杏吧原创

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: Washing-up liquid consists of particles of surfactant which act on the
surfaces of objects to remove greasy dirt. The surfactant molecule has a
hydrophobic hydrocarbon 鈥渢ail鈥� and a hydrophilic ionic 鈥渉ead鈥�. 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 (see
Diagram). The micelles hold dirt and grease in suspension and help
prevent it being redeposited.FIG-mg19506301.GIF

While micelles trap dirt at the solid/liquid interface, bubbles in ionic
surfactants can help trap dirt at the liquid/air interface so they do play a
part in the cleaning process.

However, in the anionic surfactants present 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 and anionic surfactants.

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. 鈥淗ousewives鈥� 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boiling clothes and household linen (later superseded by mechanical stirring),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.