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Emulsion
Emulsion
Emulsion
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  1. Two immiscible liquids, not yet emulsified
  2. An emulsion of phase II dispersed in phase I
  3. The unstable emulsion progressively separates
  4. The surfactant (outline around particles) positions itself on the interfaces between Phase II and Phase I, stabilizing the 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 more narrowly refers to 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 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.[1]

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.

IUPAC

A fluid system in which liquid droplets are dispersed in a liquid.

Note 1: The definition is based on the definition in ref.[2]

Note 2: The droplets may be amorphous, liquid-crystalline, or any
mixture thereof.

Note 3: The diameters of the droplets constituting the dispersed phase
usually range from approximately 10 nm to 100 μm; i.e., the droplets
may exceed the usual size limits for colloidal 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").

Note 5: A w/o emulsion is sometimes called an inverse emulsion.
The term "inverse emulsion" is misleading, suggesting incorrectly that
the emulsion has properties that are the opposite of those of an emulsion.
Its use is, therefore, not recommended.[3]

Etymology

[edit]

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).[4]

Appearance and properties

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Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface".[5] 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 (shorter-wavelength) light will be scattered more, and the emulsion will appear bluer – this is called the "Tyndall effect".[6] 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.[citation needed]

Two special classes of emulsions – microemulsions and nanoemulsions, with droplet sizes below 100 nm – appear translucent.[7] 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.[8] 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.[7] 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.[citation needed]

Common emulsions are inherently unstable and, thus, do not tend to form spontaneously. Energy input – through shaking, stirring, homogenizing, or exposure to powerful ultrasound[9] – 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.[7]

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.[10]

Instability

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Emulsion stability refers to the ability of an emulsion to resist change in its properties over time.[11][12] 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.[13] 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.[11] 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.[14] Sedimentation is the opposite phenomenon of creaming and normally observed in water-in-oil emulsions.[5] 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.[15] 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.[14]

Monitoring physical stability

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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.[16]

Accelerating methods for shelf life prediction

[edit]

The kinetic process of destabilization can be rather long – up to several months, or even years for some products.[17] 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).[18] 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.[citation needed]

Mechanical methods of acceleration, including vibration, centrifugation, and agitation, can also be used.[19]

These methods are almost always empirical, without a sound scientific basis.[citation needed]

Emulsifiers

[edit]

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".[20] Surfactants are compounds that are typically amphiphilic, meaning they have a polar or hydrophilic (water-soluble) part and a non-polar (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.[21]

The following are examples of food emulsifiers:

  • Egg yolk – in which the main emulsifying and thickening agent is lecithin.
  • Mustard[22] – 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
  • 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.
    • Processed cheese uses acids such as phosphates to chelate away calcium, which allows cheese casein to work as an emulsifier. The phosphate is considered an emulsifying agent; the actual emulsifier is the casein already present in cheese.
  • Applesauce - Sometimes used in baking as an alternative to egg yolk or fats to make up for dietary restrictions such as allergies or being vegan

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.[23]

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.[24]

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

[edit]

A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification:[5]

  • 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

[edit]

In food

[edit]
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:

Water-in-oil emulsions are less common in food, but still exist:

Other foods can be turned into products similar to emulsions, for example meat emulsion is a suspension of meat in liquid that is similar to true emulsions.

In health care

[edit]

In pharmaceutics, hairstyling, personal hygiene, and cosmetics, emulsions are frequently used. 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.[25][26] The first five are topical dosage forms, and may be used on the surface of the skin, transdermally, ophthalmically, rectally, or vaginally. A highly liquid emulsion may also be used orally, or may be injected in some cases.[25]

Microemulsions are used to deliver vaccines and kill microbes.[27] Typical emulsions used in these techniques are nanoemulsions of soybean oil, with particles that are 400–600 nm in diameter.[28] The process is not chemical, as with other types of antimicrobial treatments, but mechanical. The smaller the droplet the greater the surface tension and thus the greater the force required to merge with other lipids. The oil is emulsified with detergents using a high-shear mixer to stabilize the emulsion so that when emulsion nano-droplets encounter the lipids in the cell membrane or cell envelope of bacteria or viruses, they force those lipids to merge with the nano-droplets. On a mass scale, this effectively disintegrates the membrane and kills the pathogen. The soybean oil emulsion does not harm normal human cells, or the cells of most other higher organisms, with the exceptions of sperm cells and blood cells, which are vulnerable to nanoemulsions due to the peculiarities of their membrane structures. For this reason, these nanoemulsions are not used intravenously (IV). The most effective application of this type of nanoemulsion is for the disinfecting of surfaces. Some types of nanoemulsion have been shown to effectively destroy HIV-1 and tuberculosis pathogens on non-porous surfaces.[citation needed]

Applications in pharmaceutical industry

[edit]
  • Oral drug delivery: Emulsions may provide an efficient means of administering drugs that are poorly soluble or have low bioavailability or dissolution rates, increasing both dissolution rates and absorption to increase bioavailability and improve bioavailability. By increasing surface area provided by an emulsion, dissolution rates and absorption rates of drugs are increased, improving their bioavailability.[29]
  • Topical formulations: Emulsions are widely utilized as bases for topical drug delivery formulations such as creams, lotions and ointments. Their incorporation allows lipophilic as well as hydrophilic drugs to be mixed together for maximum skin penetration and permeation of active ingredients.[30]
  • Parenteral drug delivery: Emulsions serve as carriers for intravenous or intramuscular administration of drugs, solubilizing lipophilic ones while protecting from degradation and decreasing injection site irritation. Examples include propofol as a widely used anesthetic and lipid-based solutions used for total parenteral nutrition delivery.[31]
  • Ocular Drug Delivery: Emulsions can be used to formulate eye drops and other ocular drug delivery systems, increasing drug retention time in the eye and permeating through corneal barriers more easily while providing sustained release of active ingredients and thus increasing therapeutic efficacy.[32]
  • Nasal and Pulmonary Drug Delivery: Emulsions can be an ideal vehicle for creating nasal sprays and inhalable drug products, enhancing drug absorption through nasal and pulmonary mucosa while providing sustained release with reduced local irritation.[33]
  • Vaccine Adjuvants: Emulsions can serve as vaccine adjuvants by strengthening immune responses against specific antigens. Emulsions can enhance antigen solubility and uptake by immune cells while simultaneously providing controlled release, amplifying an immunological response and thus amplifying its effect.[34]
  • Taste Masking: Emulsions can be used to encase bitter or otherwise unpleasant-tasting drugs, masking their taste and increasing patient compliance - particularly with pediatric formulations.[34]
  • Cosmeceuticals: Emulsions are widely utilized in cosmeceuticals products that combine cosmetic and pharmaceutical properties. These emulsions act as carriers for active ingredients like vitamins, antioxidants and skin lightening agents to provide improved skin penetration and increased stability.[35]

In firefighting

[edit]

Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (class B fires). 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 aqueous 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 aqueous film-forming foam need cover only the surface of the fuel to achieve vapor mitigation.[36]

Chemical synthesis

[edit]
Main article: Emulsion polymerization

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 latexes (rubbers) are also produced by this process.

See also

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Citations

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  1. ^ Khan, A. Y.; Talegaonkar, S.; Iqbal, Z.; Ahmed, F. J.; Khar, R. K. (2006). "Multiple emulsions: An overview". Current Drug Delivery. 3 (4): 429–443. doi:10.2174/156720106778559056. PMID 17076645.
  2. ^ IUPAC (1997). "Emulsion". Compendium of Chemical Terminology (The "Gold Book"). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.E02065. ISBN 978-0-9678550-9-7. Archived from the original on 10 March 2012.
  3. ^ Slomkowski, Stanislaw; Alemán, José V.; Gilbert, Robert G.; Hess, Michael; Horie, Kazuyuki; Jones, Richard G.; Kubisa, Przemyslaw; Meisel, Ingrid; Mormann, Werner; Penczek, Stanisław; Stepto, Robert F. T. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry. 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03. S2CID 96812603.
  4. ^ Harper, Douglas. "Online Etymology Dictionary". EtymOnline.com. Retrieved 2 November 2019.
  5. ^ a b c Loi, Chia Chun; Eyres, Graham T.; Birch, E. John (2018), "Protein-Stabilised Emulsions", Reference Module in Food Science, Elsevier, doi:10.1016/b978-0-08-100596-5.22490-6, ISBN 9780081005965
  6. ^ Remington, Joseph Price (2010) [1990]. Gennaro, Alfonso R. (ed.). Remington's Pharmaceutical Sciences. Mack Publishing Co. (original from Northwestern University). p. 281. ISBN 9780912734040.
  7. ^ a b c Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. (2006). "Nanoemulsions: Formation, structure, and physical properties" (PDF). Journal of Physics: Condensed Matter. 18 (41): R635 – R666. Bibcode:2006JPCM...18R.635M. doi:10.1088/0953-8984/18/41/R01. S2CID 11570614. Archived from the original (PDF) on 12 January 2017.
  8. ^ Leong, T. S.; Wooster, T. J.; Kentish, S. E.; Ashokkumar, M. (2009). "Minimising oil droplet size using ultrasonic emulsification" (PDF). Ultrasonics Sonochemistry. 16 (6): 721–727. Bibcode:2009UltS...16..721L. doi:10.1016/j.ultsonch.2009.02.008. hdl:11343/129835. PMID 19321375.
  9. ^ Kentish, S.; Wooster, T. J.; Ashokkumar, M.; Balachandran, S.; Mawson, R.; Simons, L. (2008). "The use of ultrasonics for nanoemulsion preparation". Innovative Food Science & Emerging Technologies. 9 (2): 170–175. doi:10.1016/j.ifset.2007.07.005. hdl:11343/55431.
  10. ^ Cittadini, Aurora; Munekata, Paulo Eduardo Sichetti; Pateiro, Mirian; Sarriés, María V.; Domínguez, Rubén; Lorenzo, José M. (2022). "Encapsulation techniques to increase lipid stability". Food Lipids. pp. 413–459. doi:10.1016/B978-0-12-823371-9.00010-1. ISBN 978-0-12-823371-9.
  11. ^ a b McClements, David Julian (16 December 2004). Food Emulsions: Principles, Practices, and Techniques, Second Edition. Taylor & Francis. pp. 269 ff. ISBN 978-0-8493-2023-1.
  12. ^ Silvestre, M. P. C.; Decker, E. A.; McClements, D. J. (1999). "Influence of copper on the stability of whey protein stabilized emulsions". Food Hydrocolloids. 13 (5): 419. doi:10.1016/S0268-005X(99)00027-2.
  13. ^ Fuhrmann, Philipp L.; Sala, Guido; Stieger, Markus; Scholten, Elke (1 August 2019). "Clustering of oil droplets in o/w emulsions: Controlling cluster size and interaction strength". Food Research International. 122: 537–547. doi:10.1016/j.foodres.2019.04.027. ISSN 0963-9969. PMID 31229109.
  14. ^ a b Loi, Chia Chun; Eyres, Graham T.; Birch, E. John (2019). "Effect of mono- and diglycerides on physical properties and stability of a protein-stabilised oil-in-water emulsion". Journal of Food Engineering. 240: 56–64. doi:10.1016/j.jfoodeng.2018.07.016. ISSN 0260-8774. S2CID 106021441.
  15. ^ McClements, David Julian (27 September 2007). "Critical Review of Techniques and Methodologies for Characterization of Emulsion Stability". Critical Reviews in Food Science and Nutrition. 47 (7): 611–649. doi:10.1080/10408390701289292. ISSN 1040-8398. PMID 17943495. S2CID 37152866.
  16. ^ Dowding, Peter J.; Goodwin, James W.; Vincent, Brian (30 November 2001). "Factors governing emulsion droplet and solid particle size measurements performed using the focused beam reflectance technique". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 192 (1): 5–13. doi:10.1016/S0927-7757(01)00711-7. ISSN 0927-7757.
  17. ^ "Emulsifying Guide: Advanced Techniques & Industrial Application". Xiangxiang Daily. 5 August 2024.
  18. ^ Masmoudi, H.; Le Dréau, Y.; Piccerelle, P.; Kister, J. (31 January 2005). "The evaluation of cosmetic and pharmaceutical emulsions aging process using classical techniques and a new method: FTIR" (PDF). International Journal of Pharmaceutics. 289 (1): 117–131. doi:10.1016/j.ijpharm.2004.10.020. ISSN 0378-5173. PMID 15652205.
  19. ^ Editorial Board Entrée (2006). Emulsions. Begel House. doi:10.1615/AtoZ.e.emulsions. ISBN 978-1-56700-456-4. Retrieved 16 June 2023. {{cite book}}: |work= ignored (help)
  20. ^ "Emulsions: making oil and water mix". AOCS.org. American Oil Chemists' Society. Retrieved 1 January 2021.
  21. ^ Cassidy, L. "Emulsions: Making oil and water mix". AOCS.org. American Oil Chemists' Society.
  22. ^ Pomerantz, Riva (15 November 2017). "Kosher in the Lab". Ami. No. 342.
  23. ^ Bertsch, Pascal; Steingoetter, Andreas; Arnold, Myrtha; Scheuble, Nathalie; Bergfreund, Jotam; Fedele, Shahana; Liu, Dian; Parker, Helen L.; Langhans, Wolfgang; Rehfeld, Jens F.; Fischer, Peter (30 August 2022). "Lipid emulsion interfacial design modulates human in vivo digestion and satiation hormone response". Food & Function. 13 (17): 9010–9020. doi:10.1039/D2FO01247B. ISSN 2042-650X. PMC 9426722. PMID 35942900.
  24. ^ Faiola, Anne-Marie (21 May 2008). "Using Emulsifying Wax". TeachSoap.com. Retrieved 22 July 2008.
  25. ^ a b Aulton, Michael E., ed. (2007). Aulton's Pharmaceutics: The Design and Manufacture of Medicines (3rd ed.). Churchill Livingstone. pp. 92–97, 384, 390–405, 566–569, 573–574, 589–596, 609–611. ISBN 978-0-443-10108-3.
  26. ^ Troy, David A.; Remington, Joseph P.; Beringer, Paul (2006). Remington: The Science and Practice of Pharmacy (21st ed.). Philadelphia: Lippincott Williams & Wilkins. pp. 325–336, 886–887. ISBN 978-0-7817-4673-1.
  27. ^ "Adjuvant Vaccine Development". Archived from the original on 5 July 2008.
  28. ^ "Nanoemulsion vaccines show increasing promise". Eurekalert! Public News List. University of Michigan Health System. 26 February 2008. Retrieved 22 July 2008.
  29. ^ Sharma, Anubhav (26 April 2023). "Role of Surfactant in Emulsion Stabilization: A Comprehensive Overview". Witfire. Retrieved 27 April 2023.
  30. ^ Apostolidis, Eftychios; Stoforos, George N.; Mandala, Ioanna (April 2023). "Starch physical treatment, emulsion formation, stability, and their applications". Carbohydrate Polymers. 305 120554. doi:10.1016/j.carbpol.2023.120554. ISSN 0144-8617. PMID 36737219. S2CID 255739614.
  31. ^ Hazt, Bianca; Pereira Parchen, Gabriela; Fernanda Martins do Amaral, Lilian; Rondon Gallina, Patrícia; Martin, Sandra; Hess Gonçalves, Odinei; Alves de Freitas, Rilton (April 2023). "Unconventional and conventional Pickering emulsions: Perspectives and challenges in skin applications". International Journal of Pharmaceutics. 636 122817. doi:10.1016/j.ijpharm.2023.122817. hdl:10198/16535. ISSN 0378-5173. PMID 36905974. S2CID 257474428.
  32. ^ Ding, Jingjing; Li, Yunxing; Wang, Qiubo; Chen, Linqian; Mao, Yi; Mei, Jie; Yang, Cheng; Sun, Yajuan (April 2023). "Pickering high internal phase emulsions with excellent UV protection property stabilized by Spirulina protein isolate nanoparticles". Food Hydrocolloids. 137 108369. doi:10.1016/j.foodhyd.2022.108369. ISSN 0268-005X. S2CID 254218797.
  33. ^ Udepurkar, Aniket Pradip; Clasen, Christian; Kuhn, Simon (March 2023). "Emulsification mechanism in an ultrasonic microreactor: Influence of surface roughness and ultrasound frequency". Ultrasonics Sonochemistry. 94 106323. Bibcode:2023UltS...9406323U. doi:10.1016/j.ultsonch.2023.106323. ISSN 1350-4177. PMC 9945801. PMID 36774674.
  34. ^ a b Hong, Xin; Zhao, Qiaoli; Liu, Yuanfa; Li, Jinwei (13 August 2021). "Recent advances on food-grade water-in-oil emulsions: Instability mechanism, fabrication, characterization, application, and research trends". Critical Reviews in Food Science and Nutrition. 63 (10): 1406–1436. doi:10.1080/10408398.2021.1964063. ISSN 1040-8398. PMID 34387517. S2CID 236998385.
  35. ^ Xu, Tian; Jiang, Chengchen; Huang, Zehao; Gu, Zhengbiao; Cheng, Li; Hong, Yan (January 2023). "Formation, stability and the application of Pickering emulsions stabilized with OSA starch/chitosan complexes". Carbohydrate Polymers. 299 120149. doi:10.1016/j.carbpol.2022.120149. ISSN 0144-8617. PMID 36876777. S2CID 252553332.
  36. ^ Friedman, Raymond (1998). Principles of Fire Protection Chemistry and Physics. Jones & Bartlett Learning. ISBN 978-0-87765-440-7.

General and cited references

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Emulsion

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from Grokipedia
An emulsion is a thermodynamically unstable mixture of two or more immiscible liquids, in which one liquid is dispersed as droplets of microscopic or ultramicroscopic size within the other, appearing macroscopically homogeneous but microscopically heterogeneous. These systems require emulsifying agents, such as , to stabilize them by reducing interfacial tension and preventing droplet coalescence or separation. Emulsions are classified primarily by the nature of their continuous and dispersed phases: oil-in-water (O/W) emulsions feature oil droplets suspended in water, as seen in and , while water-in-oil (W/O) emulsions have water droplets dispersed in oil, exemplified by and . More complex forms, such as multiple emulsions (e.g., water-in-oil-in-water), also exist for specialized uses like controlled release. Preparation typically involves mechanical agitation or high-shear processes to achieve droplet sizes ranging from nanometers to micrometers, with stability influenced by factors like , , and emulsifier concentration. Emulsions play a critical role across diverse industries due to their ability to combine incompatible phases effectively. In the food sector, they enhance texture, stability, and in products like sauces, ice creams, and dressings. In pharmaceuticals and , they facilitate , improve , and form bases for creams and lotions. Industrial applications extend to paints, adhesives, and processing, where emulsions enable efficient and reaction control.

Etymology and Fundamentals

Etymology

The term "emulsion" originates from the Latin verb emulgere, meaning "to milk out," a reference to the process of extracting milk and the resulting milky appearance of mixtures where one liquid is dispersed in another, as seen in natural examples like milk itself. This etymological root reflects the early association of the concept with dairy products, where fat globules are suspended in water, and the word entered scientific and medical discourse in the early 17th century through French émulsion around 1610, initially describing such oil-and-water mixtures. By the 19th century, as colloidal science emerged, the term evolved in chemical contexts to denote a stable colloidal suspension of one immiscible liquid dispersed as droplets within another, formalized through foundational work by Thomas Graham around 1860, who distinguished colloids from true solutions and highlighted emulsions as a key subclass.

Definition and Classification

An emulsion is a type of , which refers to a heterogeneous where particles of one substance, ranging in size from 1 nm to 1 μm, are dispersed evenly throughout another substance without dissolving, resulting in a suspension under certain conditions. In the context of emulsions, this colloidal system specifically involves the dispersion of droplets of one liquid within another immiscible liquid, driven by the need to minimize unfavorable interactions at their interface. Emulsions are defined as thermodynamically unstable mixtures of two or more immiscible liquids, where one liquid—the dispersed phase—forms small droplets distributed throughout the other liquid—the continuous phase. This instability arises primarily from the high interfacial tension between the immiscible liquids, which represents the excess free energy per unit area at their boundary and favors to reduce the total interfacial area. Conventional macroemulsions, the most common type, feature droplet sizes typically ranging from 0.1 to 100 μm, which are visible under a and contribute to the system's opacity. Emulsions are primarily classified based on the nature of the dispersed and continuous phases. Oil-in-water (O/W) emulsions have droplets dispersed in a continuous aqueous phase, such as , while water-in- (W/O) emulsions have water droplets in a continuous phase, like . Multiple emulsions, such as water-in--in-water (W/O/W), involve nested dispersions where smaller droplets of one liquid are encapsulated within larger droplets of another, enabling controlled release applications. Special cases include nanoemulsions, with droplet sizes below 100 nm, which remain thermodynamically unstable but achieve kinetic stability through high energy input during formation, and microemulsions, which are thermodynamically stable, isotropic systems with droplet sizes around 10–100 nm that form spontaneously due to low interfacial tension induced by .

Physical Characteristics

Appearance

Emulsions typically exhibit a milky or opaque appearance as a result of the , where dispersed droplets scatter visible , creating a cloudy visual effect when a passes through the . This scattering occurs because the droplets are larger than the of visible (approximately 400–700 nm), leading to multiple scattering events that prevent from transmitting directly and instead diffuse it in various directions. The specific visual traits vary depending on the emulsion type. Oil-in-water (O/W) emulsions, such as , often appear and fluid due to the scattering of light by oil droplets dispersed in an aqueous continuous phase. In contrast, water-in-oil (W/O) emulsions, like or , present a creamier or more solid-like texture and appearance, with water droplets scattered in a continuous oil phase that contributes to a denser, less translucent look. Over time, processes like creaming or can alter this opacity, potentially leading to clearer layers as droplets aggregate and rise or settle. Several factors influence the overall appearance of emulsions. Droplet size plays a key role: larger droplets (typically >100 nm) enhance and opacity, while smaller ones (e.g., <100 nm in nanoemulsions) reduce intensity, resulting in greater clarity or transparency. Additionally, the concentration of the dispersed phase affects visual properties; as it increases up to the close-packing limit of about 74% volume fraction, the emulsion becomes more opaque due to higher density from closely packed droplets.

Key Properties

Emulsions exhibit a range of rheological behaviors depending on their composition and concentration. Dilute emulsions, where the volume fraction of the dispersed phase (φ) is low, typically display Newtonian flow, characterized by a constant viscosity independent of shear rate. In contrast, concentrated emulsions often behave as non-Newtonian fluids, showing shear-thinning properties where viscosity decreases under increasing shear rates due to droplet deformation and alignment. The overall viscosity of an emulsion increases with the dispersed phase volume fraction; for dilute suspensions, this relationship is described by the Einstein equation: η=η0(1+2.5ϕ) \eta = \eta_0 (1 + 2.5 \phi) where η is the emulsion viscosity, η₀ is the viscosity of the continuous phase, and φ is the volume fraction of the dispersed phase (valid for φ < 0.05). Interfacial properties play a critical role in emulsion formation and maintenance. Emulsifiers adsorb at the oil-water interface, significantly reducing surface tension from typical values of around 50 mN/m to as low as 1-10 mN/m, which facilitates droplet breakup during emulsification. For charged emulsions, the zeta potential of droplets, arising from ionized groups or adsorbed ions, influences electrostatic repulsion between droplets; values greater than |30| mV typically indicate sufficient stability against coalescence by providing a repulsive barrier. Thermal and electrical properties of emulsions vary markedly with the type of emulsion. Oil-in-water (O/W) emulsions are generally conductive due to the aqueous continuous phase, which allows ion mobility, whereas water-in-oil (W/O) emulsions are non-conductive as the oil phase insulates the dispersed water droplets. Density differences between phases drive buoyancy effects, with dispersed droplets rising (creaming) or sinking (sedimentation) based on whether their density exceeds or is less than that of the continuous phase, influencing phase separation tendencies. Optically, these density gradients can contribute to light scattering and turbidity variations, though emulsions often appear opaque due to refractive index mismatches at interfaces.

Stability Aspects

Instability Mechanisms

Emulsions are thermodynamically unstable systems that tend to separate into their constituent phases over time due to various physical processes driven by gravity, interfacial forces, and diffusion. The primary instability mechanisms include creaming or sedimentation, flocculation, coalescence, , and phase inversion, each contributing to the breakdown by altering droplet distribution or integrity. Creaming occurs when less dense droplets rise to the surface, while sedimentation involves denser droplets settling at the bottom, both governed by gravitational forces in emulsions where the dispersed phase density differs from the continuous phase. The velocity of this movement for individual spherical droplets in dilute emulsions follows : v=2r2(ρ1−ρ2)g9ηv = \frac{2r^2 (\rho_1 - \rho_2) g}{9 \eta} where vv is the settling or creaming velocity, rr is the droplet radius, ρ1\rho_1 and ρ2\rho_2 are the densities of the dispersed and continuous phases, gg is gravitational acceleration, and η\eta is the continuous phase viscosity. Larger droplets cream faster due to the quadratic dependence on radius, exacerbating separation in polydisperse systems. Flocculation refers to the reversible aggregation of droplets without merging, primarily due to attractive van der Waals forces overcoming repulsive barriers between droplets. This process forms loose clusters that can accelerate creaming by increasing effective droplet size while maintaining individual droplet integrity. Coalescence involves the irreversible merging of flocculated droplets through the drainage and rupture of the thin liquid film separating them, often initiated by van der Waals attractions. The rate depends on film stability, with higher interfacial tension and viscoelastic films slowing the process. Ostwald ripening arises from the diffusion of dispersed phase molecules from smaller to larger droplets, driven by differences in solubility caused by curvature effects. The Laplace pressure, ΔP=2γr\Delta P = \frac{2\gamma}{r}, where γ\gamma is the interfacial tension and rr is the droplet radius, is higher in smaller droplets, increasing their internal pressure and solubility according to the Kelvin equation. This leads to gradual growth of larger droplets and shrinkage of smaller ones, broadening the size distribution over time. Phase inversion is the transition from an oil-in-water (O/W) to a water-in-oil (W/O) emulsion or vice versa, triggered by changes in composition, such as increasing the dispersed phase volume fraction beyond a critical point (typically around 0.74 for catastrophic inversion). Transitional inversion occurs when emulsifier affinity shifts, often with temperature, altering the curvature of the interfacial film. Environmental factors significantly influence these mechanisms by modulating inter-droplet interactions. Elevated temperature reduces continuous phase viscosity and accelerates , enhancing diffusion rates in and flocculation while promoting coalescence through increased kinetic energy. Variations in pH affect the charge on emulsified interfaces, altering electrostatic repulsion; for instance, at the isoelectric point, reduced repulsion facilitates flocculation and coalescence. Higher ionic strength screens electrostatic charges, compressing the double layer and weakening repulsion, which promotes flocculation and accelerates creaming in charged emulsions.

Monitoring and Prediction Methods

Monitoring emulsion stability relies on a suite of experimental techniques that probe droplet size distribution, aggregation tendencies, and interfacial properties. Turbidity measurements, utilizing light scattering principles, quantify the optical density of emulsions to infer droplet size distributions and detect instability through changes in scattering intensity; for instance, increased turbidity signals flocculation or coalescence as larger droplets form. Optical and electron microscopy provide direct visualization of droplet morphology and flocculation events, allowing observation of processes like bridging or coalescence at the microscale, often enhanced by image analysis for quantitative droplet sizing. Rheometry assesses viscosity and viscoelastic responses, where elevations in shear viscosity or the onset of non-Newtonian behavior indicate droplet aggregation, offering insights into structural changes during storage. Zeta potential analysis, commonly performed via electroacoustic methods, evaluates electrostatic repulsion by measuring electrophoretic mobility; absolute values exceeding 30 mV typically signify robust charge-based stability against coalescence. Prediction of emulsion shelf life employs accelerated testing and theoretical modeling to forecast long-term behavior from condensed experiments. Accelerated shelf-life protocols, such as temperature cycling to hasten diffusion-driven creaming or centrifugation to mimic gravitational separation, enable extrapolation of stability timelines using Arrhenius-based kinetics, reducing real-time testing durations from years to weeks. Mathematical frameworks like model colloidal interactions by summing attractive van der Waals forces and repulsive electrostatic potentials, with the total interaction energy expressed as Vtotal=VvdW+VelecV_{\text{total}} = V_{\text{vdW}} + V_{\text{elec}} where a high energy barrier (often >20 kT) predicts resistance to aggregation. Population balance equations further simulate droplet size evolution by accounting for birth and death rates of droplets due to breakage, coalescence, and Ostwald ripening, solving integro-differential forms to project distribution shifts over time. Post-2020 advancements introduce non-invasive and automated tools for enhanced monitoring and forecasting. Ultrasonic spectroscopy facilitates real-time, contactless assessment of droplet size and concentration by analyzing sound wave attenuation and velocity, detecting phase separation or flocculation without sample perturbation, particularly useful for opaque industrial emulsions. AI-driven image analysis processes microscopic or macroscopic visuals via convolutional neural networks to predict stability in real time, classifying destabilization patterns like creaming with accuracies exceeding 90% by training on annotated datasets of droplet dynamics. Recent machine learning models, such as interpretable frameworks trained on multi-laboratory literature data, have been developed to predict lipid emulsion stability in parenteral nutrition as of November 2025, offering generalizable predictions across diverse clinical formulations.

Components and Processes

Emulsifiers

Emulsifiers are surface-active agents, primarily , that enable the formation of emulsions by adsorbing at the interface between immiscible liquids, such as oil and water, to stabilize dispersed droplets. These amphiphilic molecules feature both hydrophilic (water-attracting) and lipophilic (oil-attracting) components, allowing them to bridge the two phases. The (HLB) scale, developed by William C. Griffin in 1949, quantifies this property on a numerical range from 0 (highly lipophilic) to 20 (highly hydrophilic); for instance, emulsifiers with HLB values greater than 8 are typically used for oil-in-water (O/W) emulsions, while those below 6 suit water-in-oil (W/O) types. Emulsifiers are categorized into natural and synthetic types, with polymers and particle-based stabilizers serving as additional classes for enhanced stabilization. Natural emulsifiers, such as —a mixture extracted from soybeans or egg yolks—offer and are widely used in food and pharmaceutical applications due to their mild nature. Emerging natural approaches include Pickering emulsions, stabilized by solid particles like protein nanoparticles or nanocrystals, which provide irreversible adsorption and improved environmental . Synthetic emulsifiers include nonionic like Tween 80 (, HLB ≈ 15), which promotes O/W emulsions, and Span 80 (, HLB ≈ 4.3), favored for W/O systems; these provide precise control over emulsion properties but may raise concerns regarding long-term safety. Polymeric emulsifiers, exemplified by —a microbial —contribute steric stabilization by creating a thick, entangled layer around droplets, preventing aggregation through physical hindrance rather than charge effects. The core functions of emulsifiers involve lowering interfacial tension (γ) to allow droplet formation and creating protective barriers against instability. Adsorption of emulsifiers at the interface follows the Gibbs adsorption isotherm, which relates surface excess (Γ) to changes in tension: Γ=−1RTdγdln⁡C\Gamma = -\frac{1}{RT} \frac{d\gamma}{d \ln C} Here, RR is the gas constant, TT is the absolute temperature, and CC is the emulsifier concentration in the bulk phase; this equation demonstrates how increased adsorption reduces γ, easing the mechanical energy required for emulsification. Additionally, emulsifiers assemble into viscoelastic interfacial films that encase droplets, imparting steric repulsion or mechanical strength to inhibit coalescence—the merging of droplets into larger ones. Selecting an appropriate emulsifier requires evaluating compatibility with the emulsion's phases to ensure rapid and complete interfacial coverage, as mismatches can lead to ineffective stabilization. In regulated sectors like food production, emulsifiers must secure approval, such as Generally Recognized as Safe (GRAS) status from the U.S. Food and Drug Administration, confirming their safety for intended use without extensive toxicity data. Environmental considerations are increasingly pivotal, with a shift toward biodegradable alternatives—like plant-derived proteins or biosurfactants—to reduce persistence in ecosystems and align with sustainability goals, addressing limitations of traditional synthetics.

Emulsification Mechanisms

Emulsification involves the mechanical or physicochemical disruption of immiscible liquids to form droplets of one phase dispersed in another, typically requiring emulsifiers to reduce interfacial tension and promote droplet breakup while aiding initial stabilization. The process balances disruptive forces against cohesive interfacial forces, with the goal of achieving desired droplet sizes and uniform distribution. High-energy and low-energy approaches differ in their reliance on external mechanical input versus internal phase behavior. High-energy methods apply intense mechanical forces to break larger droplets into smaller ones, often producing emulsions with droplet diameters ranging from nanometers to micrometers. High-pressure homogenization forces the through narrow valves or orifices under pressures of 500–2,000 bar (7,250–29,000 ), generating , shear, and that reduce droplets to 0.1–10 μm. This technique is widely used in industrial settings for its in creating fine, uniform emulsions. Ultrasonication employs high-intensity waves (typically 20 kHz) to induce acoustic , where collapsing bubbles generate localized shear and shock waves that disrupt droplets, yielding sizes as small as 100 nm depending on energy input and duration. Microfluidization directs the through fixed-geometry interaction chambers under (up to 20,000 ), where colliding streams produce intense shear forces in narrow channels, resulting in monodisperse droplets often below 1 μm and narrower size distributions compared to conventional homogenization. Low-energy methods leverage spontaneous or thermally induced phase changes, minimizing external mechanical input and relying on emulsifier properties for droplet formation. Phase inversion temperature (PIT) involves heating the mixture to alter the emulsifier's hydrophile-lipophile balance (HLB), causing a temporary inversion from oil-in-water to water-in-oil or vice versa, followed by rapid cooling to trap fine droplets (typically 20–200 nm) in the desired configuration. Spontaneous emulsification occurs via displacement, where a water-miscible containing oil and emulsifier is injected into a continuous aqueous phase, leading to rapid , , and into nano-sized droplets (10–100 nm) without significant agitation. Key process parameters influence droplet breakup and emulsion quality. Energy input governs disruption, quantified by the We=ρv2dγWe = \frac{\rho v^2 d}{\gamma}, where ρ\rho is the continuous phase , vv is the , dd is the droplet , and γ\gamma is the interfacial tension; values exceeding a critical threshold (often 4–12 depending on the ) enable inertial forces to overcome for breakup. The order of phase addition affects initial droplet formation: for oil-in-water emulsions, the oil (dispersed) phase is typically added gradually to the aqueous (continuous) phase under stirring to prevent coalescence. Scale-up from laboratory to industrial production presents challenges, including inconsistent mixing and leading to larger droplets or agglomeration, as well as difficulties in maintaining uniform energy dissipation across larger volumes, often requiring adjusted equipment like anchor stirrers to achieve turbulent flow ( >10,000).

Practical Applications

Food and Nutrition

Emulsions are integral to , enabling the incorporation of fats and oils into aqueous-based products to achieve desirable sensory attributes and nutritional benefits. In the , oil-in-water (O/W) emulsions predominate, dispersing hydrophobic within a continuous aqueous phase to form stable mixtures essential for everyday consumables. These systems not only influence texture and but also facilitate the delivery of essential nutrients, making them a cornerstone of processed and natural foods alike. Prominent examples of food emulsions include , a classic O/W system comprising 70–80% oil droplets stabilized by the in yolk, which acts as a natural emulsifier to prevent and ensure a smooth, viscous consistency. exemplifies a natural O/W emulsion, with approximately 3.5% present as globules ranging from 0.2 to 15 μm in diameter, enveloped by a phospholipid-protein that maintains dispersion in the aqueous serum phase. represents a more complex partially coalesced emulsion, where globules (typically 10–16% of the mix) adsorb to air-water interfaces during freezing, enabling overrun levels of 50–100% that impart lightness and creaminess to the final product. Emulsions enhance food by creating creamy textures and uniform distribution, which improve sensory appeal without altering core flavors, as seen in dressings and spreads. They also boost nutrient , particularly for fat-soluble vitamins such as A, D, E, and K, by increasing their and absorption in the through . Additionally, homogenization processes in products reduce globule size to below 1 μm, minimizing creaming and thereby extending by up to several weeks while preserving product uniformity. Nutritionally, low-fat emulsion formulations often employ modified starches, such as octenyl (OSA)-modified variants, to replicate the and stability of full-fat systems, allowing reduced content (e.g., 30–50%) without compromising texture in products like or sauces. Regulatory concerns arise with synthetic or allergen-containing emulsifiers; for instance, soy , a common additive derived from soybeans, can trigger IgE-mediated reactions in sensitive individuals due to residual soy proteins, prompting labeling requirements under frameworks like the U.S. Food Allergen Labeling and Consumer Protection Act. Post-2010 trends in the food sector emphasize clean-label alternatives, favoring natural emulsifiers like , , or unmodified starches to meet consumer demands for transparency and minimal processing, with market growth for such ingredients projected at 7–8% annually through 2035.

Healthcare and Pharmaceuticals

Emulsions play a vital role in healthcare and pharmaceuticals, particularly as vehicles for and nutritional support. Intravenous lipid emulsions, such as Intralipid, are widely used for in patients unable to receive adequate calories enterally, providing s and energy-dense formulations typically containing 10–30% oil from sources like or . Intralipid specifically consists of 20% emulsified with egg phospholipids and glycerin, administered via intravenous infusion to prevent deficiency. In topical applications, oil-in-water (O/W) emulsions form the basis of creams and lotions, such as moisturizers, where the external aqueous phase facilitates easy spreading and absorption while delivering oil-soluble active ingredients to the skin. Nanoemulsions enhance by improving the of poorly water-soluble compounds, such as , a chemotherapeutic agent with inherently low aqueous limiting its oral absorption. These submicron droplets (typically 20–200 nm) increase and permeation across biological barriers, achieving up to 70% oral for in optimized formulations compared to less than 8% for conventional preparations. Controlled release is another key benefit, achieved through droplet encapsulation where drugs are entrapped within emulsion cores, enabling sustained delivery and reducing dosing frequency; for instance, nanoemulsion-loaded alginate capsules have demonstrated prolonged release of lipophilic actives over hours to days. Advancements in emulsion technology include self-emulsifying systems (SEDDS), isotropic mixtures of oils, , and cosurfactants that spontaneously form fine O/W emulsions upon dilution in gastrointestinal fluids, significantly boosting oral of lipophilic drugs by up to 20-fold in some cases. Regulatory standards ensure safety for injectables, with the (USP <729>) stipulating that intravenous lipid emulsions must have a volume-weighted mean droplet below 500 nm and no more than 0.05% of fat globules exceeding 5 μm to minimize risks like . In vaccine development, squalene-based emulsions like MF59 serve as adjuvants in , enhancing immune responses; post-2020 research has elucidated mechanisms such as uric acid release from muscle cells to amplify immunogenicity, supporting broader applications in seasonal and pandemic flu formulations.

Firefighting and Safety

Emulsions play a critical role in , particularly through foam formulations designed for suppressing Class B fires involving flammable liquids such as and polar solvents. Aqueous film-forming foams (AFFF), a type of emulsion-based suppressant, utilize to create a thin aqueous that spreads across the surface, suppressing vapors and preventing ignition or re-ignition by acting as a physical barrier to oxygen. The primary mechanisms of AFFF involve emulsification, where the foam blanket incorporates oil into water droplets, smothering the fire by excluding air and providing evaporative cooling to reduce fuel temperatures below ignition points. This emulsification process disrupts the fuel's continuity, while the foam's structure enhances coverage over irregular surfaces like spill areas. For alcohol-resistant variants (AR-AFFF), designed for polar fuels, expansion ratios typically reach up to 8:1 when proportioned at 3% concentration, allowing efficient deployment with lower water volumes compared to standard foams. Environmental concerns have driven significant updates in AFFF composition, with per- and polyfluoroalkyl substances (PFAS)-based emulsifiers phased out due to their environmental persistence and risks. EU REACH regulations, including Commission Regulation (EU) 2025/1988 adopted in October 2025, restrict PFAS in firefighting foams, entering into force on October 23, 2025, with full prohibition applying from October 23, 2030, and labeling requirements starting October 23, 2026. Alternatives include protein-based foams, derived from natural hydrolysates for stable blanketing without fluorination, and fluorotelomer-based foams using shorter-chain C6 chemistries to reduce persistence while maintaining film-forming efficacy. These foams are increasingly applied in response scenarios, where rapid deployment suppresses ignited spills on water or land, minimizing spread and environmental damage.

Industrial and Chemical Uses

Emulsions play a pivotal role in , particularly through , a process where monomers are dispersed in an aqueous phase and polymerized using radical initiators to form stable particles. This method is widely used to produce paints, which consist of dispersions like acrylic or styrene-acrylic copolymers that provide and when applied to surfaces. Similarly, synthetic rubbers such as rubber (SBR) are manufactured via free radical in water, enabling the creation of high-molecular-weight with controlled particle sizes for applications in tires and adhesives. The aqueous environment in these processes facilitates heat dissipation and reduces , allowing for efficient production at industrial scales. In industrial applications, emulsions enhance recovery through surfactant-polymer (SP) flooding, where lower interfacial tension between and , and polymers increase solution to mobilize trapped in reservoirs. This technique can improve recovery factors by up to 25% in heterogeneous formations by forming stable emulsions that block high-permeability zones and redirect flow. Emulsions are also essential in fluids, typically oil-in- formulations that provide to reduce and cooling to dissipate heat during operations like milling and turning. These fluids prevent and improve by forming a lubricating film at the tool-workpiece interface while flushing away chips. Beyond these, emulsions find use in cosmetics as oil-in-water systems, such as lotions, where oil droplets are dispersed in a continuous aqueous phase to deliver moisturizing agents while ensuring a non-greasy feel and easy spreadability on skin. Asphalt emulsions, consisting of droplets stabilized in water with emulsifiers, are applied in road paving for tack coats and surface treatments, promoting between pavement layers and enabling cold-mix applications that reduce energy consumption. Recent eco-friendly innovations include bio-based emulsifiers derived from renewable sources like or biosurfactants, which are increasingly used in to stabilize dye dispersions, improve color uptake, and minimize environmental impact by replacing petroleum-based alternatives.

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