An emulsion is a mixture
of two or more liquid
s that are normally immiscible
(unmixable or unblendable) owing to liquid-liquid phase separation
. Emulsions are part of a more general class of two-phase systems of matter
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 vinaigrette
s, homogenized milk
, liquid biomolecular condensate
s, and some cutting fluid
s for metal working
Two liquids can form different types of emulsions. As an example, oil and water can form, first, an oil-in-water emulsion, in which the oil is the dispersed phase, and water is the continuous phase. Second, they can form a water-in-oil emulsion, in which water is the dispersed phase and oil is the continuous phase. Multiple emulsions are also possible, including a "water-in-oil-in-water" emulsion and an "oil-in-water-in-oil" emulsion.
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. When molecules are ordered during liquid-liquid phase separation, they form liquid crystal
s rather than emulsions
, used by all living organisms, are one example of molecules able to form either emulsions
(e.g: spherical micelle
) or liquid crystal
s (lipid bilayer membranes
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
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
micelles (a type of secreted biomolecular condensate
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
r – this is called the "Tyndall effect
". 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.
Two special classes of emulsions – microemulsion
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 nanometer
s (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 surfactant
s, co-surfactants, and co-solvent
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
, 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
stable, while translucent nanoemulsions are kinetically
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.
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
, 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
, or under the influence of the centripetal force
induced when a centrifuge
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.
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 Stoke's 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
, 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
. 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.
An emulsifier (also known as an "emulgent") is a substance that stabilizes an emulsion by increasing its kinetic stability
. Emulsifiers are part of a broader group of compounds known as surfactant
s, or "surface active agents". Surfactants (emulsifiers) are compounds that are typically amphiphilic
, meaning they have a polar or hydrophilic
(i.e. water-soluble) part and a non-polar (i.e. hydrophobic or lipophilic
) part. Because of this, emulsifiers tend to have more or less solubility either in water or in oil. 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
. In fact, ''lecithos'' is the Greek word for egg yolk.
– where a variety of chemicals in the mucilage
surrounding the seed hull act as emulsifiers
* Soy lecithin
is another emulsifier and thickener
* Pickering stabilization
– uses particles under certain circumstances
* Sodium phosphates
– not directly an emulsifier, but modifies behavior of other molecules, e.g. casein
* Mono- and diglycerides
– a common emulsifier found in many food products (coffee creamers, ice-creams, spreads, breads, cakes)
* Sodium stearoyl lactylate
(diacetyl tartaric acid esters of mono- and diglycerides) – an emulsifier used primarily in baking
* Simple cellulose – a particulate emulsifier
derived from plant material using only water
– those with both hydrophilic and hydrophobic regions, e.g. sodium caseinate
, as in meltable cheese product
s are another class of surfactant, and will interact physically with both oil
, 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
. Many different emulsifiers are used in pharmacy
to prepare emulsions such as creams
s. Common examples include emulsifying wax
, polysorbate 20
, 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-
size" droplets within the outer phase. A well-known example of this phenomenon, the "ouzo effect
", happens when water is poured into a strong alcoholic anise
-based beverage, such as ouzo
, 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 – the emulsifying agent 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
, which are hydrocolloids, as well as PEG (or 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
Oil-in-water emulsions are common in food products:
* Crema (foam) in espresso
– coffee oil in water (brewed coffee), unstable colloid
and Hollandaise sauce
s – these are oil-in-water emulsions stabilized with egg yolk lecithin
, or with other types of food additives, such as sodium stearoyl lactylate
* Homogenized milk
– an emulsion of milk fat in water, with milk proteins as the emulsifier
– 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:
– an emulsion of water in butterfat
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.
, personal hygiene
, and cosmetics
, emulsions are frequently used. These are usually oil and water emulsions but dispersed, and which is continuous depends in many cases on the pharmaceutical formulation
. These emulsions may be called cream
s (balms), paste
s, or liquid
s, depending mostly on their oil-to-water ratios, other additives, and their intended route of administration
The first 5 are topical dosage form
s, and may be used on the surface of the skin
ly, or vagina
lly. A highly liquid emulsion may also be used oral
ly, or may be injected
in some cases.
Microemulsions are used to deliver vaccine
s and kill microbe
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
treatments, but mechanical. The smaller the droplet the greater the surface tension
and thus the greater the force required to merge with other lipids
. The oil is emulsified with detergents using a high-shear mixer
to stabilize the emulsion so, when they encounter the lipids in the cell membrane
or envelope of bacteria
es, they force the lipids to merge with themselves. On a mass scale, in effect this disintegrates the membrane and kills the pathogen. The soybean oil emulsion does not harm normal human cells, or the cells of most other higher organisms
, with the exceptions of sperm cells
and blood cells
, which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not currently used intravenous
ly (IV). The most effective application of this type of nanoemulsion is for the disinfection
of surfaces. Some types of nanoemulsions have been shown to effectively destroy HIV-1
pathogens on non-porous
Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (class B fire
s). Such agents encapsulate the fuel in a fuel-water emulsion, thereby trapping the flammable vapors in the water phase. This emulsion is achieved by applying an aqueous
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.
Emulsions are used to manufacture polymer dispersions – polymer production in an emulsion 'phase' has a number of process advantages, including prevention of coagulation of product. Products produced by such polymerisations may be used as the emulsions – products including primary components for glues and paints. Synthetic latex
es (rubbers) are also produced by this process.
* Emulsion dispersion
* Emulsified fuel
* Liquid whistle
* Pickering emulsion
* Water-in-water emulsion
* ''Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H.S., Ed.; Academic Press: New York, NY, USA, 2000; Volume 5, pp. 501–575''
Category:Condensed matter physics
Category:Drug delivery devices