This article is about mixtures of liquids. For the light-sensitive mixture used in photography, see .
definitionFluid system in which liquid droplets are dispersed in a liquid.
Note 1: The definition is based on the definition in ref.
Note 2: The droplets may be amorphous, liquid-crystalline, or any
Note 3: The diameters of the droplets constituting the
usually range from approximately 10 nm to 100 μm; i.e., the droplets
may exceed the usual size limits for particles.
Note 4: An emulsion is termed an oil/water (o/w) emulsion if the
dispersed phase is an organic material and the continuous 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").
An emulsion is a of two or more that are normally (unmixable or unblendable). Emulsions are part of a more general class of two-phase systems of called . 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 ) is in the other (the continuous phase). Examples of emulsions include , homogenized , , and some for . and its modified forms are also a good example of recent unconventional helping in stabilizing emulsion systems.
The word "emulsion" comes from the Latin mulgeo, mulgere "to milk", as milk is an emulsion of fat and water, along with other components.
Two liquids can form different types of emulsions. As an example, oil and water can form, first, an oil-in-water emulsion, wherein the oil is the dispersed phase, and water is the dispersion medium. (, used by all complex living organisms, are one example of this.) Second, they can form a water-in-oil emulsion, wherein water is the dispersed phase and oil is the external 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 liquid matrix (called the “dispersion medium”) are usually assumed to be .
The term "emulsion" is also used to refer to the photo-sensitive side of . Such a consists of colloidal particles dispersed in a matrix. are similar to photographic emulsions, except that they are used in particle physics to detect high-energy .
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 light as it passes through the emulsion. Emulsions appear 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 – this is called the "".If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear more . This phenomenon is easily observable when comparing , which contains little fat, to , which contains a much higher concentration of milk fat. One example would be a mixture of water and oil.
Two special classes of emulsions – 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 of light is composed of wavelengths between 390 and 750 (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 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 , co-surfactants, and co-. The required surfactant concentration in a 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, , or exposure to power – 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 , an unstable emulsion that will quickly separate unless shaken almost continuously. There are important exceptions to this rule – are stable, while translucent nanoemulsions are 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. In general, the 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 geometric structure of an emulsion mixture of two lyophobic liquids with a large concentration of the secondary component is fractal: Emulsion particles unavoidably form dynamic inhomogeneous structures on small length scale. The geometry of these structures is fractal. The size of elementary irregularities is governed by a universal function which depends on the volume content of the components. The fractal dimension of these irregularities is 2.5.
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: , , , and . Flocculation occurs when there is an attractive force between the droplets, so they form flocs, like bunches of grapes. 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 , where the droplets rise to the top of the emulsion under the influence of , or under the influence of the induced when a is used. 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.
An appropriate "surface active agent" (or "") can increase the kinetic stability of an emulsion so that the size of the droplets does not change significantly with time. It is then said to be stable. For example, oil-in-water emulsions containing and milk protein as showed that stable oil droplet size over 28 days storage at 25°C.
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.
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 inter-facial tension in the case of non-ionic surfactants or, on a broader scope, interactions of forces inside 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 to accelerate 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 . One class of emulsifiers is known as "surface active agents", or . Emulsifiers are compounds that typically 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:
are another class of surfactant, and will interact physically with both and , thus stabilizing the interface between the oil and water droplets in suspension. This principle is exploited in , to remove for the purpose of . Many different emulsifiers are used in to prepare emulsions such as and . Common examples include , , and .
Sometimes the inner phase itself can act as an emulsifier, and the result is a nanoemulsion, where the inner state disperses into "" droplets within the outer phase. A well-known example of this phenomenon, the "", happens when water is poured into a strong alcoholic -based beverage, such as , , , , or . The anisolic compounds, which are soluble in , then form nano-size droplets and emulsify within the water. The resulting color of the drink is opaque and milky white.
See also: explained, in the Simple English Wikipedia
A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification:
Oil-in-water emulsions are common in food products:
Water-in-oil emulsions are less common in food, but still exist:
Other foods can be turned into products similar to emulsions, for example is a suspension of meat in liquid that is similar to true emulsions.
In , , , and , emulsions are frequently used. These are usually oil and water emulsions but dispersed, and which is continuous depends in many cases on the . These emulsions may be called , ointments, (balms), , , or , depending mostly on their oil-to-water ratios, other additives, and their intended . The first 5 are , and may be used on the surface of the , , , , or . A highly liquid emulsion may also be used , or may be in some cases. Popular medications occurring in emulsion form include, , cream, , and .
Microemulsions are used to deliver and kill . Typical emulsions used in these techniques are nanoemulsions of , with particles that are 400–600 nm in diameter. The process is not chemical, as with other types of treatments, but mechanical. The smaller the droplet the greater the and thus the greater the force required to merge with other . The oil is emulsified with detergents using a to stabilize the emulsion so, when they encounter the lipids in the or envelope of or , 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 , with the exceptions of and , which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not currently used (IV). The most effective application of this type of nanoemulsion is for the of surfaces. Some types of nanoemulsions have been shown to effectively destroy and pathogens on non- surfaces.
Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (). 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 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 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 (rubbers) are also produced by this process.