This article is about mixtures of liquids. For the light-sensitive mixture used in photography, seePhotographic emulsion.
Two immiscible liquids, not yet emulsified
An emulsion of Phase II dispersed in Phase I
The unstable emulsion progressively separates
Thesurfactant (outline around particles) positions itself on the interfaces between Phase II and Phase I, stabilizing the emulsion
Anemulsion is amixture of two or moreliquids that are normallyimmiscible (unmixable or unblendable) owing to liquid-liquidphase separation. Emulsions are part of a more general class of two-phase systems ofmatter calledcolloids. Although the termscolloid andemulsion are sometimes used interchangeably,emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid (the dispersedphase) isdispersed in the other (the continuous phase). Examples of emulsions includevinaigrettes, homogenizedmilk, liquidbiomolecular condensates, and somecutting fluids formetal 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.[1]
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 bestatistically distributed to produce roughly spherical droplets.
A fluid system in which liquid droplets are dispersed in a liquid.
Note 1: The definition is based on the definition in ref.[2]
Note 2: The droplets may be amorphous, liquid-crystalline, or any mixture thereof.
Note 3: The diameters of the droplets constituting thedispersed phase usually range from approximately 10 nm to 100 μm; i.e., the droplets may exceed the usual size limits forcolloidal particles.
Note 4: An emulsion is termed an oil/water (o/w) emulsion if the dispersed phase is an organic material and thecontinuous phase is water or an aqueous solution and is termed water/oil (w/o) if the dispersed phase is water or an aqueous solution and the continuous phase is an organic liquid (an "oil").
Note 5: A w/o emulsion is sometimes called an inverse emulsion. The term "inverse emulsion" is misleading, suggesting incorrectly that the emulsion has properties that are the opposite of those of an emulsion. Its use is, therefore, not recommended.[3]
The word "emulsion" comes from the Latinemulgere "to milk out", fromex "out" +mulgere "to milk", as milk is an emulsion of fat and water, along with other components, includingcolloidalcasein micelles (a type of secretedbiomolecular condensate).[4]
Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface".[5] Emulsions tend to have a cloudy appearance because the manyphase interfacesscatter light as it passes through the emulsion. Emulsions appearwhite 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 appearbluer – this is called the "Tyndall effect".[6] If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear moreyellow. This phenomenon is easily observable when comparingskimmed milk, which contains little fat, tocream, which contains a much higher concentration of milk fat. One example would be a mixture of water and oil.[7]
Two special classes of emulsions –microemulsions and nanoemulsions, with droplet sizes below 100 nm – appear translucent.[8] 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 thevisible spectrum of light is composed of wavelengths between 390 and 750nanometers (nm), if the droplet sizes in the emulsion are below about 100 nm, the light can penetrate through the emulsion without being scattered.[9] 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 ofsurfactants, co-surfactants, and co-solvents.[8] 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]
Common emulsions are inherently unstable and, thus, do not tend to form spontaneously. Energy input – through shaking, stirring,homogenizing, or exposure to powerultrasound[10] – 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 ofvinaigrette, an unstable emulsion that will quickly separate unless shaken almost continuously. There are important exceptions to this rule – microemulsions arethermodynamically stable, while translucent nanoemulsions arekinetically stable.[8]
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) (seeEmulsifier, below) present.[11]
Emulsion stability refers to the ability of an emulsion to resist change in its properties over time.[12][13] There are four types of instability in emulsions:flocculation,coalescence,creaming/sedimentation, andOstwald 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.[14] 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 ofbuoyancy, or under the influence of thecentripetal force induced when acentrifuge is used.[12] 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.[15] Sedimentation is the opposite phenomenon of creaming and normally observed in water-in-oil emulsions.[5] 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 followsStokes' 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 asuspension, can be studied in terms ofzeta 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.[16] For example, oil-in-water emulsions containingmono- and diglycerides and milk protein as surfactant showed that stable oil droplet size over 28 days storage at 25 °C.[15]
The stability of emulsions can be characterized using techniques such as light scattering, focused beam reflectance measurement, centrifugation, andrheology. Each method has advantages and disadvantages.[17]
The kinetic process of destabilization can be rather long – up to several months, or even years for some products.[18] 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).[19] 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]
Mechanical methods of acceleration, including vibration, centrifugation, and agitation, can also be used.[20]
These methods are almost always empirical, without a sound scientific basis.[citation needed]
Anemulsifier is a substance that stabilizes an emulsion by reducing the oil-waterinterface tension. Emulsifiers are a part of a broader group of compounds known assurfactants, or "surface-active agents".[21] Surfactants are compounds that are typicallyamphiphilic, meaning they have a polar orhydrophilic (i.e., water-soluble) part and a non-polar (i.e., hydrophobic orlipophilic) 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.[22]
Examples of food emulsifiers are:
Egg yolk – in which the main emulsifying and thickening agent islecithin.
Mustard[23] – where a variety of chemicals in themucilage surrounding the seed hull act as emulsifiers
In food emulsions, the type of emulsifier greatly affects how emulsions are structured in the stomach and how accessible the oil is for gastriclipases, thereby influencing how fast emulsions are digested and trigger asatiety inducinghormone response.[24]
Detergents are another class of surfactant, and will interact physically with bothoil andwater, thus stabilizing the interface between the oil and water droplets in suspension. This principle is exploited insoap, to removegrease for the purpose ofcleaning. Many different emulsifiers are used inpharmacy to prepare emulsions such ascreams andlotions. Common examples includeemulsifying wax,polysorbate 20, andceteareth 20.[25]
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 alcoholicanise-based beverage, such asouzo,pastis,absinthe,arak, orraki. The anisolic compounds, which are soluble inethanol, then form nano-size droplets and emulsify within the water. The resulting color of the drink is opaque and milky white.
A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification:[5]
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 likeacacia andtragacanth, 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
An example of the ingredients used to makemayonnaise;olive oil,table salt, an egg (foryolk) and alemon (for lemon juice). The oil and water in the egg yolk do not mix, while thelecithin in the yolk serves as an emulsifier, allowing the two to be blended together.
Oil-in-water emulsions are common in food products:
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:
Other foods can be turned into products similar to emulsions, for examplemeat emulsion is a suspension of meat in liquid that is similar to true emulsions.
Microemulsions are used to delivervaccines and killmicrobes.[28] Typical emulsions used in these techniques are nanoemulsions ofsoybean oil, with particles that are 400–600 nm in diameter.[29] The process is not chemical, as with other types ofantimicrobial treatments, but mechanical. The smaller the droplet the greater thesurface tension and thus the greater the force required to merge with otherlipids. The oil is emulsified with detergents using ahigh-shear mixer to stabilize the emulsion so, when they encounter the lipids in thecell membrane or envelope ofbacteria orviruses, 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 otherhigher organisms, with the exceptions ofsperm cells andblood cells, which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not currently usedintravenously (IV). The most effective application of this type of nanoemulsion is for thedisinfection of surfaces. Some types of nanoemulsions have been shown to effectively destroyHIV-1 andtuberculosis pathogens on non-porous surfaces.
Oral drug delivery: Emulsions may provide an efficient means of administering drugs that are poorly soluble or have lowbioavailability 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.[30]
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.[31]
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.[32]
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.[33]
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.[34]
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.[35]
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.[35]
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.[36]
Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (class B fires). 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 anaqueous 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 asaqueous film-forming foam need cover only the surface of the fuel to achieve vapor mitigation.[37]
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. Syntheticlatexes (rubbers) are also produced by this process.
^Khan, A. Y.; Talegaonkar, S; Iqbal, Z; Ahmed, F. J.; Khar, R. K. (2006). "Multiple emulsions: An overview".Current Drug Delivery.3 (4):429–43.doi:10.2174/156720106778559056.PMID17076645.
^Joseph Price Remington (1990). Alfonso R. Gennaro (ed.).Remington's Pharmaceutical Sciences. Mack Publishing Company (Original from Northwestern University) (Digitized 2010). p. 281.ISBN9780912734040.
^Kentish, S.; Wooster, T.J.; Ashokkumar, M.; Balachandran, S.; Mawson, R.; Simons, L. (2008). "The use of ultrasonics for nanoemulsion preparation".Innovative Food Science & Emerging Technologies.9 (2):170–175.doi:10.1016/j.ifset.2007.07.005.hdl:11343/55431.
^Silvestre, M.P.C.; Decker, E.A.; McClements, D.J. (1999). "Influence of copper on the stability of whey protein stabilized emulsions".Food Hydrocolloids.13 (5): 419.doi:10.1016/S0268-005X(99)00027-2.
^abLoi, Chia Chun; Eyres, Graham T.; Birch, E. John (2019). "Effect of mono- and diglycerides on physical properties and stability of a protein-stabilised oil-in-water emulsion".Journal of Food Engineering.240:56–64.doi:10.1016/j.jfoodeng.2018.07.016.ISSN0260-8774.S2CID106021441.
^Dowding, Peter J.; Goodwin, James W.; Vincent, Brian (2001-11-30). "Factors governing emulsion droplet and solid particle size measurements performed using the focused beam reflectance technique".Colloids and Surfaces A: Physicochemical and Engineering Aspects.192 (1):5–13.doi:10.1016/S0927-7757(01)00711-7.ISSN0927-7757.
^abAulton, Michael E., ed. (2007).Aulton's Pharmaceutics: The Design and Manufacture of Medicines (3rd ed.).Churchill Livingstone. pp. 92–97, 384,390–405,566–69,573–74,589–96,609–10, 611.ISBN978-0-443-10108-3.
^Troy, David A.; Remington, Joseph P.; Beringer, Paul (2006).Remington: The Science and Practice of Pharmacy (21st ed.). Philadelphia:Lippincott Williams & Wilkins. pp. 325–336,886–87.ISBN978-0-7817-4673-1.
-solution.jpg)
