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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
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.
==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==▼
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▲==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 – 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>
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===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 °C.<ref name=":1" />
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===Accelerating methods for shelf life prediction===
The kinetic process of destabilization can be rather long – up to several months, or even years for some products.<ref>{{Cite book |last=
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>
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* [[Soy lecithin]] is another emulsifier and thickener
* [[Pickering emulsion|Pickering stabilization]] – 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]]
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* Surface tension theory – according to this theory, emulsification takes place by reduction of interfacial tension between two phases
* Repulsion theory –
* 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
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* '''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}}</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}}</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>
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==See also==
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