Can Soap Get Dirty?
When you see lovely little bars of lemon-thyme orlavender hand soaps on the rim of a sink, you knowthey are there to make you feel as fresh as agardenia-scented daisy. We all know washing ourhands is important, but, like washcloths and towels, can the bars of hand soap we use to clean ourselvesbecome dirty as well?
Soaps are simply mixtures of sodium or potassium salts derived from fatty acids and alkalisolutions during a process called saponification. Each soap molecule is made of a long, non-polar, hydrophobic (repelled by water) hydrocarbon chain (the "tail") capped by a polar, hydrophilic (water-soluble) "salt" head. Because soap molecules have both polar and non-polar properties, they're great emulsifiers, which means they can disperse one liquid intoanother.
When you wash your dirty hands with soap and water, the tails of the soap molecules arerepelled by water and attracted to oils, which attract dirt. The tails cluster together and formstructures called micelles, trapping the dirt and oils. The micelles are negatively charged andsoluble in water, so they repel each other and remain dispersed in water—and can easily bewashed away.
So, yes, soap does indeed get dirty. That's sort of how it gets your hands clean: by latchingonto grease, dirt and oil more strongly than your skin does. Of course, when you're usingsoap, you're washing all those loose, dirt-trapping, dirty soap molecules away, but a bar ofsoap sitting on the bathroom counter or liquid soap in a bottle can also be contaminatedwith microorganisms.
This doesn't seem to be much of a problem, though. In the few studies that have been done onthe matter, test subjects were given bars of soap laden with E. coli and other bacteria andinstructed to wash up. None of the studies found any evidence of bacteria transfer from thesoap to the subjects' hands. (It should be noted that two of these studies were conducted byProcter & Gamble and the Dial Corp., though no contradictory evidence has beenfound.)
Dirty soap can't clean itself, though. A contaminated bar of soap gets cleaned via the samemechanical action that helps clean you up when you wash your hands: good ol' fashionedscrubbing. The friction from rubbing your hands against the soap, as well as the flushingaction of running water, removes any harmful microorganisms from both your hands and thesoap and sends them down the drain.