An
emulsion (IPA: /ɪˈmʌlʃən/) is a mixture of two or more immiscible (unblendable) liquids. Emulsions are part of a more general class of two-phase systems of matter called
colloidA colloid is a type of chemical mixture in which one substance is dispersed evenly throughout another.The particles of the dispersed substance are only suspended in the mixture, unlike in a solution, in which they are completely dissolved...
s. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the dispersed and the continuous phase are
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
.In an emulsion, one liquid (the dispersed
phaseIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
) is
dispersedDispersion may refer to:In physics:*Dispersion *Dispersion *Dielectric dispersion*Dispersive mass transfer, in fluid dynamics, the spreading of mass from areas of high to low concentration**Atmospheric pollution dispersion...
in the other (the continuous
phaseIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
).
Examples of emulsions include
vinaigrettesVinaigrette is a mixture of vinegar and oil, often flavored with herbs, spices, and other ingredients...
, the photo-sensitive side of
photographic filmPhotographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
,
milkMilk is an opaque white liquid produced by the mammary glands of mammals. It provides the primary source of nutrition for young mammals before they are able to digest other types of food. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce...
and
cutting fluidCutting fluids are various fluids that are used in machining to cool and lubricate the cutting tool. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, and mists. They may be made from petroleum distillates, animal fats, plant oils, or other raw...
for metal working.
Appearance and Properties
Emulsions tend to have a cloudy appearance, because the many phase interfaces (the boundary between the phases is called the interface)
scatterScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
light that passes through the emulsion. Emulsions are unstable and thus do not form spontaneously. Energy input through shaking, stirring,
homogenizingHomogenization , or homogenisation, is any of several processes used to make a chemical mixture the same throughout.-Definition:...
, or spray processes are needed to form an emulsion. Over time, emulsions tend to revert to the stable state of the phases comprising the emulsion. Surface active substances (
surfactantSurfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.- Etymology :The term surfactant is a blend of surface active agent...
s) can increase the kinetic stability of emulsions greatly so that, once formed, the emulsion does not change significantly over years of storage.
VinaigretteVinaigrette is a mixture of vinegar and oil, often flavored with herbs, spices, and other ingredients...
is an example of an unstable emulsion that will quickly separate unless shaken continuously. This phenomenon is called
coalescenceCoalescence is the process by which two or more droplets or particles merge during contact to form a single daughter droplet . It can take place in many processes, ranging from meteorology to astrophysics. For example, it is both involved in the formation of raindrops as well as planetary and star...
, and happens when small droplets recombine to form bigger ones. Emulsions can also suffer from
creamingCreaming, in the laboratory sense, is the migration of a substance in an emulsion, under the influence of buoyancy, to the top of a sample while the particles of the substance remain separated, as compared to flocculation or breaking...
, the migration of one of the substances to the top of the emulsion under the influence of
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
or
centripetal forceCentripetal force is a force that makes a body follow a curved path; it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path...
when a
centrifugeA centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
is used.
There are three types of emulsion instability:
flocculationFlocculation is, in the field of chemistry, a process where colloids come out of suspension in the form of floc or flakes. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution.-Term...
, where the particles form clumps;
creamingCreaming may refer to:In cooking and baking*Cooking in cream*Creaming , the process of emulsifying butter and sugar in bakingIn chemistry* Creaming , a process of separationOther uses:...
, where the particles concentrate towards the surface (or bottom, depending on the relative density of the two phases) of the mixture while staying separated; and breaking and
coalescenceCoalescence may refer to:* Coalescence , the merging of genetic lineages backwards time to a most recent common ancestor* Coalescence , the merging of two or more words into one...
where the particles coalesce and form a layer of liquid.
Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the
Bancroft ruleThe Bancroft rule states: "The phase in which an emulsifier is more soluble constitutes the continuous phase."It was named after Wilder Dwight Bancroft, an American physical chemist....
applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well; for example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water).
The basic color of emulsions is
white White is a color, the perception which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
. If the emulsion is dilute, the
Tyndall effectThe Tyndall effect is an effect of light scattering by colloidal particles or particles in suspension. It is named after the 19th century Irish scientist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so...
will scatter the light and distort the color to
blueBlue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
; if it is concentrated, the color will be distorted towards
yellowYellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–580 nm is yellow, as is light with a suitable mixture of somewhat longer and shorter wavelengths...
. This phenomenon is easily observable on comparing skimmed milk (with no or little fat) to
creamCream is a dairy product that is composed of the higher-butterfat layer skimmed from the top of milk before homogenization. In un-homogenized milk, over time, the lighter fat rises to the top. In the industrial production of cream this process is accelerated by using centrifuges called "separators"...
(high concentration of milk fat).
MicroemulsionMicroemulsions are clear, stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins...
s and nanoemulsions tend to appear clear due to the small size of the disperse phase.
Emulsifier
An emulsifier (also known as an emulgent) is a substance which stabilizes an emulsion, frequently a
surfactantSurfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.- Etymology :The term surfactant is a blend of surface active agent...
. Examples of food emulsifiers are
egg yolkAn egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...
(where the main emulsifying chemical is
lecithinLecithin is any of a group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids...
), honey, and
mustardMustard seeds are the smallest seeds of the various mustard plants. The seeds are about 3 mm in diameter, and may be colored from yellowish white to black. They are important spices in many regional cuisines. The seeds can come from three different plants: black mustard , brown Indian mustard...
, where a variety of chemicals in the
mucilageMucilage is a polar glycoprotein; an exopolysaccharide; a polymer produced by most plants and some microorganisms.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as tannins and...
surrounding the seed hull act as emulsifiers;
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s and low-molecular weight emulsifiers are common as well. Soy lecithin is another emulsifier and thickener. In some cases, particles can stabilize emulsions as well through a mechanism called
Pickering stabilizationA Pickering emulsion is an emulsion that is stabilized by solid particles which adsorb onto the interface between the two phases. This type of emulsion was named after its discoverer, S.U...
. Both
mayonnaiseMayonnaise is a thick condiment. White or yellowish-white in color, it is a stable emulsion of oil, salt and vinegar, or lemon juice which uses egg yolks as an emulsifier...
and
Hollandaise sauceHollandaise sauce is an emulsion of butter and lemon juice or vinegar using egg yolks as the emulsifying agent, usually seasoned with salt and a little black pepper or cayenne pepper. It is a French sauce, so named because it was believed to have mimicked a Dutch sauce. Hollandaise sauce is well...
are oil-in-water emulsions that are stabilized with egg yolk
lecithinLecithin is any of a group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids...
.
DetergentA detergent is a material intended to assist cleaning. The term is sometimes used to differentiate between soap and other surfactants used for cleaning...
s are another class of surfactant, and will physically interact with both
oilCooking oil is purified fat of plant origin, which is liquid at room temperature.Some of the many different kinds of edible vegetable oils include: olive oil, palm oil, soybean oil, canola oil, pumpkin seed oil, corn oil, sunflower oil, safflower oil, peanut oil, grape seed oil, sesame oil, argan...
and
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, thus stabilizing the interface between oil or water droplets in suspension. This principle is exploited in
soapSoap is an anionic surfactant used in conjunction with water for washing and cleaning, which historically comes either in solid bars or in the form of a viscous liquid....
to remove
greaseYellow grease is a term from the rendering industry. Yellow grease is distinct from brown grease, as yellow grease is typically used-frying oils from deep fryers, whereas brown grease is sourced from grease interceptors...
for the purpose of cleaning. A wide variety of emulsifiers are used in
pharmacyPharmacy is the health profession that links the health sciences with the chemical sciences, and it is charged with ensuring the safe and effective use of medication....
to prepare emulsions such as
creams- Description:A cream is a topical preparation usually for application to the skin. Creams for application to mucus membranes such as those of the rectum or vagina are also used...
and
lotionA lotion is a low- to medium-viscosity, topical preparation intended for application to unbroken skin; creams and gels have a higher viscosity. Most lotions are oil-in-water emulsions using a substance such as Cetearyl alcohol to keep the emulsion together, but water-in-oil lotions are also...
s. Common examples include
emulsifying waxEmulsifying 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....
,
cetearyl alcoholCetostearyl alcohol, cetearyl alcohol or cetylstearyl alcohol is a mixture of fatty alcohols, consisting predominantly of cetyl and stearyl alcohols and is classified as a fatty alcohol. It is used as an emulsion stabilizer, opacifying agent, and foam boosting surfactant, as well as an aqueous and...
, polysorbate 20, and
ceteareth 20The INCI names Ceteareth-n refer to polyoxyethylene ethers of a mixture of high molecular mass saturated fatty alcohols...
.
Sometimes the inner phase itself can act as an emulsifier, and the result is
nanoemulsion - the inner state disperses into nano-size droplets within the outer phase. A well-known example of this phenomenon, the
ouzo effectThe ouzo effect is a phenomenon seen when water is added to ouzo and other anise-flavored liqueurs and spirits : a cloudy oil-in-water microemulsion forms...
, happens when
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
is poured in a strong alcoholic
aniseis a flowering plant in the family Apiaceae native to the eastern Mediterranean region and southwest Asia known for its flavor that resembles liquorice, fennel, and tarragon.- Biology :Anise is an herbaceous annual plant growing to tall...
-based beverage, such as
ouzoOuzo is an anise-flavored spirit that is widely consumed in Greece and Cyprus. Its taste is quite similar to arak/raki in the Levant and Turkey, pastis or absinthe , Sambuca , mastika . It can be consumed neat or mixed with water....
,
pastisPastis is an anise-flavored liqueur and apéritif from France, typically containing 40–45% alcohol by volume, although alcohol-free varieties exist.-Origins:...
,
arakArak or araq is a clear, colourless, unsweetened aniseed-flavoured distilled alcoholic drink, produced and consumed in the Eastern Mediterranean and Northern African countries, Lebanon,Iran, Syria, Israel, the Palestinian Territories, Iraq, Jordan, and Egypt. The word comes from Arabic ˤaraq عرق...
or
rakiRaki may refer to:Alcoholic Drinks:*Raki , an anise-flavored spirit popular in Turkey.*Rakia, an alcoholic beverage popular throughout the Balkans.*An alternate name for the Cretan beverage Tsikoudia.Places:...
. The anisolic compounds, which are soluble in
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
, now form nano-sized droplets and emulgate within the water. The colour of such diluted drink is opaque and milky.
In medicine
In
pharmaceuticsPharmaceutics is the discipline of pharmacy that deals with all facets of the process of turning a new chemical entity into a medication able to be safely and effectively used by patients in the community. Pharmaceutics is the science of dosage form design...
,
hairstylingHairstyling products are used to change the texture or shape of the hair, or to hold it in place in a certain style. Applied properly, most styling products will not damage the hair apart from drying it out; most styling products contain alcohols, which can dissolve oils...
, personal hygiene and
cosmeticsCosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, permanent waves, colored contact lenses, hair colors, [hair sprays] and gels,...
, emulsions are frequently used. These are usually oil and water emulsions, but which is dispersed and which is continuous depends on the
pharmaceutical formulationPharmaceutical formulation, in pharmaceutics, is the process in which different chemical substances, including the active drug, are combined to produce a final medicinal product.-Stages and timeline:...
. These emulsions may be called
cream- Description:A cream is a topical preparation usually for application to the skin. Creams for application to mucus membranes such as those of the rectum or vagina are also used...
s,
ointmentAn ointment is a viscous semisolid preparation used topically on a variety of body surfaces. These include the skin and the mucus membranes of the eye , vagina, anus, and nose. An ointment may or may not be medicated.- Description :...
s,
linimentLiniment, from the Latin linere, to anoint, is a medicated topical preparation for application to the skin. Preparations of this type are also called balm...
s (balms),
pasteIn physics, a paste is a substance that behaves as a solid until a sufficiently large load or stress is applied, at which point it flows like a fluid. In rheological terms, a paste is an example of a Bingham plastic fluid....
s,
filmThin films are thin material layers ranging from fractions of a nanometre to several micrometres in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
s or
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
s, depending mostly on their oil and water proportions and their
route of administrationA route of administration in pharmacology and toxicology is the path by which a drug, fluid, poison, or other substance is brought into contact with the body....
. The first 4 are
topicalIn medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears....
dosage formA dosage form is the physical form of a dose of a chemical compound used as a drug or medication intended for administration or consumption...
s, and may be used on the surface of the
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
,
transdermalTransdermal means "through the skin". In common use it may refer to:* Transdermal patch, used for medicine delivery* Transdermal implant, used for medical or aesthetic purposes...
ly,
ophthalmicOphthalmic can refer to:* Ophthalmology* Ophthalmic nerve* Ophthalmic artery* Ophthalmic veins...
ally, rectally or
vaginaThe vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
lly. A very liquidy emulsion may also be used
oralThe word oral may refer to:As an adjective:* the mouth.* speech communication as opposed to writing.* oral schools use oralism to teach the deaf....
ly, or it may be
injectedAn injection is an infusion method of putting fluid into the body, usually with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body...
using various routes (typically intravenously or intramuscularly). Popular medicated emulsions include calamine lotion,
cod liver oilCod liver oil is a nutritional supplement derived from liver of cod fish. It has high levels of the omega-3 fatty acids, EPA and DHA, and very high levels of vitamin A and vitamin D. It is widely taken to ease the symptoms of arthritis as well as other health benefits...
,
PolysporinPolysporin is an antibiotic ointment produced by Johnson & Johnson used in the prevention of infection and speeding the healing of wounds. The original formulation contains bacitracin and polymyxin B....
,
cortisolCortisol is a corticosteroid hormone or glucocorticoid produced by the adrenal cortex, that is part of the adrenal gland . It is usually referred to as the "stress hormone" as it is involved in response to stress and anxiety, controlled by CRH...
cream, Canesten and Fleet.
Microemulsions are used to deliver
vaccineA vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains a small amount of an agent that resembles a microorganism...
s and kill microbes. Typically, the emulsions used in these techniques are nanoemulsions of
soybean oilIn processing soybeans for oil extraction and subsequent soy flour production, selection of high quality, sound, clean, dehulled yellow soybeans is very important. Soybeans having a dark colored seed coat, or even beans with a dark hilum will inadvertently leave dark specks in the flour, and are...
, with particles that are 400-600 nm in diameter. The process is not chemical, as with other types of
antimicrobialAn antimicrobial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans, as well as destroying viruses. Antimicrobial drugs either kill microbes or prevent the growth of microbes...
treatments, but physical. The smaller the droplet, the greater the
surface tensionSurface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid .Applying Newtonian physics to the forces that arise due to surface tension accurately predicts many liquid behaviors...
and thus the greater the force to merge with other lipids. The oil is emulsified using a
high shear mixerA high shear mixer disperses, or transports, one phase or ingredient into a main continuous phase , with which it would normally be immiscible...
with detergents to stabilize the emulsion, so when they encounter the lipids in the
membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
or envelope of
bacteriaThe cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics....
or
virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es, they force the lipids to merge with themselves. On a mass scale, this effectively disintegrates the membrane and kills the pathogen. This soybean oil emulsion does not harm normal human cells nor the cells of most other higher organisms. The exceptions are
sperm cellsA sperm, from the ancient Greek word σπέρμα and and more commonly known as a sperm cell, is the haploid cell that is the male gamete. It joins an ovum to form a zygote. A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.Sperm cells contribute...
and blood cells, which are vulnerable to nanoemulsions due to their membrane structures. For this reason, these nanoemulsions are not currently used intravenously. The most effective application of this type of nanoemulsion is for the
disinfectionDisinfectants are antimicrobial agents that are applied to non-living objects to destroy microorganisms, the process of which is known as disinfection....
of
surfaceIn mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R
3 — for example, the surface of a ball...
s. Some types of nanoemulsions have been shown to effectively destroy HIV-1 and various
tuberculosisTuberculosis is a common and often deadly infectious disease caused by mycobacteria...
pathogens, for example, on non-porous surfaces.
In fire fighting
Emulsifying agents are effective at extinguishing fires on small thin layer spills of flammable liquids (Class B fires). Extinguishment is achieved by encapsulating the fuel in a fuel-water emulsion thereby trapping the flammable vapors in the water phase. This emulsion is achieved by applying an aqueous surfactant solution to the fuel through a high pressure nozzle.
Emulsifiers are not effective at extinguishing large Class B fuel in depth fires. This is because the amount of agent needed for extinguishment is a function of the volume of the fuel whereas agents such as aqueous film forming foam (AFFF) need only cover the surface of the fuel to achieve vapor mitigation.