About Emulsions

Students learn to explain that soap, water and oil together form an emulsion with the soap acting as an emulsifier.

Possibly the most important industrial role for surfactants is the formation of emulsions. An emulsion is a dispersion of one liquid in a second, immiscible liquid. If you ever shaken up salad dressing, you have made emulsion. Milk and cream are emulsions, as are medicinal creams such as moisturisers.

Emulsions are multiphase systems, even though they often look like they are just one phase. The phases in an emulsion are normally called the continuous phase and the disperse phase.

When you shake in emulsion very hard, you make the droplet size smaller. Shaking is an inefficient way of making an emulsion; when you shake oil and water with some surfactant you usually make an emulsion in which you can still see the individual droplets. The droplets formed by shaking a normally 100 to 500 µm in size, and are visible to the naked eye (1000 µm = 1 mm). Using a proper emulsion mixer it is possible to achieve a droplet size of 100 to 1000 nm (1000 nm = 1 µm).

The small droplets in an emulsion scatter the light passing through it. The result is that the emulsion appears to be either an opaque grey or white. This effect is similar to a bowl of sugar - each individual grain is transparent (just like an emulsion droplet), but a collection of grains appears white.

Light is scattered from an emulsion as it passes through it. If the droplets are reflective, then the scattering is as depicted above. If the droplets are not reflective, then light will be scattered by refraction instead.