Foam Characteristics
q. Do the bubbles that form when washing with soap or detergent actually play a part in the cleaning process? Do the items being washed get cleaner the more bubbles there are?


a. Colloidal Agents consist of particles of surfactant which act on the surfaces of objects to remove greasy dirt. The surfactant molecule has a hydrophobic hydrocarbon "tail" and a hydrophilic ionic "head". This enables grease to be trapped by the tails in structures called micelles. The grease or dirt is on the inside of a sphere of heads that point outwards into the watery solution with the tails pointing inwards. The micelles hold dirt and grease in suspension and help prevent it being redeposited.

While micelles trap dirt at the solid/liquid interface, bubbles can help trap dirt at the liquid/air interface so they do play a part in the cleaning process. However, in car-washing detergents, dishwasher powders and window cleaning detergents, there are no bubbles. This makes for easy rinsing which would sometimes be better for dishes too as it would prevent a build-up of washing-up liquid which may be unpleasant. Most commercial washing-up liquids are frothy because the consumer thinks that the more bubbles there are, the cleaner the dishes, but this is not the case. We would be better off with fewer bubbles.


a. When soap was the only washing medium available, foam indicated that the calcium in hard water had been precipitated and enough soap remained in solution to accomplish the washing process. "Housewives" rightly perceived that a bubbly wash was a good wash. Early detergents were rejected because they did not foam enough, and manufacturers still add special foaming ingredients. At one stage detergent advertising was as much about foam as about detergence.

Cleaning is achieved by mechanical motion which used to be induced by boiling clothes and household linen (later superseded by mechanical stirring), and by rinsing, the latter process being every bit as important as the former.


© Copyright New Scientist, IPC Magazines Limited 1997