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An emulsion is a mixture of two or more liquids that are normally
immiscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...
(unmixable or unblendable) owing to liquid-liquid
phase separation Phase separation is the creation of two distinct phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. Colloids are formed by phase separation, though no ...
. Emulsions are part of a more general class of two-phase systems of
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic parti ...
called
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
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) is dispersed in the other (the continuous phase). Examples of emulsions include vinaigrettes, homogenized milk, liquid
biomolecular condensate In biochemistry, biomolecular condensates are a class of membrane-less organelles and organelle subdomains, which carry out specialized functions within the cell. Unlike many organelles, biomolecular condensate composition is not controlled by ...
s, and some
cutting fluid Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), ...
s for
metal working Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale ...
. 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. 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 statistically distributed to produce roughly spherical droplets. The term "emulsion" is also used to refer to the photo-sensitive side of
photographic film Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine t ...
. Such a
photographic emulsion Photographic emulsion is a light-sensitive colloid used in film-based photography. Most commonly, in silver-gelatin photography, it consists of silver halide crystals dispersed in gelatin. The emulsion is usually coated onto a substrate of glas ...
consists of silver halide colloidal particles dispersed in a
gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
matrix. Nuclear emulsions are similar to photographic emulsions, except that they are used in particle physics to detect high-energy elementary particles.


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 A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
al
casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins ( αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in human ...
micelles (a type of secreted
biomolecular condensate In biochemistry, biomolecular condensates are a class of membrane-less organelles and organelle subdomains, which carry out specialized functions within the cell. Unlike many organelles, biomolecular condensate composition is not controlled by ...
).


Appearance and properties

Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface". Emulsions tend to have a cloudy appearance because the many phase interfaces 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 (low-wavelength) light will be scattered more, and the emulsion will appear
blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ob ...
r – this is called the "
Tyndall effect The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the ...
". 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 Skimmed milk (British English), or skim milk (American English), is made when all the milkfat is removed from whole milk. It tends to contain around 0.1% fat. Background Historically, skimmed milk was used for fattening pigs, and was recommended ...
, which contains little fat, to
cream Cream is a dairy product composed of the higher-fat layer skimmed from the top of milk before homogenization. In un-homogenized milk, the fat, which is less dense, eventually rises to the top. In the industrial production of cream, this process ...
, which contains a much higher concentration of milk fat. One example would be a mixture of water and oil. Two special classes of emulsions –
microemulsion Microemulsions are clear, thermodynamically stable isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the "oil" may actually be ...
s and nanoemulsions, with droplet sizes below 100 nm – appear translucent. 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 nanometers (nm), if the droplet sizes in the emulsion are below about 100 nm, the light can penetrate through the emulsion without being scattered. 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 surfactants, co-surfactants, and co- solvents. 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. Common emulsions are inherently unstable and, thus, do not tend to form spontaneously. Energy input – through shaking, stirring,
homogenizing Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, size, ...
, or exposure to power ultrasound – 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, an unstable emulsion that will quickly separate unless shaken almost continuously. There are important exceptions to this rule – microemulsions are thermodynamically stable, while translucent nanoemulsions are kinetically stable. 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.


Instability

Emulsion stability refers to the ability of an emulsion to resist change in its properties over time. There are four types of instability in emulsions:
flocculation Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from pr ...
,
coalescence Coalescence may refer to: * Coalescence (chemistry), the process by which two or more separate masses of miscible substances seem to "pull" each other together should they make the slightest contact * Coalescence (computer science), the merging o ...
, creaming/
sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
, 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. 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 Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
, or under the influence of the
centripetal force A centripetal force (from Latin ''centrum'', "center" and ''petere'', "to seek") is a force that makes a body follow a curved path. Its direction is always orthogonal to the motion of the body and towards the fixed point of the instantaneous ...
induced when a
centrifuge A centrifuge is a device that uses centrifugal force to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities (e.g. cream from milk) or l ...
is used. Creaming is a common phenomenon in dairy and non-dairy beverages (i.e. milk, coffee milk,
almond milk Almond milk is a plant-based milk with a watery texture and nutty flavor manufactured from almonds, although some types or brands are flavored in imitation of cow's milk. It does not contain cholesterol or lactose and is low in saturated fat. Alm ...
, soy milk) and usually does not change the droplet size. Sedimentation is the opposite phenomenon of creaming and normally observed in water-in-oil emulsions. 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, can be studied in terms of
zeta potential Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...
, 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. For example, oil-in-water emulsions containing mono- and diglycerides and milk protein as surfactant showed that stable oil droplet size over 28 days storage at 25°C.


Monitoring physical stability

The stability of emulsions can be characterized using techniques such as light scattering, focused beam reflectance measurement, centrifugation, and
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
. Each method has advantages and disadvantages.


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. 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). 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. Mechanical methods of acceleration, including vibration, centrifugation, and agitation, can also be used. These methods are almost always empirical, without a sound scientific basis.


Emulsifiers

An emulsifier is a substance that stabilizes an emulsion by reducing the oil-water
interface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders ...
. Emulsifiers are a part of a broader group of compounds known as surfactants, or "surface-active agents". Surfactants are compounds that are typically
amphiphilic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
, meaning they have a polar or
hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are no ...
(i.e. water-soluble) part and a non-polar (i.e. hydrophobic or 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. Examples of food emulsifiers are: * Egg yolk – in which the main emulsifying and thickening agent is
lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...
. * Mustard – where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers *
Soy lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...
is another emulsifier and thickener * Pickering stabilization – uses particles under certain circumstances *
Sodium phosphates Sodium phosphate is a generic term for a variety of salts of sodium (Na+) and phosphate (PO43−). Phosphate also forms families or condensed anions including di-, tri-, tetra-, and polyphosphates. Most of these salts are known in both anhydrou ...
– not directly an emulsifier, but modifies behavior of other molecules, e.g.
casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins ( αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in human ...
* Mono- and diglycerides – a common emulsifier found in many food products (coffee creamers, ice creams, spreads, breads, cakes) * Sodium stearoyl lactylate *
DATEM DATEM (diacetyl tartaric acid ester of mono- and diglycerides, also E472e) is an emulsifier primarily used in baking to strengthen the gluten network in dough. It is added to crusty breads, such as rye, to impart a springy, chewy texture. It is ...
(diacetyl tartaric acid esters of mono- and diglycerides) – an emulsifier used primarily in baking * Proteins – those with both hydrophilic and hydrophobic regions, e.g. sodium caseinate, as in meltable
cheese product Processed cheese (also known as process cheese, cheese food, prepared cheese, cheese product, or plastic cheese) is a food product made from cheese and unfermented dairy ingredients mixed with emulsifiers. Additional ingredients, such as vegeta ...
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 Satiety ( ) is a state or condition of fullness gratified beyond the point of satisfaction, the opposite of hunger. It is a state which induces meal termination.Hetherington, M.Sensory-specific satiety and its importance in meal termination ''Neur ...
inducing hormone response.
Detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are mor ...
s are another class of surfactant, and will interact physically with both
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and water, thus stabilizing the interface between the oil and water droplets in suspension. This principle is exploited in soap, to remove grease for the purpose of
cleaning Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for aesthetic, hygienic, functional, environmental, or safety purposes. ...
. Many different emulsifiers are used in pharmacy to prepare emulsions such as
creams Cream is a dairy product composed of the higher-fat layer skimmed from the top of milk before homogenization. In un-homogenized milk, the fat, which is less dense, eventually rises to the top. In the industrial production of cream, this process ...
and
lotion Lotion is a low-viscosity topical preparation intended for application to the skin. By contrast, creams and gels have higher viscosity, typically due to lower water content. Lotions are applied to external skin with bare hands, a brush, a clean ...
s. Common examples include
emulsifying wax Emulsifying wax is a cosmetic emulsifying ingredient. The ingredient name is often followed by the initials NF, indicating that it conforms to the specifications of the National Formulary. Emulsifying wax is created when a wax material (either a ve ...
,
polysorbate 20 Polysorbate 20 (common commercial brand names include Kolliphor PS 20, Scattics, Alkest TW 20, Tween 20, and Kotilen-20) is a polysorbate-type nonionic surfactant formed by the ethoxylation of sorbitan monolaurate. Its stability and relative no ...
, and ceteareth 20. Sometimes the inner phase itself can act as an emulsifier, and the result is a nanoemulsion, where the inner state disperses into "
nano- Nano (symbol n) is a unit prefix meaning "one billionth". Used primarily with the metric system, this prefix denotes a factor of 10−9 or . It is frequently encountered in science and electronics for prefixing units of time and length. ...
size" droplets within the outer phase. A well-known example of this phenomenon, the "
ouzo effect The ouzo effect (also louche effect and spontaneous emulsification) is a milky ('' louche'') oil-in-water emulsion that is formed when water is added to ouzo and other anise-flavored liqueurs and spirits, such as pastis, rakı, arak, sambuca a ...
", happens when water is poured into a strong alcoholic
anise Anise (; '), also called aniseed or rarely anix is a flowering plant in the family Apiaceae native to Eurasia. The flavor and aroma of its seeds have similarities with some other spices and herbs, such as star anise, fennel, licorice, and t ...
-based beverage, such as
ouzo Ouzo ( el, ούζο, ) is a dry anise-flavored aperitif that is widely consumed in Greece. It is made from rectified spirits that have undergone a process of distillation and flavoring. Its taste is similar to other anise liquors like pastis, s ...
,
pastis Pastis (; oc, Pastís, ; or ) is an anise-flavoured spirit and apéritif traditionally from France, typically containing less than 100 g/L sugar and 40–45% ABV (alcohol by volume). Origins Pastis was first commercialized by Paul Ricard i ...
,
absinthe Absinthe (, ) is an anise-flavoured spirit derived from several plants, including the flowers and leaves of ''Artemisia absinthium'' ("grand wormwood"), together with green anise, sweet fennel, and other medicinal and culinary herbs. Historical ...
, arak, or 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: * 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
acacia ''Acacia'', commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia. The genus nam ...
and
tragacanth Tragacanth is a natural gum obtained from the dried sap of several species of Middle Eastern legumes of the genus ''Astragalus'', including '' A. adscendens'', '' A. gummifer'', '' A. brachycalyx'', and '' A. tragacantha''. Some of these species ...
, which are hydrocolloids, as well as PEG (
polyethylene glycol Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular we ...
), glycerine, and other polymers like CMC (
carboxymethyl cellulose Carboxymethyl cellulose (CMC) or cellulose gum is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodiu ...
), all increase the viscosity of the medium, which helps create and maintain the suspension of globules of dispersed phase


Uses


In food

Oil-in-water emulsions are common in food products: *
Mayonnaise Mayonnaise (; ), colloquially referred to as "mayo" , is a thick, cold, and creamy sauce or dressing commonly used on sandwiches, hamburgers, composed salads, and French fries. It also forms the base for various other sauces, such as tartar s ...
and
Hollandaise sauce Hollandaise sauce ( or ; ), also called Dutch sauce, is a mixture of egg yolk, melted butter, and lemon juice (or a white wine or vinegar reduction). It is usually seasoned with salt, and either white pepper or cayenne pepper. It is well kn ...
s – these are oil-in-water emulsions stabilized with egg yolk
lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...
, 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 Butter is a dairy product made from the fat and protein components of churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 80% butterfat. It is used at room temperature as a spread, melted as a condiment ...
– an emulsion of water in butterfat *
Margarine Margarine (, also , ) is a spread used for flavoring, baking, and cooking. It is most often used as a substitute for butter. Although originally made from animal fats, most margarine consumed today is made from vegetable oil. The spread was or ...
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 A hairstyle, hairdo, haircut or coiffure refers to the styling of hair, usually on the human scalp. Sometimes, this could also mean an editing of facial or body hair. The fashioning of hair can be considered an aspect of personal grooming ...
,
personal hygiene Hygiene is a series of practices performed to preserve health. According to the World Health Organization (WHO), "Hygiene refers to conditions and practices that help to maintain health and prevent the spread of diseases." Personal hygiene refer ...
, and
cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protect ...
, 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 Pharmaceutical formulation, in pharmaceutics, is the process in which different chemical substances, including the active drug, are combined to produce a final medicinal product. The word ''formulation'' is often used in a way that includes dosage ...
. These emulsions may be called
cream Cream is a dairy product composed of the higher-fat layer skimmed from the top of milk before homogenization. In un-homogenized milk, the fat, which is less dense, eventually rises to the top. In the industrial production of cream, this process ...
s,
ointment A topical medication is a medication that is applied to a particular place on or in the body. Most often topical medication means application to body surfaces such as the skin or mucous membranes to treat ailments via a large range of classes ...
s,
liniment Liniment (from la, linere, meaning "to anoint"), also called embrocation and heat rub, is a medicated topical preparation for application to the skin. Some liniments have viscosity similar to that of water; others are lotion or balm; still other ...
s (balms), pastes, films, or liquids, depending mostly on their oil-to-water ratios, other additives, and their intended
route of administration A route of administration in pharmacology and toxicology is the way by which a drug, fluid, poison, or other substance is taken into the body. Routes of administration are generally classified by the location at which the substance is applied. ...
. The first 5 are topical
dosage form Dosage forms (also called unit doses) are pharmaceutical drug products in the form in which they are marketed for use, with a specific mixture of active ingredients and inactive components (excipients), in a particular configuration (such as a cap ...
s, and may be used on the surface of the skin,
transdermal Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution. Examples include transdermal patches used for medicine delivery. The drug is administered in the form of a patch or ointm ...
ly, ophthalmically,
rectal The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the le ...
ly, or vaginally. A highly liquid emulsion may also be used
oral The word oral may refer to: Relating to the mouth * Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid ** Oral administration of medicines ** Oral examination (also known as an oral exam or or ...
ly, or may be injected in some cases. Microemulsions are used to deliver vaccines and kill
microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s. Typical emulsions used in these techniques are nanoemulsions of soybean oil, with particles that are 400–600 nm in diameter. The process is not chemical, as with other types of
antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals a ...
treatments, but mechanical. The smaller the droplet the greater the
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to ...
and thus the greater the force required to merge with other lipids. The oil is emulsified with detergents using a
high-shear mixer A high-shear mixer disperses, or transports, one phase or ingredient (liquid, solid, gas) into a main continuous phase (liquid), with which it would normally be immiscible. A rotor or impeller, together with a stationary component known as a stato ...
to stabilize the emulsion so, when they encounter the lipids in the cell membrane or envelope of bacteria or viruses, 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 The evolution of biological complexity is one important outcome of the process of evolution. Evolution has produced some remarkably complex organisms – although the actual level of complexity is very hard to define or measure accurately in biolog ...
, with the exceptions of
sperm cells A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, ...
and
blood cells A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), ...
, which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not currently used intravenously (IV). The most effective application of this type of nanoemulsion is for the
disinfection A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than s ...
of surfaces. Some types of nanoemulsions have been shown to effectively destroy
HIV-1 The subtypes of HIV include two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in the sooty mangabey ...
and tuberculosis pathogens on non-
porous Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
surfaces.


In firefighting

Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (
class B fire In fire classes, a Class B fire is a fire in flammable liquids or flammable gases, petroleum greases, tars, oils, oil-based paints, solvents, lacquers, or alcohols. For example, propane, natural gas, gasoline and kerosene fires are types o ...
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 An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
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 aqueous film-forming foam need cover only the surface of the fuel to achieve vapor mitigation.


Chemical synthesis

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 latexes (rubbers) are also produced by this process.


See also

*
Emulsion dispersion An emulsion dispersion is thermoplastics or elastomers suspended in a liquid state by means of emulsifiers. Preparation Emulsions are thermodynamically unstable liquid/liquid dispersions that are stabilized. Emulsion dispersion is not about reac ...
* Emulsified fuel *
Homogenizer A homogenizer is a piece of laboratory or industrial equipment used for the homogenization of various types of material, such as tissue, plant, food, soil, and many others. Many different models have been developed using various physical technologi ...
*
Liquid whistle A liquid whistle is a static mixer which passes fluid at high pressure through an orifice and over a blade. This subjects the fluid to high turbulence and may result in mixing or emulsification An emulsion is a mixture of two or more liquids th ...
*
Miniemulsion A miniemulsion (also known as nanoemulsion) is a special type of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases (for example, oil and water), one or more surfactants and, possibly, one or more co ...
*
Pickering emulsion A Pickering emulsion is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the water and oil phases. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, b ...
*
Rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
*
Water-in-water emulsion Water-in-water (W/W) emulsion is a system that consists of droplets of water-solvated molecules in another continuous aqueous solution; both the droplet and continuous phases contain different molecules that are entirely water-soluble. As such, whe ...


References


Other sources

* * ''Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H.S., Ed.; Academic Press: New York, NY, USA, 2000; Volume 5, pp. 501–575'' {{Authority control Chemical mixtures Colloidal chemistry Colloids Dosage forms Drug delivery devices Soft matter