An emulsion is a mixture of two or more liquids that are usually unmixable but under specific transforming processes will adopt a macroscopic homogeneous aspect and a microscopic heterogeneous one. In an emulsion, one liquid is dispersed in the other. There are several types of emulsions :

• An emulsion oil in water (O/W) is composed of an oil phase dispersed in an aqueous one. It is known as a direct emulsion. Stabilization of O/W emulsion is often performed with hydrophilic-hydrophobic particles. The hydrophilic end of the emulsifier molecule has an affinity for water, and the hydrophobic end is drawn to the fat/oil. By vigorously mixing the emulsifier with the water and oil, it creates a stable emulsion. For example, milk is oil in the water type of emulsion. In this mixture, fat globules are dispersed in the water.
• Emulsion water in oil (W/O) is composed of an aqueous phase dispersed in the oil phase. A water in oil emulsion is much fattier than a direct emulsion. Margarine is a water in oil emulsion.
• Other emulsions such as oil in water in oil, or water in oil in water exist as well. Blood is also an emulsion. It consists of negatively charged colloidal particles, which are albuminoid substances.

Emulsions are a sub-class of colloids, which are two-phase systems of matter.

Although the terms colloid and emulsion are at times used indistinctly, emulsion applies only when both phases, dispersed and continuous, are liquids. A colloid is a mixture of a compound that is in solid, liquid, or gas state and a liquid. The critical difference between colloid and emulsion is that colloid can form when any state of matter (solid, gas, or liquid) combine with a liquid. In contrast, the emulsion has two liquid components that are initially immiscible with each other.

Emulsions, as liquids, do not demonstrate a static internal structure. Emulsions are thermodynamically unstable as both the dispersed and continuous phases can revert as separate phases, oil, and water; by fusion or the coalescing of droplets. Industries use emulsifying agents (surfactants)to maintain a static structure.[1] Usually, the phase in which the surfactant exhibits the greatest solubility becomes the continuous phase. Thus, hydrophilic surfactants foster O/W emulsions, whereas lipophilic surfactants promote W/O emulsions.

Emulsions are frequently used in pharmaceuticals, personal hygiene products, and cosmetics. These are usually oil and water emulsions, albeit dispersed. These emulsions are called creams, ointments, balms, pastes, films, or liquids, depending on their oil-to-water ratios, the addition of other additives, and their intended administration route. Emulsions allow the encapsulation of an active ingredient in the dispersed phase, to protect it from degradation and to preserve its activity in a sustained manner. They are used to make medications more palatable, to improve their effectiveness via dosage control of active ingredients, and to provide better aesthetics for topical drugs such as ointments. Intravenous and parenteral emulsions may be used for nutritive therapy applications when a patient is unable to consume food or receive nutrition. Fat emulsions serve as dietary complements for patients who cannot get the required fat solely from their diet. For oral administration, the compound may be given in the form of a tablet, capsule, granule, or powder.

Nanoemulsions are nano-sized emulsions, manufactured for improving the delivery of active pharmaceutical ingredients.[2] Microemulsions form spontaneously and are thermodynamically stable. Thermal stability is not true for nanoemulsions, which are somewhat more stable than standard emulsions, but only kinetically stable. The two systems differ significantly since nanoemulsions form by mechanical shear, and microemulsion phases form by self-assembly.

Microemulsions are a vehicle by which to deliver vaccines and kill microbes. Emulsions utilized in these techniques are soybean oil nanoemulsions, with particles in the range of 400 to 600 nm in diameter. The process of creating these microemulsions is not chemical, as with other types of antimicrobial treatments, but mechanical. The smaller the droplet, the more significant the surface tension, and thus the more significant the force required to merge with other lipids.[3] The oil is emulsified using a high-shear mixer to stabilize the emulsion so, when the droplets come in contact with lipids in a cell membrane or the envelope of a bacteria or virus, they cause the lipids to merge with themselves. This action disintegrates the membrane and kills the pathogen. The soybean oil emulsion is not toxic to healthy human cells, or the cells of most other higher organisms, with an exception to sperm cells and blood cells, which are vulnerable to nanoemulsions due to the singularity of their membrane structures. These nanoemulsions are not currently used intravenously to prevent irreparable sterility. The most useful application of this type of nanoemulsion is for the disinfection of surfaces. Some nanoemulsions have demonstrated effective destruction of HIV-1 and tuberculosis pathogens on non-porous surfaces.[4]

The submicron emulsions, an isotropic mixture of drug, lipids, and surfactants, with hydrophilic co-solvents and droplet diameters ranging from 10 to 500 nm, are of increasing interest in medicine due to their kinetic stability, high solubilizing capacity, and tiny globule size.[5] They are designed mainly for applications in controlled or sustained drug delivery, targeted delivery, taste masking, bioavailability enhancement, and enzyme immobilization.[6]

Emulsions are in widespread use, and acquiring sufficient knowledge about them is prudent for all health care workers and personnel. Nurses, pharmacists, and physicians all deal with emulsions in a variety of formats and can provide a better service if informed about their characteristics.