emulsion emulsion an emulsion is a mixture of two or more liquids 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 called colloids. 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 condensates, and some cutting fluids for metal working. two immiscible liquids, not yet emulsifiedan emulsion of phase ii dispersed in phase ithe unstable emulsion progressively separatesthe surfactant (outline around particles) positions itself on the interfaces between phase ii and phase i, stabilizing the emulsion 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. such a photographic emulsion consists of silver halide colloidal particles dispersed in a gelatin 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 colloidal casein micelles (a type of secreted biomolecular condensate). appearance and properties iupac a fluid 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 anymixture thereof. note 3: the diameters of the droplets constituting the dispersed phaseusually range from approximately 10 nm to 100 μm; i.e., the dropletsmay exceed the usual size limits for colloidal particles. note 4: an emulsion is termed an oil/water (o/w) emulsion if thedispersed phase is an organic material and the continuous phase iswater or an aqueous solution and is termed water/oil (w/o) if the dispersedphase is water or an aqueous solution and the continuous phase is anorganic liquid (an "oil"). note 5: a w/o emulsion is sometimes called an inverse emulsion.the term "inverse emulsion" is misleading, suggesting incorrectly thatthe emulsion has properties that are the opposite of those of an emulsion.its use is, therefore, not recommended. 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 bluer – 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 – microemulsions 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, 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, coalescence, creaming/sedimentation, 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 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. 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, 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. emulsifiers an emulsifier is a substance that stabilizes an emulsion by reducing the oil-water interface tension. emulsifiers are a part of a broader group of compounds known as surfactants, or "surface-active agents". surfactants 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. 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. mustard – 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 datem (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 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 inducing hormone response. detergents are another class of surfactant, and will interact physically with both oil 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. many different emulsifiers are used in pharmacy to prepare emulsions such as creams and lotions. 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, pastis, absinthe, 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 and tragacanth, which are hydrocolloids, as well as peg (polyethylene glycol), glycerine, and other polymers like cmc (carboxymethyl cellulose), all increase the viscosity of the medium, which helps create and maintain the suspension of globules of dispersed phase uses in food an example of the ingredients used to make mayonnaise; olive oil, table salt, an egg (for yolk) and a lemon (for lemon juice). the oil and water in the egg yolk do not mix, while the lecithin in the yolk serves as an emulsifier, allowing the two to be blended together. oil-in-water emulsions are common in food products: mayonnaise and hollandaise sauces – 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 vinaigrette – an emulsion of vegetable oil in vinegar, if this is prepared using only oil and vinegar (i.e., without an emulsifier),