Emulsion

An emulsion is a

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.

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).^[2]

Appearance and properties

Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface".^[3] 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".^[4] 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.^[5]

Two special classes of emulsions –

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.[9]

Instability

Emulsion stability refers to the ability of an emulsion to resist change in its properties over time.^[10][11] 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.^[12] 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.^[10] 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.^[13] Sedimentation is the opposite phenomenon of creaming and normally observed in water-in-oil emulsions.^[3] 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.^[14] 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.^[13]

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.^[15]

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.^[16] 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).^[17] 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.^[18]

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

Emulsifiers

An emulsifier is a substance that stabilizes an emulsion by reducing the oil-water

• Egg yolk – in which the main emulsifying and thickening agent is lecithin
  .
• Mustard^[21] – 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
• 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

A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification:[3]

• 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

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), an unstable emulsion results

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

• Butter – an emulsion of water in butterfat
• Margarine

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

In

• 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.^[28]
• 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.^[29]
• 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.^[30]
• 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.^[31]
• 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.^[32]
• 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.^[33]
• 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.^[33]
• 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.^[34]

In firefighting

Emulsifying agents are effective at extinguishing fires on small, thin-layer spills of flammable liquids (

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

References

Other sources

• Philip Sherman;
  ISBN 9780080102900
  .
• Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H.S., Ed.; Academic Press: New York, NY, USA, 2000; Volume 5, pp. 501–575

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