Friday, March 11, 2005

Apple #46: Soap


So I was about to wash the dishes, and I thought, "How does soap work, anyway?" I have a dim idea that it somehow traps dirt so that you can then rinse it away, but is this idea correct, and how does the trapping part happen? And does it really get rid of the dirt, or am I just falling prey to some stupid, enormous ad campaign? And is dish soap that much different from hand soap, and shampoo, and laundry detergent, and all the rest?

  • Soap works essentially by combining things with opposite natures. All soap is made of an acid plus a base. The acid is a fat, which is often a combination of a fatty acid and a triglyceride. The base is sodium hydroxide. When you mix the acid and the base together, the fat part splits, separating the fatty acids from the triglycerides. The fatty acids then combine with the hydroxides. All this, by the way, makes a kind of salt. Yes, soap is really a salt.
  • The process of cleaning is easiest to understand if you think of soap molecules as having a head and a tail. The head part of the soap is the triglycerides, which attract and hold onto water. The tail part of the soap is the fatty acids, which don't like water. Dirt and grime don't like water either.
  • When you run soap under water, the head of each soap molecule attracts and clings to water. When you rub the wet soap over dirt, the dirt will cling to the fatty acids because they're both in agreement that they don't like water. So now the tail and the dirt are clinging to each other, but at the same time, the head of the molecule is holding onto water.
  • When you pour more water over the whole thing, the head of the molecule with the water on it goes right along with the other water, and even though the tail and the dirt would otherwise not budge, it's too late for them, they are connected to the head of the molecule, and they are carried off with it.

Antibacterial soaps

  • The same process applies to bacteria. That is, regular old soap will also snag and carry off bacteria in the same way.
  • Antibacterial soaps, which are supposed to be that much better, really aren't. They do have an extra agent in them that is designed to kill bacteria. However, you have to wash your hands for at least two minutes for these agents to work, and most people don't want to spend two minutes scrubbing their hands.
  • You also don't want to kill every single type of bacteria on your hands. Some bacteria can be helpful. And bacteria can develop a resistance to antibacterials, which means that new strains of the bacteria could no longer be killed by the antibacerials. So if you do manage to kill the bacteria in the first place, you might actually be causing more problems.
  • Also, some of the germy things on your hands are viruses, not bacteria. Antibacterial agents will not kill viruses.
  • The upshot is, washing your hands thoroughly with regular soap will accomplish the same level of benefit, without the possibility of creating uber-bacteria.

Other soaps

  • Soaps can be calibrated, so to speak, depending on how you'll use it. Soaps that will be used on your body are made to be milder than those that will clean things that are not on your body. And if you're not going to touch it at all, soap can be made as harsh as it needs to be, to get the job done.
  • So, yes, you will get better results if you use dish soap on dishes and shampoo on your hair. If you use shampoo on your dishes, for example, your dishes might not get so clean.

Improved soap

  • The fatty acids in soaps can cause problems. Some of them don't link up to the molecule chain, and because they don't like water, it can be hard to rinse them away. When the fatty acids won't rinse away, you get soap scum. This is what gives you a ring in your bathtub, a film that dulls your hair, or a gradual graying of your clothes. If you have hard water, which contains extra minerals, the fatty acids will cling to those to form more salts, which basically means the soap scum is even worse.
  • So it's essentially soap scum that's led chemists to experiment with all kinds of synthetic building blocks for soap to work as effectively as possible, depending on what type of surface you're trying to clean, what kind of dirt you're trying to get rid of, and what's already in your water.
  • Another reason chemists have added synthetic items is to give soap some extra benefits, many of which we take for granted but which are really not things that soap will do. If your soap does any of the following things, it's because somebody's added an extra something, which means it is technically no longer soap but a synthetic detergent, and which also means that the manufacturer has to list the ingredients on the package:
    • deodorizes
    • moisturizes
    • is an antiperspirant
    • fights acne
    • cures dandruff
    • gives a clean, fresh scent
    • makes you more attractive

Oh, and by the way, before all this experimenting started, soap used to be made from animal fats and wood ashes.

And yes, my dishes are still waiting for me to wash them. Although the dirt, which hates water, is very glad I have not yet washed them...

Howstuffworks, "
Is antibacterial soap any better than regular soap?"
Poison Ivy, Oak, & Sumac Information Center,
How soaps work,
How Does Soap Clean?
US Food and Drug Administration, Office of Cosmetics and Colors Fact Sheet, "
Soap," February 3, 1995

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