w emulsions: Controlling cluster size and interaction strength|journal=Food Research International|volume=122|pages=537–547|doi=10.1016/j.foodres.2019.04.027|pmid=31229109|issn=0963-9969|doi-access=free}}</ref> Coalescence occurs when droplets bump into each other and combine to form a larger droplet, so the average droplet size increases over time. Emulsions can also undergo creaming, where the droplets rise to the top of the emulsion under the influence of [[buoyancy]], or under the influence of the [[centripetal force]] induced when a [[centrifuge]] is used.<ref name=":0" /> Creaming is a common phenomenon in dairy and non-dairy beverages (i.e. milk, coffee milk, [[almond milk]], soy milk) and usually does not change the droplet size.<ref name=":1">{{Cite journal|last1=Loi|first1=Chia Chun|last2=Eyres|first2=Graham T.|last3=Birch|first3=E. John|date=2019|title=Effect of mono- and diglycerides on physical properties and stability of a protein-stabilised oil-in-water emulsion|journal=Journal of Food Engineering|volume=240|pages=56–64|doi=10.1016/j.jfoodeng.2018.07.016|s2cid=106021441|issn=0260-8774}}</ref> Sedimentation is the opposite phenomenon of creaming and normally observed in water-in-oil emulsions.<ref name=":2" /> Sedimentation happens when the dispersed phase is denser than the continuous phase and the gravitational forces pull the denser globules towards the bottom of the emulsion. Similar to creaming, sedimentation follows [[Stokes' law]].

An appropriate surface active agent (or surfactant) can increase the kinetic stability of an emulsion so that the size of the droplets does not change significantly with time. The stability of an emulsion, like a [[Suspension (chemistry)|suspension]], can be studied in terms of [[zeta potential]], which indicates the repulsion between droplets or particles. If the size and dispersion of droplets does not change over time, it is said to be stable.<ref>{{Cite journal|last=Mcclements|first=David Julian|date=2007-09-27|title=Critical Review of Techniques and Methodologies for Characterization of Emulsion Stability|journal=Critical Reviews in Food Science and Nutrition|volume=47|issue=7|pages=611–649|doi=10.1080/10408390701289292|issn=1040-8398|pmid=17943495|s2cid=37152866}}</ref> For example, oil-in-water emulsions containing [[Mono- and diglycerides of fatty acids|mono- and diglycerides]] and milk protein as surfactant showed that stable oil droplet size over 28 days storage at 25&nbsp;°C.<ref name=":1" />

===Monitoring physical stability===
The stability of emulsions can be characterized using techniques such as light scattering, focused beam reflectance measurement, centrifugation, and [[rheology]]. Each method has advantages and disadvantages.<ref>{{Cite journal|last1=Dowding|first1=Peter J.|last2=Goodwin|first2=James W.|last3=Vincent|first3=Brian|date=2001-11-30|title=Factors governing emulsion droplet and solid particle size measurements performed using the focused beam reflectance technique|journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects|volume=192|issue=1|pages=5–13|doi=10.1016/S0927-7757(01)00711-7|issn=0927-7757}}</ref>

===Accelerating methods for shelf life prediction===
The kinetic process of destabilization can be rather long&nbsp;– up to several months, or even years for some products.<ref>{{Cite book |last=Xiangxiang |first=Daily |url=https://xiangxiangdaily.com/emulsifying-guide-advanced-techniques-industrial-application/ |title=Emulsifying Guide: Advanced Techniques & Industrial Application |date=2024-08-05 |language=en}}</ref> Often the formulator must accelerate this process in order to test products in a reasonable time during product design. Thermal methods are the most commonly used – these consist of increasing the emulsion temperature to accelerate destabilization (if below critical temperatures for phase inversion or chemical degradation).<ref>{{Cite journal|last1=Masmoudi|first1=H.|last2=Dréau|first2=Y. Le |last3=Piccerelle |first3=P. |last4=Kister |first4=J.|date=2005-01-31|title=The evaluation of cosmetic and pharmaceutical emulsions aging process using classical techniques and a new method: FTIR|journal=International Journal of Pharmaceutics|volume=289|issue=1|pages=117–131 |doi=10.1016/j.ijpharm.2004.10.020|pmid=15652205|issn=0378-5173|url=https://hal.archives-ouvertes.fr/hal-03543083/file/The%20evaluation%20of%20cosmetic%20and%20pharmaceutical%20emulsions%20%20YLD%20Masmoudi.pdf}}</ref> Temperature affects not only the viscosity but also the interfacial tension in the case of non-ionic surfactants or, on a broader scope, interactions between droplets within the system. Storing an emulsion at high temperatures enables the simulation of realistic conditions for a product (e.g., a tube of sunscreen emulsion in a car in the summer heat), but also accelerates destabilization processes up to 200 times.{{Citation needed|date= February 2018}}

Mechanical methods of acceleration, including vibration, centrifugation, and agitation, can also be used.<ref>{{cite web |last1=Editorial Board Entrée |title=Emulsions |url=https://www.thermopedia.com/content/274/ |website=Thermopedia |access-date=16 June 2023}}</ref>

These methods are almost always empirical, without a sound scientific basis.{{Citation needed|date= February 2018}}

==Emulsifiers==
An '''emulsifier''' is a substance that stabilizes an emulsion by reducing the oil-water [[interface tension]]. Emulsifiers are a part of a broader group of compounds known as [[surfactant]]s, or "surface-active agents".<ref>{{cite web |title=Emulsions: making oil and water mix |url=https://www.aocs.org/stay-informed/inform-magazine/featured-articles/emulsions-making-oil-and-water-mix-april-2014?SSO=True |publisher=www.aocs.org |access-date=1 January 2021}}</ref> Surfactants are compounds that are typically [[Amphiphile|amphiphilic]], meaning they have a polar or [[Hydrophile|hydrophilic]] (i.e., water-soluble) part and a non-polar (i.e., hydrophobic or [[Lipophilicity|lipophilic]]) part. Emulsifiers that are more soluble in water (and, conversely, less soluble in oil) will generally form oil-in-water emulsions, while emulsifiers that are more soluble in oil will form water-in-oil emulsions.<ref>Cassidy, L. (n.d.). Emulsions: Making oil and water mix. Retrieved from https://www.aocs.org/stay-informed/inform-magazine/featured-articles/emulsions-making-oil-and-water-mix-april-2014</ref>

Examples of food emulsifiers are:

* [[Egg yolk]]&nbsp;– in which the main emulsifying and thickening agent is [[lecithin]].
* [[Mustard seed|Mustard]]<ref>{{cite magazine|author=Riva Pomerantz|date=Nov 15, 2017|title=KOSHER IN THE LAB|magazine=[[Ami (magazine)|Ami]]|issue=342}}</ref>&nbsp;– where a variety of chemicals in the [[mucilage]] surrounding the seed hull act as emulsifiers
* [[Soy lecithin]] is another emulsifier and thickener
* [[Pickering emulsion|Pickering stabilization]]&nbsp;– uses particles under certain circumstances
* [[Mono- and diglycerides of fatty acids|Mono- and diglycerides]] – a common emulsifier found in many food products (coffee creamers, ice creams, spreads, breads, cakes)
* [[Sodium stearoyl lactylate]]
* [[DATEM]] (diacetyl tartaric acid esters of mono- and diglycerides)&nbsp;– an emulsifier used primarily in baking 
* [[Protein]]s – those with both hydrophilic and hydrophobic regions, e.g. sodium [[casein]]ate. [[Processed cheese]] use acids to [[chelate]] away calcium, which allows cheese casein to work as an emulsifier.

In food emulsions, the type of emulsifier greatly affects how emulsions are structured in the stomach and how accessible the oil is for gastric [[lipases]], thereby influencing how fast emulsions are digested and trigger a [[satiety]] inducing [[hormone]] response.<ref>{{cite journal |last1=Bertsch |first1=Pascal |last2=Steingoetter |first2=Andreas |last3=Arnold |first3=Myrtha |last4=Scheuble |first4=Nathalie |last5=Bergfreund |first5=Jotam |last6=Fedele |first6=Shahana |last7=Liu |first7=Dian |last8=Parker |first8=Helen L. |last9=Langhans |first9=Wolfgang |last10=Rehfeld |first10=Jens F. |last11=Fischer |first11=Peter |title=Lipid emulsion interfacial design modulates human in vivo digestion and satiation hormone response |journal=Food & Function |date=30 August 2022 |volume=13 |issue=17 |pages=9010–9020 |doi=10.1039/D2FO01247B |pmid=35942900 |pmc=9426722 |language=en |issn=2042-650X}}</ref>

[[Detergent]]s are another class of surfactant, and will interact physically with both [[cooking oil|oil]] and [[water]], thus stabilizing the interface between the oil and water droplets in suspension. This principle is exploited in [[soap]], to remove [[yellow grease|grease]] for the purpose of [[cleaning agent|cleaning]]. Many different emulsifiers are used in [[pharmacy]] to prepare emulsions such as [[cream (pharmaceutical)|creams]] and [[lotion]]s. Common examples include [[emulsifying wax]], [[polysorbate 20]], and [[ceteareth|ceteareth 20]].<ref>{{cite web|url=http://www.teachsoap.com/emulsifywax.html|title=Using Emulsifying Wax|access-date=2008-07-22|author=Anne-Marie Faiola|date=2008-05-21|website=TeachSoap.com}}</ref>

Sometimes the inner phase itself can act as an emulsifier, and the result is a nanoemulsion, where the inner state disperses into "[[nano-]]size" droplets within the outer phase. A well-known example of this phenomenon, the "[[ouzo effect]]", happens when water is poured into a strong alcoholic [[anise]]-based beverage, such as [[ouzo]], [[pastis]], [[absinthe]], [[Arak (distilled beverage)|arak]], or [[Rakı|raki]]. The anisolic compounds, which are soluble in [[ethanol]], then form nano-size droplets and emulsify within the water. The resulting color of the drink is opaque and milky white.

==Mechanisms of emulsification==
A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification:<ref name=":2" />

* Surface tension theory – according to this theory, emulsification takes place by reduction of interfacial tension between two phases
* Repulsion theory – According to this theory, the emulsifier creates a film over one phase that forms globules, which repel each other. This repulsive force causes them to remain suspended in the dispersion medium
* Viscosity modification – emulgents like [[Gum arabic|acacia]] and [[tragacanth]], which are hydrocolloids, as well as PEG ([[polyethylene glycol]]), glycerine, and other polymers like CMC ([[carboxymethyl cellulose]]), all increase the viscosity of the medium, which helps create and maintain the suspension of globules of dispersed phase

==Uses==
===In food===
[[File:Ingredients maonesa.jpg|thumb|right|An example of the ingredients used to make [[mayonnaise]]; [[olive oil]], [[salt|table salt]], an egg (for [[yolk]]) and a [[lemon]] (for lemon juice). The oil and water in the egg yolk do not mix, while the [[lecithin]] in the yolk serves as an emulsifier, allowing the two to be blended together.]]

Oil-in-water emulsions are common in food products:

* [[Mayonnaise]] and [[Hollandaise sauce]]s – these are oil-in-water emulsions stabilized with egg yolk [[lecithin]], or with other types of food additives, such as [[sodium stearoyl lactylate]]
* [[Homogenized milk]] – an emulsion of milk fat in water, with milk proteins as the emulsifier 
* [[Vinaigrette]] – an emulsion of vegetable oil in vinegar, if this is prepared using only oil and vinegar (i.e., without an emulsifier), an unstable emulsion results

Water-in-oil emulsions are less common in food, but still exist:

* [[Butter]] – an emulsion of water in butterfat
* [[Margarine]]

Other foods can be turned into products similar to emulsions, for example [[meat emulsion]] is a suspension of meat in liquid that is similar to true emulsions.

===In health care===
In [[pharmaceutics]], [[Hairstyling product|hairstyling]], [[personal hygiene]], and [[cosmetics]], emulsions are frequently used. These are usually oil and water emulsions but dispersed, and which is continuous depends in many cases on the [[pharmaceutical formulation]]. These emulsions may be called [[cream (pharmaceutical)|cream]]s, [[ointment]]s, [[liniment]]s (balms), [[paste (rheology)|paste]]s, [[Thin film|film]]s, or [[liquid]]s, depending mostly on their oil-to-water ratios, other additives, and their intended [[route of administration]].<ref name="Aulton">{{cite book|editor=Aulton, Michael E.|edition=3rd|title=Aulton's Pharmaceutics: The Design and Manufacture of Medicines|publisher=[[Churchill Livingstone]]|year=2007|isbn=978-0-443-10108-3|pages=92–97, 384, 390–405, 566–69, 573–74, 589–96, 609–10, 611}}</ref><ref name="Remington">{{Cite book|last1=Troy|first1=David A.|last2=Remington|first2=Joseph P.|last3=Beringer|first3=Paul|title=Remington: The Science and Practice of Pharmacy|edition=21st|year=2006|publisher=[[Lippincott Williams & Wilkins]]|location=Philadelphia|isbn=978-0-7817-4673-1|pages=325–336, 886–87}}</ref> The first five are [[topical]] [[dosage form]]s, and may be used on the surface of the [[human skin|skin]], [[transdermal]]ly, [[Eye drop|ophthalmically]], [[rectal]]ly, or [[vagina]]lly. A highly liquid emulsion may also be used [[oral administration|oral]]ly, or may be [[Injection (medicine)|injected]] in some cases.<ref name="Aulton"/>

Microemulsions are used to deliver [[vaccine]]s and kill [[microbe]]s.<ref>{{cite web|url=http://www.nano.med.umich.edu/Platforms/Adjuvant-Vaccine-Development.html|title=Adjuvant Vaccine Development|access-date=2008-07-23|url-status=dead|archive-url=https://web.archive.org/web/20080705134014/http://nano.med.umich.edu/Platforms/Adjuvant-Vaccine-Development.html|archive-date=2008-07-05}}</ref> Typical emulsions used in these techniques are nanoemulsions of [[soybean oil]], with particles that are 400–600&nbsp;nm in diameter.<ref>{{cite web|url=http://www.eurekalert.org/pub_releases/2008-02/uomh-nvs022608.php|title=Nanoemulsion vaccines show increasing promise|access-date=2008-07-22|website=Eurekalert! Public News List|publisher=University of Michigan Health System|date=2008-02-26}}</ref> The process is not chemical, as with other types of [[antimicrobial]] treatments, but mechanical. The smaller the droplet the greater the [[surface tension]] and thus the greater the force required to merge with other [[lipids]]. The oil is emulsified with detergents using a [[high-shear mixer]] to stabilize the emulsion so, when they encounter the lipids in the [[cell membrane]] or envelope of [[Cell envelope|bacteria]] or [[virus]]es, they force the lipids to merge with themselves. On a mass scale, in effect this disintegrates the membrane and kills the pathogen. The soybean oil emulsion does not harm normal human cells, or the cells of most other [[higher organisms]], with the exceptions of [[Spermatozoon|sperm cells]] and [[blood cells]], which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not currently used [[intravenous]]ly (IV). The most effective application of this type of nanoemulsion is for the [[disinfection]] of surfaces. Some types of nanoemulsions have been shown to effectively destroy [[HIV-1]] and [[tuberculosis]] pathogens on non-[[porous]] surfaces.

==== Applications in pharmaceutical industry ====
* '''Oral drug delivery:''' Emulsions may provide an efficient means of administering drugs that are poorly soluble or have low [[bioavailability]] or dissolution rates, increasing both dissolution rates and absorption to increase bioavailability and improve bioavailability. By increasing surface area provided by an emulsion, dissolution rates and absorption rates of drugs are increased, improving their bioavailability.<ref>{{Cite web |last=Sharma |first=Dr Anubhav |date=2023-04-26 |title=Role of Surfactant in Emulsion Stabilization: A Comprehensive Overview |url=https://thewitfire.in/2023/04/26/role-of-surfactant-in-emulsion-stabilization-a-comprehensive-overview/ |access-date=2023-04-27 |website=Witfire |language=en-US}}</ref>
* '''Topical formulations:''' Emulsions are widely utilized as bases for topical drug delivery formulations such as creams, lotions and ointments. Their incorporation allows lipophilic as well as hydrophilic drugs to be mixed together for maximum skin penetration and permeation of active ingredients.<ref>{{Cite journal |last1=Apostolidis |first1=Eftychios |last2=Stoforos |first2=George N. |last3=Mandala |first3=Ioanna |date=April 2023 |title=Starch physical treatment, emulsion formation, stability, and their applications |url=http://dx.doi.org/10.1016/j.carbpol.2023.120554 |journal=Carbohydrate Polymers |volume=305 |pages=120554 |doi=10.1016/j.carbpol.2023.120554 |pmid=36737219 |s2cid=255739614 |issn=0144-8617|url-access=subscription }}</ref>
* '''Parenteral drug delivery:''' Emulsions serve as carriers for intravenous or intramuscular administration of drugs, solubilizing lipophilic ones while protecting from degradation and decreasing injection site irritation. Examples include propofol as a widely used anesthetic and lipid-based solutions used for total parenteral nutrition delivery.<ref>{{Cite journal |last1=Hazt |first1=Bianca |last2=Pereira Parchen |first2=Gabriela |last3=Fernanda Martins do Amaral |first3=Lilian |last4=Rondon Gallina |first4=Patrícia |last5=Martin |first5=Sandra |last6=Hess Gonçalves |first6=Odinei |last7=Alves de Freitas |first7=Rilton |date=April 2023 |title=Unconventional and conventional Pickering emulsions: Perspectives and challenges in skin applications |url=http://dx.doi.org/10.1016/j.ijpharm.2023.122817 |journal=International Journal of Pharmaceutics |volume=636 |pages=122817 |doi=10.1016/j.ijpharm.2023.122817 |pmid=36905974 |s2cid=257474428 |issn=0378-5173|hdl=10198/16535 |hdl-access=free }}</ref>
* '''Ocular Drug Delivery:''' Emulsions can be used to formulate eye drops and other ocular drug delivery systems, increasing drug retention time in the eye and permeating through corneal barriers more easily while providing sustained release of active ingredients and thus increasing therapeutic efficacy.<ref>{{Cite journal |last1=Ding |first1=Jingjing |last2=Li |first2=Yunxing |last3=Wang |first3=Qiubo |last4=Chen |first4=Linqian |last5=Mao |first5=Yi |last6=Mei |first6=Jie |last7=Yang |first7=Cheng |last8=Sun |first8=Yajuan |date=April 2023 |title=Pickering high internal phase emulsions with excellent UV protection property stabilized by Spirulina protein isolate nanoparticles |url=http://dx.doi.org/10.1016/j.foodhyd.2022.108369 |journal=Food Hydrocolloids |volume=137 |pages=108369 |doi=10.1016/j.foodhyd.2022.108369 |s2cid=254218797 |issn=0268-005X|url-access=subscription }}</ref>
* '''Nasal and Pulmonary Drug Delivery:''' Emulsions can be an ideal vehicle for creating nasal sprays and inhalable drug products, enhancing drug absorption through nasal and pulmonary mucosa while providing sustained release with reduced local irritation.<ref>{{Cite journal |last1=Udepurkar |first1=Aniket Pradip |last2=Clasen |first2=Christian |last3=Kuhn |first3=Simon |date=March 2023 |title=Emulsification mechanism in an ultrasonic microreactor: Influence of surface roughness and ultrasound frequency |url=http://dx.doi.org/10.1016/j.ultsonch.2023.106323 |journal=Ultrasonics Sonochemistry |volume=94 |pages=106323 |doi=10.1016/j.ultsonch.2023.106323 |pmid=36774674 |pmc=9945801 |issn=1350-4177}}</ref>
* '''Vaccine Adjuvants:''' Emulsions can serve as vaccine adjuvants by strengthening immune responses against specific antigens. Emulsions can enhance antigen solubility and uptake by immune cells while simultaneously providing controlled release, amplifying an immunological response and thus amplifying its effect.<ref name="Hong 1406–1436">{{Cite journal |last1=Hong |first1=Xin |last2=Zhao |first2=Qiaoli |last3=Liu |first3=Yuanfa |last4=Li |first4=Jinwei |date=2021-08-13 |title=Recent advances on food-grade water-in-oil emulsions: Instability mechanism, fabrication, characterization, application, and research trends |url=http://dx.doi.org/10.1080/10408398.2021.1964063 |journal=Critical Reviews in Food Science and Nutrition |volume=63 |issue=10 |pages=1406–1436 |doi=10.1080/10408398.2021.1964063 |pmid=34387517 |s2cid=236998385 |issn=1040-8398|url-access=subscription }}</ref>
* '''Taste Masking:''' Emulsions can be used to encase bitter or otherwise unpleasant-tasting drugs, masking their taste and increasing patient compliance - particularly with pediatric formulations.<ref name="Hong 1406–1436"/>
* '''Cosmeceuticals:''' Emulsions are widely utilized in cosmeceuticals products that combine cosmetic and pharmaceutical properties. These emulsions act as carriers for active ingredients like vitamins, antioxidants and skin lightening agents to provide improved skin penetration and increased stability.<ref>{{Cite journal |last1=Xu |first1=Tian |last2=Jiang |first2=Chengchen |last3=Huang |first3=Zehao |last4=Gu |first4=Zhengbiao |last5=Cheng |first5=Li |last6=Hong |first6=Yan |date=January 2023 |title=Formation, stability and the application of Pickering emulsions stabilized with OSA starch/chitosan complexes |url=http://dx.doi.org/10.1016/j.carbpol.2022.120149 |journal=Carbohydrate Polymers |volume=299 |pages=120149 |doi=10.1016/j.carbpol.2022.120149 |pmid=36876777 |s2cid=252553332 |issn=0144-8617|url-access=subscription }}</ref>

=== In firefighting ===
Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids ([[Fire classes|class B fire]]s). Such agents encapsulate the fuel in a fuel-water emulsion, thereby trapping the flammable vapors in the water phase. This emulsion is achieved by applying an [[Aqueous solution|aqueous]] surfactant solution to the fuel through a high-pressure nozzle. Emulsifiers are not effective at extinguishing large fires involving bulk/deep liquid fuels, because the amount of emulsifier agent needed for extinguishment is a function of the volume of the fuel, whereas other agents such as [[Fire-fighting foam|aqueous film-forming foam]] <!-- (AFFF) --> need cover only the surface of the fuel to achieve vapor mitigation.<ref>{{cite book|title=Principles of Fire Protection Chemistry and Physics |author=Friedman, Raymond |isbn= 978-0-87765-440-7|year=1998|publisher=[[Jones & Bartlett Learning]]}}</ref>

===Chemical synthesis===
{{Main|Emulsion polymerization}}

Emulsions are used to manufacture polymer dispersions – polymer production in an emulsion 'phase' has a number of process advantages, including prevention of coagulation of product. Products produced by such polymerisations may be used as the emulsions – products including primary components for glues and paints. Synthetic [[latex]]es (rubbers) are also produced by this process.

==See also==
{{Div col|colwidth=18em}}
* {{annotated link|Emulsion dispersion}}
* {{annotated link|Emulsified fuel}}
* {{annotated link|Homogenizer}}
* {{annotated link|Liquid whistle}}
* {{annotated link|Miniemulsion}}
* {{annotated link|Pickering emulsion}}
* {{annotated link|Rheology}}
* {{annotated link|Water-in-water emulsion}}
{{div col end}}

== Citations ==
{{Reflist|30em}}

== General and cited references ==
* {{cite book|author1=Philip Sherman|author2-link=British Society of Rheology|author2=British Society of Rheology|title=Rheology of emulsions: proceedings of a symposium held by the British Society of Rheology ... Harrogate, October 1962|url=https://books.google.com/books?id=UJ0FAQAAIAAJ|year=1963|publisher=Macmillan|isbn=9780080102900}}
* ''Handbook of Nanostructured Materials and Nanotechnology''; Nalwa, H.S., ed.; Academic Press: New York, NY, USA, 2000; Volume 5, pp. 501–575

== External links ==
* {{Wiktionary-inline}}

{{Dosage forms|state=expanded}}
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[[Category:Chemical mixtures]]
[[Category:Colloidal chemistry]]
[[Category:Colloids]]
[[Category:Dosage forms]]
[[Category:Drug delivery devices]]
[[Category:Soft matter]]