Why does dishwashing soap make bubbles?

28 November 2010

Question

Hi Chris -

I'm a big fan in the States listening to the podcast.  My 10 year old son has a question about soap bubbles.  His question is simple, and I suspect difficult to answer.  Exactly why does dishwashing soap make bubbles?

I understand about the soap sort of denaturing the fatty organic matter (software is my strong suit, clearly not chemistry).  I don't understand why the soap would be more prone to make bubbles that water or lemonade.

Many thanks,

Scott

BTW, when I play the podcast for my kids and their friends as I'm driving around, they are always amused and wonder about the whole 'naked' thing. I tell them about the stripping down science, but ultimately resort to the fact that you all are British as an explanation.  Do you have any guidance for what to tell kids about
your show?

Answer

Dave - It is related to surface tension, but it doesn't work very well with pure water because a bubble is a very, very thin layer of water; And with normal water, the surface tension is so strong that it tends to be incredibly unstable.

This means that It will just all form down into a droplet and pop the bubble before it's even really formed.

But what you do to make a soap bubble is add some kind of detergent or soap. Soap or detergent molecules tend to have a "head" region, which really loves water and is very charged, and a "tail" region, which is very, very oily and doesn't like water at all.

So the tail region is very, very good for dissolving fats when you're doing washing up because all the tails stick in the fat.

But that would mean you'd have something - like oil - which doesn't mix with water. So you surround it with lots of water-loving heads, so it's then soluble in water and you can dissolve the fats and clean your washing up.

But these head regions also tend to stick on the surface, so all the water-hating tails tend to cover all the surface of water, and deeper inside the water, you tend to get kind of little vesicles, so little spheres with all the tails pointing inwards in the water-loving heads surrounding the outside.

This means that if you increase the surface, just some of these vesicles kind of get pulled up the surface and they can keep creating surface really, really easily.

They stabilise these very, very thin films of water, soap bubbles. And so, because you've got all this detergent in there, you get this great big thin films of water with a layer of soap on one side, then a little layer of water, then a layer of soap which is really quite stable, and will last a good couple of minutes.

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