Most recent answer: 10/22/2007

How does soap work to eliminate dirt?
- Masa
Focusad, Toronto
Masa -

Actually, soap doesn’t really do much with dirt (although scrubbing helps!). What soap gets at is oils. To explain this, I’ll have to start by giving you a little bit of chemistry...

Nearly all compounds fall into one of two categories: hydrophilic (’water-loving’) and hydrophobic (’water-hating’). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. But if you’ve ever tried mixing oil with water, you’ve probably noticed that it doesn’t work very well. Hydrophilic and hydrophobic compounds just don’t mix. (Ok, there’s more to the chemistry than just this, but this is the short version.)

This is why it’s so hard to clean grease off of dirty dishes. The grease won’t mix with the water in your sink, so it won’t come off. This is where soap comes in. Soap molecules are kind of special in that they have a hydrophilic part and a hydrophobic part. So one end of the soap molecule can mix with water and the other end can mix with oil.

When you mix soap into the water in your sink, the soap molecules arrange themselves into tiny clusters (called ’micelles’). The water-loving (hydrophilic) part of the soap molecules points outwards, forming the outer surface of the micelle. The oil-loving (hydrophobic) parts group together on the inside, where they don’t come into contact with the water at all.

Now you add the greasy dishes. The hydrophobic oil molecules on the dishes still don’t mix with the water. Instead, it gets trapped in the center of the soap micelles, where all of the molecules around it are hydrophobic too. Then you can rinse away the soapy-water, taking all of the trapped oil molecules with it.

If you want a neat way of seeing this at home, try taking a clear glass of water and adding a thin layer of oil. You should be able to see them separate pretty clearly. Now add a squirt of liquid dishsoap and stir it up. The oil doesn’t separate at all any more! It’s all trapped in little clusters of soap molecules, which can dissolve in water just fine.


(published on 10/22/2007)

Follow-Up #1: "hydrophilic" biodiesel?

How is bio diesel hydrophilic ??
- Ernie Angrilli
Beats me. It’s oily, and the Wikipedia article that calls it ’hydrophilic’ also says it’s ’immiscible with water’. That sounds contradictory to me.

some weeks later:

Aha- I wrote that to Wikipedia, and they changed ’hydrophilic’ to ’hydrophobic’, which means the opposite and is thus consistent with ’immiscible with water’. 

So I guess it’s hydrophobic, which is what it sounds like it should be given its chemistry.

Anyway, that’ll learn ya to trust the Web.

Mike W. (w. assistance from Tom)

(published on 10/22/2007)