Under normal conditions the phases of the emulsion are immiscible, unstable. The phases will separate if no energy is applied to them, such as continuous stirring, or if an emulsifier is not added to keep them stable. Otherwise, the emulsion would “cut”.
Emulsions are widely and frequently used in all fields of industry and daily life. There are medicines, creams, personal hygiene lotions, soaps and detergents, among other examples, which consist of emulsions.
Characteristics of a chemical emulsion
They are formed by immiscible liquids
Emulsions are colloids formed by mixing two or more liquids that are mutually characterized by being immiscible or insoluble in each other.
They contain two phases
A dispersed phase and a continuous one. The boundary between these phases is called the interface.
They present the Tyndall effect
Emulsions exhibit the physical phenomenon called the Tyndall effect. This is responsible for the dispersant phase particles scattering light, giving the emulsions a cloudy appearance. That is why it is easy to observe the suspended particles in the liquid medium.
Emulsions contain a mixture of inhomogeneous liquids. Therefore, they tend to have a cloudy appearance due to the aforementioned Tyndall effect.
When the emulsions are white, it is because all the light is scattered in equal proportions. Otherwise, its colors depend to a great extent on how its particles absorb radiation from the visible spectrum.
If they do not contain an emulsifying agent, emulsions tend to separate and, being liquid, their internal structure is not static: they exhibit Brownian motion. When the two liquid-liquid phases are separated, they are no longer emulsions and the molecules of the mixture can be rearranged into two distinctive and independent phases.
Emulsions can be separated into their two phases by deactivating the emulsifying agent, subjecting the mixture to heating, freezing, or centrifugation. A demulsification is said to take place.
A chemical emulsion can be formed naturally, spontaneously, or by the mechanical means of stirring the mixture of two or more immiscible or poorly soluble liquids.
The emulsion can be kept stable by adding an emulsifying agent or surfactant, which confers mechanical stability to the mixture; for example, egg yolk in mayonnaise, or casein in milk.
The emulsifier is characterized by being amphipathic, that is, by having a hydrophilic end of the molecule (soluble in water) and another hydrophobic end (insoluble in water). This allows them to be soluble in both water and oil, making it easier for them to interact with the dispersed phase and the continuous phase at the same time.
Emulsifiers can form a film around the dispersed phase droplets, preventing them from coalescing and coalescing.
Phases of a chemical emulsion
Emulsions contain two phases: a dispersed and a continuous one.
It is present in drops immersed in the continuous phase. Said drops can be amorphous, spherical, crystalline or a mixture of these. Their sizes are very tiny, even ultramicroscopic, since their diameter varies between 10 nm (nanometers) to 100 µm (microns).
There are large droplets that exceed the size limits of colloidal particles. However, the size of the droplets is smaller than that of the particles in the suspensions.
The continuous phase is the dispersion medium, analogous to the solvent in a solution with the solute.
An example would be mixing a little oil with water. When stirred, an emulsion forms in which oil is the dispersed phase, while water is the continuous phase.
Consider for example the mixture of two liquids I (blue) and II (yellow).
In A (see image), both phases I and II are separated. When stirred, emulsion B is formed, where phase II is dispersed in the larger phase I. After a while the phases separate, giving rise to state C. However, if an emulsifier is added, we will have a more stable emulsion, seen in D.
Types of chemical emulsions
Emulsions can be classified according to the properties of their two phases.
It is one in which its dispersed phase is a liquid in a solid dispersing medium. As an example we have cheese and jellies.
Its dispersed or solute-like phase is liquid, which is contained in a dispersing medium that is also liquid, such as oil-in-water or water-in-oil.
Oil / water emulsion
The dispersed phase is an organic material and the continuous phase is water or an aqueous solution. As an example is milk.
Water / oil emulsion
The dispersed phase is water or an aqueous solution, while the continuous phase is an organic liquid, such as an oil. Some examples of this emulsion include butter and margarine.
As solutions, gels, or suspensions such as photographic emulsions that contain small crystals in a gelatin gel.
Microemulsions and Nanoemulsions
When the size of the droplets is less than 100 nm (nanometers), this gives them a translucent appearance and they are generally more stable.
Microemulsions form spontaneously. They may contain surfactants. They are used to administer vaccines during immunization processes.
Meanwhile, nanoemulsions need specialized equipment for their production. An example is soybean oil, used in the elimination of microbes.
Examples of chemical emulsions
Examples of emulsions include:
The chemical emulsion can also be a mixture of a liquid in a solid, such as cheese.
Like vinaigrettes, which are a mixture of oil and vinegar. Mayonnaise is also an oil and egg emulsion.
Homogenized milk is an emulsion of fat molecules or droplets in aqueous solution, which contains other components, such as milk protein or colloidal casein, forming micelles. Casein is a secreted biomolecular condensate.
Butter is an example of a water / oil type emulsion, because it is a mixture that contains water droplets in about 80% milk or cream fat.
Medicines and cosmetics
Many creams, balms, ointments, among others, are emulsions used as medicines, therapeutic products or applied for cosmetic purposes.
Used as an extinguishing agent in some fire extinguishers, it contains an emulsion of water with a foaming agent.
Emulsifiers are used for the dispersion of polymers, in the manufacture of plastics, synthetic rubbers, paints and glues. There is also the photographic emulsion, present on the photosensitive side of the film.