Emulsion

An emulsion (Latin ex and mulgēre, herausgemolken ' ) refers to a finely divided mixture of two normally immiscible liquids without any visible separation. Examples of emulsions are many cosmetics, milk, or mayonnaise.

• 4.1 surfactants ( emulsifiers)
• 4.2 solid stabilizers 4.2.1 solids properties
• 4.2.2 Advantages of a solids- stabilized emulsion

• 4.3.1 rotor -stator systems
• 4.3.2 Flow Mechanical means
• 4.3.3 Ultrasonic Generators
• 4.3.4 Micro-structured systems

Of the emulsion

In an emulsion, is a finely divided mixture of two liquids, such as oil and water, before. A liquid ( phase) forms small droplets dispersed in the other liquid. The phase forming droplets is called the internal phase or the disperse phase. The stage in which the droplets float is called external phase or continuous phase. Emulsions belong to the disperse systems and differ from mixtures of immiscible liquids, such as ethanol and water. Emulsions are usually opaque, milky liquids.

Emulsions of water and oil, a distinction in water-in -oil emulsion (W / O emulsion ) and oil -in- water emulsion ( O / W emulsion ). Another important component of the emulsions is an emulsifier (surfactant) which facilitates the formation of droplets and counteracts a separation ( phase separation).

Chemical viewing an emulsion

Many liquids can either thoroughly with water (they are hydrophilic) or can work well with oil (they are lipophilic) be miscible. Hydrophilic liquids form mainly of intermolecular forces in the form of hydrogen bonds. In contrast, lipophilic liquids mainly intermolecular van der Waals forces are formed. Are you a bit of oil in water, the oil will float. The above-mentioned forces can not train right between the two phases. The other hand forms an interfacial tension at the interface. The surface tension is the driving force to make the smallest possible interface, thus preventing the existence of an emulsion.

For the preparation and stabilization of an emulsion are surfactants, the surfactants (emulsifiers ), is necessary. The interfacial tension at the oil -water interfacial area is significantly decreased by the surfactant. They mediate between the two phases, and have a polar ( hydrophilic) and one non-polar ( lipophilic ) part. The polar part can form hydrogen bonds and bond with hydrophilic substances, while the non-polar part of the molecule forming van der Waals forces and combines with lipophilic substances.

Emulsions are still unstable systems, so they have a limited life. Carried out the so-called breaking of the emulsion since the size of the interface is reduced by coalescence of droplets into larger droplets (see Stability of emulsions). Butter, a water-in - fat emulsion is prepared from cream, a milk fat -in-water emulsion by emulsion breaking.

For additional stabilization of an emulsion hydrocolloids can be used as stabilizers. These materials increase the viscosity of the continuous phase, and thereby delay the breaking of the emulsion.

Although an emulsion of liquid exists, it can also be fixed and will then be called solid emulsion. If a suspension is emulsified, it is called a Suspo emulsion.

Physical viewing

The most important parameters in considering the phase volume ratio emulsions ( the ratio of the volume of the internal phase to each of the external ), the average particle size (Dm ) and the particle size distribution.

Phase volume ratio

To a phase volume ratio of 0.3 / 0.7 (30% internal phase, 70% of the outer phase ), the properties of the emulsion are largely dependent on the properties of the external phase. The droplets can move almost independently from each other in the outer phase, and the viscosity is approximately equal to that of the outer phase.

With increasing phase volume ratio, the properties of the inner phase clearly come more into play. If the volume fraction of the internal phase is too high, the phase angle can change. An O / W emulsion is inverted into a W / O emulsion and. One speaks of a so-called phase inversion. Inversion of an O / W emulsion can also occur by increasing the temperature, since higher temperatures weaken the hydrophilic interactions of the emulsifier with the water, so that the lipophilic interactions are relatively strengthened. Thus, an energetically more favorable situation in the system can find by the oil phase is the continuous phase in which the water phase is emulsified.

Droplet size

Emulsions are never monodisperse, rather the droplet sizes within a certain range are distributed (see also Dispersitätsanalyse ).

Therefore, can be in an emulsion higher space fulfillments realize, than would be possible in a monodisperse, hexagonal closest packing. Interstices between the larger droplets are thereby filled with smaller droplets.

The average particle diameter (Dm) in emulsions is normally between 100 nanometers and 1 millimeter. The larger the average particle diameter and the wider the particle size distribution, the greater the opacity of the milky-white emulsion. Emulsions such as milk act in reflected light bluish in transmitted light some clear red. Cause of this is the dependence of the scattering from the particle diameter (see Mie scattering, electromagnetic scattering objects in the order of the wavelength, and Rayleigh scattering).

Emulsions having a droplet diameter

• Greater than 1 micrometer is called macro-emulsions,
• In the range of 1 micrometer or less is called miniemulsion or submicron emulsion (which are thermodynamically stable )
• Below 100 nanometers called nanoemulsion

Stability of emulsions

Emulsions are thermodynamically unstable: The dispersed phase is anxious to unite by coalescence to form larger areas - while the interfacial energy between the two phases is reduced.

Emulsions are to be retained usually for a specific period ( from a few hours to several years ) and under certain conditions ( temperature, pH range ). Decomposes an emulsion, this is done in various stages which often occur simultaneously, however.

Preparation of Emulsions

By reducing the size of the droplets in the preparation of an emulsion at the interface between the two phases is increased. In this case, the surface tension must be overcome, and a new interface to be created. This requires work to be mechanically incorporated into the system. By shearing forces occurring while the droplets become smaller.

Surfactants ( emulsifiers)

Surfactants, which are often referred to as emulsifiers, the interfacial tension can be reduced drastically. The surfactant should also prevent the newly formed droplets coalesce again ( come together ). For this purpose, it must diffuse to the new interface as soon as possible. Synthetic surfactants create this in a few milliseconds. Large surfactant molecules, nor to significantly increase the viscosity (eg, starch ), require a few minutes to half an hour to completely envelop the new drop the. However, a higher viscosity also has a stabilizing effect, since the movement of the droplets and thus a possible coalescence is hindered.

An equally to be observed characteristic in the selection of a suitable surfactant is its Spreitungsgeschwindigkeit ( propagation speed ). The interface of a new droplet is initially only partially occupied by surfactant. This spreads now to that part of the interface, which was initially unassigned. So first a Tensidkonzentrations gradient at the interface, which is compensated depending on Spreitungsgeschwindigkeit more or less rapidly to a uniform Tensidverteilung. Because the concentration of the surfactant at the interface but is generally too low ( more interface requires more emulsifier) ​​, surfactant molecules must subsequently diffuse, to a maximum concentration is achieved.

Ordinarily the phase in which the emulsifier better dissolves the outer phase remains. With an HLB value of 3-6, the emulsion is a W / O emulsion, 8 to 18, an O / W emulsion ( Bancroft rule). The required amount of surfactant depends essentially on the desired droplet size ( smaller droplets more → surface → more surfactant) and the phase volume ratio. Almost always, the surfactant significantly above the corresponding Mizellbildungspunktes cmc (English: critical micelle concentration).

Solid stabilizers

An emulsion can be stabilized by the addition of certain solids. Mustard powder, for example, have long been used (in addition to taste reasons ) to stabilize mayonnaise. Solids -stabilized emulsions are often called after their discoverer Pickering emulsions - U. S. Pickering in 1907 showed that small particles that are better than wetted by oil, can stabilize O / W emulsions of water. Is important for the adequate stabilization that a mechanically stable solid film can form the dispersed phase.

Solid properties

It should fulfill the following properties of the solid:

• The solid should be a finely divided powder
• The solid particles should be packed as tightly as possible
• For the phase contact angle between water and oil on the particle must be true, otherwise the particles are either fully water or completely drawn into the oil phase and thus do not form a film on the surface longer.
• The particles should have a maximum surface roughness

In a phase contact angle of less than 90 ° caused a few exceptions O / W emulsions. If it is greater than 90 °, generally the W / O emulsions formed. If the phase contact angle exactly 90 °, then there is no curvature of the liquid meniscus. Experiments have shown, however, that this curvature plays an often underestimated role in the stability.

Advantages of solid- stabilized emulsion

Devices

To enter the labor required for emulsification in the media, there are a number of possible methods. Generally one but four different main groups can be distinguished.

Rotor -stator systems

Here, a component is moved ( or rotated ) and a component rests. Examples are:

• Fanta bowl and pestle
• Agitators
• Shaker
• Vibration mixer
• Emulsification
• Colloid mills
• Toothed-wheel dispersers

Mechanical flow means

• Flapper
• Vortex chambers
• High-pressure homogenizers, these consist of a pump and diaphragms or valves. This can for example be used atomizer.

Ultrasonic generators

Here the droplets are broken by an ultrasonic transducer and a horn.

Microstructured systems

Examples include membranes.

Microemulsions

Microemulsions are water-oil -surfactant mixtures that are thermodynamically stable in contrast to other emulsions. They are optically transparent and formed without the production of emulsions for the otherwise required high energy input. Generally used to represent one of a microemulsion co-surfactants or co-solvents. Microemulsions form only in certain regions of the phase diagrams of the ternary and quaternary material systems also.

Multiple emulsion

There are also multiple emulsions (W / O / W or O / W / O).

Applications are the stuff inclusion in the inner W or O - phase and the controlled release of specific biologically active substances from the inner phase in the food, cosmetic and pharmaceutical industries. Further, these can be used on the proportion of an aqueous phase in the O phase for the production of reduced-fat food emulsions.

Multiple emulsions can be used, inter alia, to the liquid membrane permeation ( a special form of the extraction), in the middle phase (membrane phase) acts as a filter between the inner and outer phase. The liquid membrane permeation is also used for the extraction of heavy metal traces of waste water, this is, for example, sulfuric acid in an oil phase, in dissolved form, the chelating agent, is emulsified and this emulsion is in turn emulsified in the waste water containing heavy metals is achieved by liquid-liquid extraction in the oil phase and it also converted by liquid-liquid extraction in the sulfuric acid. After separation of the oil phase, the acid -in- oil emulsion is broken in a high frequency AC field.

Photo emulsion

In the photograph is called the force applied to a substrate photosensitive layer commonly called photo emulsion. As mentioned before but it involves no emulsion, but a frozen suspension.