Making Emulsions in the Classroom

Fig.1 - Water and Oil Don't Mix

Oil and water do not dissolve in each other, a phenomenon which can be observed if a mixture of the two is left to stand for a short amount of time (see figure 1). The oil, being less dense, floats on top of the water, forming an upper layer. Students can observe the difference between these two layers by adding a few drops of food colour to such an oil-water mixture. Because it is insoluble in oil but soluble in water, the food colour clings together as distinct drops as it moves through the oil layer and then dissolves to form an even colour when it reaches the water layer.

When such a mixture of oil and water is shaken, drops of oil are suspended evenly throughout the preparation for a while, but the original layers eventually re-form. The addition of substances known as emulsifiers can, however, maintain the stability of suspensions and prevent them from settling back into layers.

Making Hair Cream – Materials and Teaching Method

Fig. 2 - Hair Cream

In both creams and ointments, the emulsifying agent is often a type of detergent, which acts to stabilise the interface between the oil and water in the mixture. Detergents have molecules with both hydrophilic (water loving) and hydrophobic (water hating) components. This results in one side of the detergent molecule binding to water and the other side binding to the oil.

In this activity, an emulsion of water and paraffin oil will be stabilised using soap flakes as the emulsifying agent. Soap flakes can be obtained from several companies, including Dri-Pak and Lux, while paraffin oil can be purchased from some hardware stores. Students will produce a thick, white cream that can be used on the hair "Elvis-style," or used as a hand cream.

The following materials and equipment are required per group of around four students:

•                ½ teaspoon soap flakes

•                10 ml hot water

•                30 ml paraffin oil

•                Plastic cup

•                Teaspoon or stirring rod

Students should be instructed to copy down the following directions, which should be followed by a teacher-led explanation.

1.             Mix the soap flakes with the hot water in a plastic cup. Stir until the soap has dissolved.

2.             Add the paraffin oil, stirring continuously.

3.             Keep stirring for the next 15 minutes or until the emulsion has completely cooled. By this time it should have formed into a thick, creamy, white mixture that resembles many commercially available preparations (see figure 2).

The following questions could be written on the board after students write up the experiment and their observations:

•                What is an emulsion?

•                Name the two liquids present in this emulsion.

•                Explain why the soap flakes act as an emulsifying agent in this mixture.

Making Mayonnaise – Materials and Teaching Method

Fig. 3 - Mayonnaise

Mayonnaise is really salad dressing with egg yolk added as an emulsifying agent (see figure 3). The protein lecithin in the egg acts to maintain the stability of the oil and water suspension by coating the oil droplets so that they can’t settle out again into an upper layer. Note that technique is important in this activity: if the oil is added too quickly it will not be properly coated by the lecithin and curdling (separation of the oil and water) will result.

The following materials and equipment are required per group of approximately four students:

•                250 ml olive oil

•                1 egg yolk

•                1 lemon

•                A small amount of water

•                A pinch of salt and pepper

Students should be instructed to copy down the following directions, which should be followed by a teacher-led explanation.

1.             Use a whisk to mix together the egg, lemon, salt and pepper.

2.             Slowly whisk in the oil.

3.             The mixture should start to thicken. If it becomes too thick a small amount of additional water can be mixed in.

Using a whisk will help to aerate and add bulk to the mixture. Students should not be concerned if the mayonnaise is not as thick as commercial mayonnaise – this is the way it should be and is in fact the consistency preferred by many chefs.

The following questions could be written on the board after students write up the experiment and their observations:

•                Name the two liquids suspended evenly throughout the mayonnaise.

•                Which ingredient acts to emulsify these two liquids?

•                Why does the oil need to be added slowly when making mayonnaise?

Emulsions Follow-Up Activities

Students could research information on the use of other emulsions in the food and pharmaceutical industry and attempt to recognise that emulsions belong to a larger group of suspensions known as colloids, which include preparations where one of the two phases can be a solid. Colloids such as paint, for instance, can be made in the classroom using chalk powder, glue and water.

References

CSIRO, 2006. "An Oily Problem." (accessed 23/7/2010).

Lechner, A., "Soaps and Emulsions." St. Louis University School of Medicine. (accessed 20/7/2010).