Nathan Smith, of Ardgowan School, asks :-

What is in soap to make it so slippery?

Kathryn McGrath, a chemist at Otago University, responded.

A cake of soap has as its main ingredient a molecule, rendered from animal fats and certain vegetable oils. The commercial process involved during rendering is called saponification. This major component of soap is generally called a surfactant (surface active agent). It may be generically classified as a salt (i.e. a compound containing both a positively and negatively charged species), where the positively charged species is often sodium and the negative charge part of the molecule is from the original fat or oil. As such this negatively charged portion of the molecule consists of a long chain of carbon and hydrogen atoms, much like a tentacle on an octopus. This surfactant, which also happens to be a base, is then the main ingredient in our cakes of soap, with water and superfatting agents comprising the remaining 25 per cent.

The surfactant molecule gives us the two basic ingredients for slipperiness built into one molecule - long chains which can easily glide over each other, in conjunction with the molecule being a base. All bases have a slippery feel to them, but it is the long chains of the surfactant that give soaps that slippery feel.

Try an experiment: rub your fingers together, you can feel a tension between them, stopping them from moving freely over each other. If you take some oil and place it between your fingers they move more smoothly than before and they feel slippery. This effect is made possible due to the weak forces holding the long chains of the oil together allowing them to move freely past each other giving us the impression that the oil is slippery. The addition of the oil acts to lubricate the gap between our fingers. Surfactants, which also have long oil-like chains, have this same property. In addition, unlike oils, surfactants are soluble in water. This is most convenient for us as it means that we get a water-soluble cleaner that lubricates as it cleans making cleaning easier and physically less abrasive.

You will notice also that as you add more water to soap it becomes increasingly fluid and you can eventually make soap bubbles, this too is due to the presence of the surfactants.

All commercial soaps and detergents are variants on this basic theme. They vary predominantly only in the type of surfactant used. Hence cakes of soaps, flowing soaps, shampoos, conditioners, dish washing liquids and laundry detergents all have a slippery feel to them due to the wonderful chemical that likes water and dirt equally, is cheap and plentiful and fun to play with; a surfactant.