How Soap Cleans

Water is often described as a universal solvent, but anyone who has tried to wash greasy hands under a running tap knows that this description has its limits. Water readily dissolves salt and sugar, yet it slides straight off oil and fat without lifting them away. The reason lies in the way water molecules behave. Each water molecule carries a slightly positive charge at one end and a slightly negative charge at the other, so the molecules cling to one another and to other charged substances. Oils and fats, by contrast, carry no such charges. They are described as non-polar, and because water and oil have such different characters they refuse to mix. To clean away oily dirt, then, something is needed to bridge the gap between the two, and that something is soap.

Soap belongs to a family of substances that chemists call surfactants, a word formed from the phrase surface-active agents. The secret of a surfactant lies in the unusual shape of its molecules. Each soap molecule has two very different ends. One end is attracted to water and is described as hydrophilic, meaning water-loving. The other end is a long chain that is repelled by water but attracted to oils and fats, and this end is described as hydrophobic, meaning water-fearing. A single molecule, in other words, is friendly to water at one end and friendly to grease at the other. This double nature is the key to how soap works.

When soap is added to water that contains greasy dirt, the molecules arrange themselves so that each end can be where it is most comfortable. The water-fearing tails bury themselves in the droplets of oil, while the water-loving heads remain pointing outward into the surrounding water. As more and more soap molecules gather around a droplet of grease, they surround it completely, forming a tiny ball with the oil trapped safely inside and a water-friendly surface on the outside. These little spheres are known as micelles. Because the outside of each micelle is attracted to water, the whole package can now be carried away in the rinse, taking the oily dirt with it. Dirt that water alone could never shift is thus lifted from the skin or the fabric and washed down the drain.

Surfactants help cleaning in a second way as well. The surface of water behaves as though it were covered by a thin, taut skin, an effect known as surface tension that is caused by water molecules pulling strongly on one another. This tension makes it hard for water to spread into the tiny crevices of a fabric or to wet a greasy surface at all. Soap weakens these attractions between water molecules, lowering the surface tension so that the water spreads out and soaks into materials far more easily. Wetter water, so to speak, reaches into places that plain water would simply bead upon and roll away from.

Traditional soap has been made for thousands of years by heating fats or oils together with a strong alkali, in a reaction known as saponification. The process splits the fat and produces the soap molecules, each with its water-loving head and grease-loving tail. Although the method is ancient, the chemistry it relies on was not understood until comparatively recent times. For most of history people simply knew that the mixture worked, without knowing why.

Ordinary soap does have a weakness. In areas where the water supply is described as hard, it contains dissolved minerals, particularly compounds of calcium and magnesium. These minerals react with soap to form an insoluble grey substance, familiar as the scum that clings to baths and basins. The scum wastes soap and leaves deposits on skin and clothing. To overcome this problem, chemists have developed synthetic surfactants, often called detergents, which are built to do the same job as soap but do not form scum in hard water. Most modern washing products rely on these engineered molecules rather than on traditional soap.

Whether the cleaning agent is an ancient bar of soap or a modern liquid detergent, the underlying principle is the same. A cleaning molecule must be able to hold hands with water on one side and with grease on the other, pulling the two together long enough for the dirt to be surrounded, lifted and rinsed away. Once this simple idea is grasped, a great deal of everyday chemistry falls into place, from the foam in a kitchen sink to the sprays used to clean up oil spills at sea. Cleaning, at heart, is a matter of persuading two substances that naturally avoid each other to meet.