Emulsion

An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation.

Examples of emulsions include vinaigrettes, homogenized milk, liquid biomolecular condensates, and some cutting fluids for metal working.

Such a photographic emulsion consists of silver halide colloidal particles dispersed in a gelatin matrix.

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.

[6] If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear more yellow.

[7] Two special classes of emulsions – microemulsions and nanoemulsions, with droplet sizes below 100 nm – appear translucent.

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.

In addition, the stability of a microemulsion is often easily compromised by dilution, by heating, or by changing pH levels.

[citation needed] Common emulsions are inherently unstable and, thus, do not tend to form spontaneously.

Energy input – through shaking, stirring, homogenizing, or exposure to power ultrasound[10] – is needed to form an emulsion.

This process can be desired, if controlled in its extent, to tune physical properties of emulsions such as their flow behaviour.

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.

[16] 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.

[15] The stability of emulsions can be characterized using techniques such as light scattering, focused beam reflectance measurement, centrifugation, and rheology.

[19] 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.

[citation needed] An emulsifier is a substance that stabilizes an emulsion by reducing the oil-water interface tension.

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 creams, ointments, liniments (balms), pastes, films, or liquids, depending mostly on their oil-to-water ratios, other additives, and their intended route of administration.

Such agents encapsulate the fuel in a fuel-water emulsion, thereby trapping the flammable vapors in the water phase.