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Food for Thought: Antibacterial Soap

February 21st, 2009 · No Comments · Cleaning

This is part two of a three-part series on antibacterial soap.

Time for your first serving of “Food for Thought“. Put your thinking caps on – here’s the science of washing your hands:

Start with water: water has a cohesive property. It likes to “stick” to itself. This explains why water forms droplets, why those water strider insects can walk on water and why water creates a meniscus (the curved shape of the top of the water when you look at it from the side, as if in a measuring glass). Water also has an adhesive property: it will “stick” to other objects. Just drip water on a vertical surface, like your shower wall, and you’ll see this in action. Droplets stay together as droplets (cohesion), and the water stays on the wall (adhesion). Water will naturally adhere to the dirt on your hands and wash it away, which explains why my “washed-with-water-only” Petri dish didn’t have much growing in it: much of the bacteria, viruses and fungi – a veritable metropolis of things that make me squirm – were simply washed away with the dirt.

Now enter soap: Soap’s job is to get water to increase its adhesive property. Most soap contains “surfactants“, a short word for “surface active agents” which do what they say they’ll do: act on the surface tension of the water. It does this by breaking the cohesion and thereby reducing the surface tension of the water.

We all know oil and water don’t mix. That’s because oil is made up of molecules that are hydrophobic, meaning not that they have a Psycho-style fear of (shower) water, but that they are repelled by water. Surfactants have two different ends, one of which is hydrophobic and the other is hydrophilic, which means it attracts water. Like objects attract: the hydrophobic end of the soap attracts the hydrophobic oil molecules, while the hydrophilic end of the soap grabs the water. This allows not only your standard issue dirt to be washed away down the drain, but also your grime and oily gunk, too, with soap acting as the middle man. The one drawback is that soap also washes away your skin’s natural oils, which explains why our hands tend to get so dry after frequent handwashing.

Ultimately, we don’t necessarily care if the bad bacteria lives or dies: we just want it away from our food and our family, running down the pipes to the water treatment plant…where they will then kill the bad guys, I suppose.

So use soap. Use plenty of water. And rub those hands together to get the dirt and germs dislodged and ready to be washed away by the magical powers of adhesion and surfactants! “Anti-bacterial” chemicals are unnecessary, ineffective, and harmful to the overall environment we call Earth.

**Be sure to visit the Monday Mission for practical tips to get rid of antibacterial soap!

Some sources for further reading:

On to part 3: The evil villain triclosan…

Other posts on antibacterial soap: