What Causes the Cleansing Action of Soap?

Soap has been used as a cleaning agent for thousands of years. Soap helps to lift grease from clothing, skin, and other materials. Soap is made by combining alkali, such as sodium hydroxide, with oils or fats. The substances react to form a sodium salt of the fatty acids in a process known as saponification. The chemical structure of soap is the secret behind its cleansing action.

Soap Chemistry

Soap is formed when fat and sodium hydroxide reacts to form glycerol and a sodium salt of fatty acids. The fatty acid salt molecule has a long chain hydrophobic (water hating) hydrocarbon tail at one end and a hydrophilic (water loving) head at the other end. As a result, one end of the soap molecule is attracted to water, and the other end is attracted to non-polar molecules such as oil and grease.

Oil and Water

As everyone knows, oil and water do not mix. However, when soap is added to the mixture, the hydrophilic end is attracted to water molecules while, at the same time, the hydrophobic end is attracted to oil and grease molecules. As more soap molecules are attracted to the oil layer, eventually a group of soap molecules surround little clumps of oil, with the tails pointing into the oil and the heads pointing outward. These tiny clumps form an emulsion of oil particles which are suspended in the water. If the oil is on a surface such as an article of clothing, the soap creates an emulsion on the surface which can be more easily washed away.

Types

The length of the molecule varies according to the type of fatty acid used to make it. The length and type of fatty acid chain tail determines the properties of the soap. For example, animal fat has a longer hydrocarbon chain and produces soap that is very hard and relatively difficult to dissolve in water. Alternatively, coconut oil has a shorter hydrocarbon chain and produces soap that is softer and lathers more easily. Soap can be made with a combination of oils to create a product with the desired properties.

Soap Scum

Soap is not perfect. "Hard" water contains relatively high levels of calcium and magnesium ions, which bind to the soap molecules to form an insoluble compound. This compound eventually precipitates and falls out of solution, forming what is commonly known as "soap scum". Synthetic detergents can be used to avoid this problem. Detergents have a different chemical structure that does not bind and precipitate with calcium and magnesium ions.

References

• Elmhurst College Virtual Chembook
• Chemistry Explained