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Sunday, February 13, 2011

Soap & Water

In the last post I talked about water molecules and about the way they behave because of the charges on their surface. Now I plan to do the same thing with soap so I can explain the marvelous ways it behaves when mixed with water! Two representations of a soap molecule are shown in the diagram on the left. The top one showing all the atoms that share or exchange electrons to form the molecule and the lower one a simplified representation of the same thing.

In general soap molecules have a charged head that feels comfortable cozying up to the oppositely charged side of water molecules, and a long tail that has no charges on it so it feels more relaxed being away from anything charged. It turns out oils, greases and fats don’t have any charges so soap molecules tend to bury their tails in them to get away from water.

A first for me today, I took a video of an experiment mixing oil and water and then adding soap. I’ve uploaded it to you tube. I hesitate to link it but it’s useful to actually see what happens and as a basis for comparison as I improve. Also it does show how the soap molecules, by burying their heads in oil droplets and exposing their charged heads to water, can form tiny particles called micelles that stay suspended in the water to form a suspension of oil in water.

You will have had the experience of grease sticking to your hands and of finding that water is no good at getting it off. Our hands exude natural oils too and they harbour dirt and bacteria. When we wash them with soap and water, you can now imagine the soap tails embedded in the oils presenting their charged heads attracted to the water and being easily flushed away with it.

There is more to this story and if you are interested - I plan post it today to expand on the marvels of soap and water! Rie