How does the emulsification work
Under one emulsion one understands a finely divided mixture of two different (normally immiscible) liquids without visible separation. Examples of emulsions are numerous cosmetics, milk, and mayonnaise.
Structure of the emulsion
An emulsion is a finely divided mixture of two liquids such as oil and water. One liquid (phase) is distributed in small droplets in the other liquid. The phase that forms droplets is called inner phase or disperse phase. The phase in which the droplets "swim" becomes outer phase or continuous phase called. Emulsions therefore belong to the disperse systems and therefore differ from completely miscible liquids, such as ethanol and water. Emulsions are usually cloudy, milky liquids.
Depending on which phase forms the droplets, one speaks of one Water-in-oil emulsion (W / O emulsion) or one Oil-in-water emulsion (O / W emulsion). Another and important component of all emulsions is Emulsifier (= Surfactant), which facilitates the formation of droplets and counteracts segregation (phase separation).
Chemical consideration of an emulsion
Many liquids can either be well miscible with water (they are hydrophilic) or can be well miscible with oil (they are lipophilic). Hydrophilic liquids mainly form intermolecular forces in the form of hydrogen bonds. At lipophilic liquids On the other hand, mainly intermolecular van der Waals forces develop. If you put some oil in water, the oil will float up. Between water and oil there is one as small as possibleInterface formed. (Fig.1a) The above-mentioned forces cannot develop properly between the two phases. In contrast, an interfacial tension develops at the interface. The interfacial tension is the driving force as small as possible To form interface and thus prevents the existence of an emulsion.
To produce and stabilize an emulsion, surface-active substances, the surfactants (= emulsifiers), are necessary. The interfacial tension at the oil-water phase interface is significantly reduced by the surfactant. They mediate between the two phases and have a polar (hydrophilic) and a non-polar (lipophilic) part. The polar part can form hydrogen bonds and bond with hydrophilic substances, while the non-polar part of the molecule develops van der Waals forces and bonds with lipophilic substances.
Nevertheless, emulsions are unstable systems and have a limited lifespan. The so-called "breaking of the emulsion" occurs because the size of the interfaces is reduced by the coalescence of droplets into larger droplets. (-> stability of emulsions)
The most important parameters when considering emulsions are Phase volume ratio (the quotient of the volume of the inner phase to each of the outer) that mean particle size (Dm), as well as the Particle size distribution.
Phase volume ratio
Up to a phase volume ratio of 0.3 / 0.7 (30% inner phase, 70% outer phase), the properties of the emulsion depend essentially on the properties of the outer phase. The droplets can move almost independently of one another in the outer phase and the viscosity also roughly corresponds to that of the outer phase.
As the phase volume ratio increases, the properties of the inner phase become significantly more important. If the volume fraction of the inner phase becomes too high, the phase position can change. An O / W emulsion becomes a W / O emulsion and vice versa. One speaks of a so-called Phase inversion. Inversion of an O / W emulsion can also occur due to an increase in temperature, since higher temperatures weaken the hydrophilic interactions of the emulsifier with the water, so that the lipophilic interactions are relatively strengthened. In this way, an energetically more favorable situation can be found in the system in that the oil phase forms the continuous phase in which the water phase is present in emulsified form.
See also: Highly concentrated emulsions
Emulsions are never monodisperse, rather the droplet sizes are distributed within a certain range (see also dispersity analysis). This is why more space can be filled in an emulsion than would be possible in a monodisperse, hexagonal close packing. Gaps between the larger droplets are filled with smaller droplets. The mean particle diameter (Dm) in emulsions is normally between 100 nanometers and 1 millimeter. The larger the mean particle diameter and the broader the particle size distribution, the greater the milky-white haze of the emulsion. Emulsions such as milk have a bluish tinge in incident light and sometimes clearly red in transmitted light.
Stability of emulsions
Emulsions should usually remain stable for a certain period of time (between a few hours and a few years) and under certain conditions (temperature range, pH range). If an emulsion breaks down, this happens in individual, but often simultaneous, phases.
- Phase: Stable emulsion
for example fat droplets are dispersed in the outer phase (water).
- Phase: Creaming or sedimentation (reversible)
Due to the force of gravity, the mixed phases are separated into the specifically lighter and the specifically heavier.
- Phase: Ostwald ripening (ripening)
- Phase: Aggregation (reversible)
the fat droplets form aggregates, the particle diameter is increased and, according to Stokes' law, the sedimentation speed of the dispersed fat droplets increases.
- Phase: Coalescence
the fat droplets combine; in extreme cases this can lead to breakage of the emulsion.
Manufacture of emulsions
By reducing the size of the droplets in the production of an emulsion, the interface between the two phases increases. The interfacial tension must be overcome and a new interface created. This requires work that must be mechanically introduced into the system. The resulting shear forces make the droplets smaller and smaller.
Surfactants, which are often also referred to as emulsifiers, can drastically reduce interfacial tensions. The surfactant is intended to prevent the newly created droplets from coalescing again (= flowing together). To do this, it has to diffuse as quickly as possible to the new interface. Synthetic surfactants do this in a few milliseconds. Large surfactant molecules that also significantly increase the viscosity (e.g. starch) need a few minutes to half an hour to completely envelop the new drop. However, a higher viscosity also has a stabilizing effect, since the movement of the droplets and thus the possibility of coalescence is made more difficult.
Another property to be considered when choosing a suitable surfactant is its spreading speed (= spreading speed). The interface of a new drop is initially only partially covered by surfactant. This now spreads to that part of the interface that was initially unoccupied. This initially creates a surfactant concentration gradient at the interface, which, depending on the spreading speed, is balanced out more or less quickly to form a uniform surfactant distribution. Because the concentration of the surfactant at the interface is too low overall (larger interface requires more emulsifier), surfactant molecules have to diffuse again until a concentration maximum is reached.
Usually the phase in which the emulsifier dissolves better remains the outer phase. With an HLB value of 3-6 the emulsion becomes a W / O emulsion, from 8-18 an O / W emulsion (Bancroft rule). The required amount of surfactant essentially depends on the desired droplet size (smaller droplets -> more surface -> more surfactant) and the phase volume ratio. The surfactant concentration is almost always well above the corresponding micelle formation point cmc (critical micelle concentration).
An emulsion can also be stabilized by adding certain solids. Mustard powder has long been used to stabilize mayonnaise, for example. Solid-stabilized emulsions are named after their discoverer S.U. Pickering (who showed in 1907 that small particles wetted better by water than oil can stabilize O / W emulsions) often Pickering emulsions called. For adequate stabilization it is important that a mechanically stable solid film can form around the dispersed phase.
The following properties of the solid should be fulfilled:
- the solid should be a finely divided powder
- the solid particles should be packed as closely as possible
- for the phase contact angle between water and oil on the particle surface must apply , because otherwise the particles will either be drawn completely into the water or completely into the oil phase and thus no longer form a film on the surface.
- the particles should have as rough a surface as possible
With a phase contact angle of less than 90 °, O / W emulsions are formed, with a few exceptions; if it is greater than 90 °, W / O emulsions are usually formed. If the phase contact angle is exactly 90 °, there is no curvature of the liquid meniscus. Experiments have shown that this curvature plays an often underestimated role for stability.
Advantages of a solid stabilized emulsion
- the emulsion is usually more resistant to changes in the chemical environment (pH value, salt concentration, etc.)
- the surfactant concentration in the emulsion can be greatly reduced
- other emulsifiers can be used than in a conventional emulsion
- the phase position can be opposite to that of a conventional emulsion of the same composition
- the rheological properties of the emulsion can be significantly changed. (Newtonian, non-Newtonian, with or without yield point)
There are a number of possible methods of entering the work required for emulsification into the medium:
- Fantasy bowl and pestle
- fast agitators
- High pressure homogenizers
- Vibratory mixer
- Ultrasonic generators
- Emulsifying centrifuges
- Colloid mills
There are no particularly small droplets in a microemulsion, but rather a water-oil-surfactant mixture which, unlike other emulsions, is thermodynamically stable. They are optically transparent and are formed without the high energy input otherwise required for the production of emulsions. Usually you need cosurfactants or cosolvents to produce a microemulsion. Microemulsions can only arise in certain areas of the phase diagrams for three or four components. Like conventional emulsions, they can change when the phase volume ratio changes.
There are also multiple emulsions (W / O / W or O / W / O). Multiple emulsions can be used, among other things, for liquid membrane permeation, in which the middle phase (membrane phase) serves as a filter between the inner and outer phases.
In photography, the light-sensitive layer applied to a substrate is commonly referred to as a photo emulsion. In the sense described above, however, this is not an emulsion, but a solidified suspension.
- Schubert H., Emulsification Technology, Behr's Verlag, Hamburg, 2005, ISBN 3-89947-086-9
- Lagaly G., Schulz O., Zimehl R., dispersions and emulsions, Steinkopff Verlag, Darmstadt, 1997, ISBN 3-7985-1087-3
- Dobiáš, B., Emulsions (Bd1, Bd2), Tenside Detergents, 1978, 1979
- Asche H. (Ed.), Technology of ointments, suspensions and emulsions. An APV seminar from September 20-22, 1982 in Darmstadt, Wissenschaftlich Verlagsgesellschaft, Stuttgart, 1984
Categories: Dispersion (Chemistry) | Soft matter
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