Emulsion Definition and Examples

Mixing Fluids That Don't Normally Mix

If you shake together oil and water, you get an emulsion.
If you shake together oil and water, you get an emulsion. ramoncovelo / Getty Images

Emulsion Definition

An emulsion is a colloid of two or more immiscible liquids where one liquid contains a dispersion of the other liquids. In other words, an emulsion is a special type of mixture made by combining two liquids that normally don't mix. The word emulsion comes from the Latin word meaning "to milk" (milk is one example of an emulsion of fat and water). The process of turning a liquid mixture into an emulsion is called emulsification.

Examples of Emulsions

  • Oil and water mixtures are emulsions when shaken together. The oil will form drops and disperse throughout the water.
  • Egg yolk is an emulsion containing the emulsifying agent lecithin.
  • Crema on espresso is an emulsion consisting of water and coffee oil.
  • Butter is an emulsion of water in fat.
  • Mayonnaise is an oil in water emulsion that is stabilized by the lecithin in egg yolk.
  • The photosensitive side of photographic film is coated with an emulsion of silver halide in gelatin.

Properties of Emulsions

Emulsions usually appear cloudy or white because light is scattered off the phase interphases between the components in the mixture. If all of the light is scattered equally, the emulsion will appear white. Dilute emulsions may appear slightly blue because low wavelength light is scattered more. This is called the Tyndall effect. It's commonly seen in skim milk. If the particle size of the droplets is less than 100 nm (a microemulsion or nanoemulsion), it's possible for the mixture to be translucent.

Because emulsions are liquids, they don't have a static internal structure. Droplets are distributed more or less evenly throughout a liquid matrix called the dispersion medium. Two liquids can form different types of emulsions. For example, oil and water can form an oil in water emulsion, where the oil droplets are dispersed in water, or they can form a water in oil emulsion, with water dispersed in oil.

Further, they can form multiple emulsions, such as water in oil in water.

Most emulsions are unstable, with components that won't mix on their own or remain suspended indefinitely.

Emulsifier Definition

A substance that stabilizes an emulsion is called an emulsifier or emulgent. Emulsifiers work by increasing the kinetic stability of a mixture. Surfactants or surface active agents are one type of emulsifiers. Detergents are an example of a surfactant. Other examples of emulsifiers include lecithin, mustard, soy lecithin, sodium phosphates, diacetyl tartaric acid ester of monoglyceride (DATEM), and sodium stearoyl lactylate.

Distinction Between Colloid and Emulsion

Sometimes the terms "colloid" and "emulsion" are used interchangeably, but the term emulsion applies when both phases of a mixture are liquids. The particles in a colloid can be any phase of matter. So, an emulsion is a type of colloid, but not all colloids are emulsions.

How Emulsification Works

There are a few mechanisms that may be involved in emulsification:

  • Emulsification may occur when the interfacial surface tension between two liquids is reduced. This is how surfactants work.
  • An emulsifier may form a film over one phase in a mixture to form globules that repel each other, allowing them to remain evenly dispersed or suspended.
  • Certain emulgents increase the viscosity of the medium, making it easier for the globules to remain suspended. Examples include the hydrocolloids acacia and tragacanth, glycerine, and the polymer carboxymethyl cellulose.