Emulsion
Mixture of immiscible liquids, one dispersed in another.
An emulsion is a mixture of two or more liquids that are normally immiscible, meaning they do not blend together due to liquid-liquid phase separation. It belongs to a broader class of two-phase systems known as colloids, though the term "emulsion" is specifically reserved for cases where both the dispersed and continuous phases are liquids. In an emulsion, one liquid (the dispersed phase) is suspended within the other (the continuous phase). Common examples include vinaigrettes, homogenized milk, liquid biomolecular condensates, and certain cutting fluids used in metalworking. Different types of emulsions can form from the same two liquids; for instance, oil and water can create an oil-in-water emulsion, where oil droplets are dispersed in water, or a water-in-oil emulsion, where water droplets are dispersed in oil. More complex multiple emulsions, such as water-in-oil-in-water or oil-in-water-in-oil, are also possible. As liquids, emulsions lack a static internal structure, and the dispersed droplets are typically assumed to be roughly spherical and statistically distributed. The word "emulsion" derives from the Latin for "to milk out," referencing milk as a natural emulsion of fat and water. Emulsions generally appear cloudy because their numerous phase interfaces scatter light; they appear white when all light is scattered equally. Dilute emulsions scatter shorter wavelengths more, appearing bluer (the Tyndall effect), while concentrated emulsions scatter longer wavelengths, appearing more yellow. Special classes like microemulsions and nanoemulsions, with droplet sizes below 100 nanometers, appear translucent because light waves are not scattered by droplets smaller than about one-quarter of the incident light's wavelength. Unlike nanoemulsions, which require specialized equipment, microemulsions form spontaneously through solubilization with surfactants, though they require much higher surfactant concentrations and are often unstable to dilution, heating, or pH changes. Most common emulsions are inherently unstable and require energy input—such as shaking, stirring, or ultrasound—to form, and they tend to revert to separate phases over time, as seen in vinaigrette. Exceptions include thermodynamically stable microemulsions and kinetically stable translucent nanoemulsions. The type of emulsion formed depends on the volume fraction of the phases and the
- field
- Colloid chemistry, physical chemistry
- known_for
- Mixture of immiscible liquids; oil-in-water and water-in-oil emulsions; emulsification process
Lore & Background
The word 'emulsion' comes from the Latin 'emulgere' meaning 'to milk out', as milk is a natural emulsion of fat and water. An emulsion is a mixture of two or more normally immiscible liquids, forming a two-phase colloid where both the dispersed and continuous phases are liquids. The dispersed phase consists of droplets statistically distributed within the continuous phase, or dispersion medium, and these droplets are typically roughly spherical. Emulsions can be of different types: oil-in-water, where oil is dispersed in water (e.g., milk), or water-in-oil, where water is dispersed in oil (e.g., butter). Multiple emulsions, such as water-in-oil-in-water or oil-in-water-in-oil, are also possible. Emulsions are inherently unstable and require energy input—such as shaking, stirring, homogenizing, or powerful ultrasound—to form, and they tend to revert to separate phases over time. Exceptions include microemulsions, which are thermodynamically stable, and nanoemulsions, which are kinetically stable. The appearance of an emulsion is typically cloudy due to light scattering at the many phase interfaces; it appears white when all light is scattered equally. Dilute emulsions scatter shorter-wavelength light more, appearing bluer (the Tyndall effect), while concentrated emulsions appear more yellow. Nanoemulsions and microemulsions with droplet sizes below about 100 nanometers appear translucent because light waves are not scattered by droplets smaller than roughly one-quarter of the incident light's wavelength. Emulsification is the process of dispersing immiscible liquids using an emulsifier, such as lecithin. The term "emulsion" also refers to the photosensitive side of photographic film, consisting of silver halide colloidal particles in a gelatin matrix, and to nuclear emulsions used in particle physics to detect high-energy particles.
Reader's Guide
Emulsions are significant because they enable the blending of immiscible liquids into stable mixtures used in foods, pharmaceuticals, and industrial processes. Stability is a major concern, with four types of instability: flocculation, coalescence, creaming/sedimentation, and Ostwald ripening. Emulsifiers (surfactants) reduce interfacial tension and increase kinetic stability. The study of emulsions has practical applications in product formulation, shelf-life prediction, and the development of nanoemulsions for specialized uses. The term 'emulsion' also refers to photographic and nuclear emulsions, which are colloidal dispersions of silver halide in gelatin, used in imaging and particle detection.
Did You Know?
- The word 'emulsion' comes from the Latin 'emulgere', meaning 'to milk out'.
- Emulsions appear white when all light is scattered equally by phase interfaces.
- Microemulsions are thermodynamically stable, while translucent nanoemulsions are kinetically stable.
- Photographic emulsion consists of silver halide colloidal particles dispersed in a gelatin matrix.
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