An emulsion is a system that has at least one immiscible liquid distributed in another liquid, or solid, in the form of tiny droplets. It is an unstable system that is made more stable by the presence of a surfactant. Emulsions have been used by humans for centuries and continue to find application in a variety of industries.
Emulsions are typically made up of a hydrophilic, aqueous portion and a hydrophobic, oil portion. These two portions constitute the internal and external phases of the emulsion. The internal phase is composed of the tiny dispersed droplets, while the external phase is made up of the rest of the materials. The more abundant phase is called the external or continuous phase. When water is the external phase and oil is dispersed, it is known as an oil-in-water emulsion (o/w). If water is the internal phase it is called a water in oil emulsion (w/o). Many factors are responsible for determining whether an emulsion is of the o/w or w/o type, including the concentration of the components, the type of emulsifier, and processing steps.
More complex emulsion systems are possible when there are numerous internal phases. These systems are called multiple emulsions and can be described as emulsion in emulsions. For instance, if water is dispersed in an oil and then further dispersed in another water phase, the system is described as a water in oil in water (w/o/w) emulsion. Other multiple emulsions can have many times this number of internal phases.
The particles that make up the emulsion internal phase are polydisperse, meaning they have variable sizes. Emulsions can be classified by the size of these particles. When the average diameter is less than 100Å the system is called a micellar emulsion. A microemulsion has a particle diameter of 100-2,000Å. Systems with larger sized particles are known as macroemulsions.
Typical emulsions have an oil phase and an aqueous phase. The oil phase is composed of non-polar materials that are not soluble in water. These include materials such as fats, oils and waxes. The aqueous phase of an emulsion is made up water and all of the other hydrophillic materials in the system.
Compounds that stabilize emulsions are called emulsifiers. Also known as surfactants, these materials help stabilize the system by reducing the interfacial tension between the two phases. Most emulsifiers have a molecular structure that includes a lipophilic portion and a hydrophilic portion. These materials are classified by the nature of their hydrophilic portion. They can be either anionic, cationic, nonionic, or amphoteric emulsifiers. An example of an anionic emulsifier is stearic acid. It is made up of a number of carbon molecules that are lipophilic attached to a hydrophilic carboxylic acid head group.
When an emulsifier is put into water the molecules have a tendency to align themselves in a manner that reduces the interaction between its hydrophilic and lipophilic ends. If enough emulsifier is present spherical structures called micelles form. These micelles are particles that have the lipophilic tails oriented toward the center and hydrophilic heads on the outer surface.
By themselves, oil and water will not mix. If they are combined in a container and shaken, the oil breaks up into smaller particles and may be dispersed momentarily. However, after the agitation is stopped, the dispersed oil particles quickly separate from the water. When an emulsifier is present, the oil particles are stabilized because they become incorporated into the interior part of the micelles. The oil particles are shielded from the water and each other. This prevents them from combining and separating with the water. The result is a more stable system.
Although emulsifiers help stabilize the interaction between the oil phase and the water phase, emulsions are still inherently unstable according to the second law of thermodynamics because the entropy of the system would be greater if all molecules were dispersed randomly. One of the basic facts about emulsions is that given enough time or energy they will separate into the original phases. The speed and efficiency at which this occurs depends on the composition of the emulsion. For example, a system of mineral oil and water forms an emulsion when agitated that immediately separates completely upon standing. If a small amount of an emulsifier is added, the system may remain stable for a few days. Other emulsions can remain stable for years.
Examples of emulsions are found throughout nature. For instance, homogenized milk is an emulsion made up of butterfat droplets dispersed in water. The emulsifier is a protein called casein. Yogurt, ice cream, and whipped cream are also emulsions. Emulsions are also used for making various products. They are the most common type of delivery system used for cosmetic products, enabling a wide variety of ingredients to be quickly and conveniently put on hair and skin. Additionally, emulsions are used for things such as cleaning products, paints, coatings, adhesives, inks, and dyes.
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