洗手皂会变脏吗？

When you see lovely little bars of lemon-thyme or lavender hand soaps on the rim¹ of a sink, you know they are there to make you feel as fresh as a gardenia-scented daisy. We all know washing our hands is important, but, like washcloths and towels, can the bars of hand soap we use to clean ourselves become dirty as well?

 

Soaps are simply mixtures of sodium² or potassium salts derived³ from fatty acids and alkali solutions during a process called saponification. Each soap molecule⁴ is made of a long, non-polar, hydrophobic (repelled⁶ by water) hydrocarbon⁷ chain (the "tail") capped by a polar, hydrophilic (water-soluble⁸) "salt" head. Because soap molecules⁹ have both polar and non-polar properties, they're great emulsifiers, which means they can disperse¹⁰ one liquid into another.

 

When you wash your dirty hands with soap and water, the tails of the soap molecules are repelled by water and attracted to oils, which attract dirt. The tails cluster together and form structures called micelles, trapping the dirt and oils. The micelles are negatively charged and soluble in water, so they repel⁵ each other and remain dispersed¹¹ in water-and can easily be washed away.

 

So, yes, soap does indeed get dirty. That's sort of how it gets your hands clean: by latching¹² onto grease, dirt and oil more strongly than your skin does. Of course, when you're using soap, you're washing all those loose, dirt-trapping, dirty soap molecules away, but a bar of soap sitting on the bathroom counter or liquid soap in a bottle can also be contaminated with microorganisms.

 

This doesn't seem to be much of a problem, though. In the few studies that have been done on the matter, test subjects were given bars of soap laden¹³ with E. coli and other bacteria and instructed to wash up. None of the studies found any evidence of bacteria transfer from the soap to the subjects' hands. (It should be noted¹⁴ that two of these studies were conducted by Procter & Gamble and the Dial Corp., though no contradictory¹⁵ evidence has been found.)

 

Dirty soap can't clean itself, though. A contaminated bar of soap gets cleaned via the same mechanical action that helps clean you up when you wash your hands: good ol' fashioned scrubbing. The friction¹⁶ from rubbing your hands against the soap, as well as the flushing action of running water, removes any harmful microorganisms from both your hands and the soap and sends them down the drain.

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