# The Naked Scientists Forum

### Author Topic: Is there a limit to how many bubbles a certain soap can make?  (Read 6411 times)

#### elegantlywasted

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##### Is there a limit to how many bubbles a certain soap can make?
« on: 15/02/2007 22:22:40 »
Last week I was visiting my sister, when her winner of a roommate used regular dishwashing soap in the dishwasher. As you can imagine this led to bubbles galore. The kitchen was full of them.

This incident reminded me of being cautioned not to use bubble bath in a jacuzzi, because of the amount of bubbles that will be created.

These thoughts lead to my question. Is there a bubble potential? A maximum amount of bubbles that soap can create? I know from doing dishes eventually the bubbles in the sink stop foaming, but how do we learn how much soap is too much? Will I ever be able to have a relaxing bubble bath while jets massage my back? These are the things I need to know.

Thanks!
« Last Edit: 18/02/2007 12:42:14 by chris »

#### daveshorts

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #1 on: 15/02/2007 22:57:31 »
A soap film is actually 2 layers of detergent molecules, back to back. They form like this because they have a head that is soluble in water and tails which are oily and insoluble. So the heads try and stick in the water and the tails try and get away in the air.

normally a lot of the detergent molecules are in solution in little balls called micelles with all the tails sticking together, sometimes around some dirt or oil - why detergents are useful for washing up.

This means that the absolute limit on amount of bubble formed would be all the detergent is used for the bubble films and none as micelles.

The thinnest you can get a soap film is about 5nm so say your wasing up liquid is half detergent 1cm3 of liquid should make 1cm/5nm/2 = 1million cm2 of bubble film - or about a 10m square. In practice you would probably be significantly worse than this because the detergent would get lost, but it gives an idea.

#### Soul Surfer

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #2 on: 15/02/2007 23:03:54 »
We use special aromatherapy oils in our jacuzzi they give a small amount of bubbles when the air is being allowed into the jets and this gives a reasonable bubble bath type of layer on the surface.  They also smell pretty nice.

I think Dave's got it wrong about the quantity of bubbles there because the detergent layer is monomolecular and the molecules are much smaller than 5nm I'm reasonably sure you would get lots more surface area of bubbles than he suggests
« Last Edit: 15/02/2007 23:06:46 by Soul Surfer »

#### daveshorts

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #3 on: 16/02/2007 08:50:39 »
The detergent layer is 2 molecules thick, and the minimum thickness of a soap film is pretty close to a bilayer with very little water in between. As a sanity check each molecule would be 2.5nm long, which means about 17 atoms long (a C-C bond is about 0.15nm)

from http://en.wikipedia.org/wiki/Surfactant
detergents seem to be 14-25 atoms long
eg.

so probably right within a factor of 2 or 3 which ain't bad

as another sanity check a cell wall is 3-10nm thick which is essentially a soap film the other way round.

surfactant molecules with their hydrophobic tails into the middle of the wall. So should be a similar thickness to a minimum thickness soap film
« Last Edit: 16/02/2007 08:52:15 by daveshorts »

#### elegantlywasted

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #4 on: 16/02/2007 22:10:14 »
Thanks guys, another question... why are more bubbles created when the water is agitated???

#### rosy

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #5 on: 17/02/2007 03:08:44 »
I think it's to do with forcing bubbles of air into the water- if you stir detergent-in-water without disturbing the surface you don't make extra bubbles.
The water will have a layer of detergent on it, tails upward away from the water, water-loving heads downward. When you agitate the water's surface you'll get air trapped under the water with molecules (largely from the surface layer) at the air-water interface (so with greasy tails inward.
As the bubble rises back to the surface it will collect another layer of detergent molecules, the other way out, from the layer on the surface, giving the double layered structure shown above.

As an aside, this is closely analogous to the way in which cells engulf food or (in the case of the mammalian immune system and probably other organisms) "enemy" cells.
(As in this animation: http://www.stolaf.edu/people/giannini/flashanimat/cellstructures/cell.swf )

#### WylieE

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #6 on: 17/02/2007 04:35:39 »
Cool,
So why does soap-water lose its bubble potential after it sits in the sink for a day?
Does the soap break down?
Or does dirt just accumulate?
Thanks,
Colleen

#### rosy

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #7 on: 17/02/2007 05:54:40 »
Hmm.. don't know.
I've never actually tried to get clean soap solution started again after leaving it to stand...when it's been left to stand in any of the places I spend time it usually has dishes in it, which are of course dirty and so transfer grease into the mixture encouraging it to form micelles.

Um, I'd expect it to form more small, stable micelles which are less likely to join up and make the bilayer sheets needed for bubbles? I'm don't think the detergent will brea down on that timescale (think how long it lasts in the bottle).

#### WylieE

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##### Re: Is there a limit to how many bubbles a certain soap can make?
« Reply #8 on: 17/02/2007 07:37:23 »
Right,
That's why I always wondered where the bubbles went- because it lasts in the bottles for . . . well, years at least- so it is probably the dirt.
Of course, in the bottle they are pretty protected from water too, so if it was breaking down due to interaction with the water it would be slower in the bottle .. .. hmm- I should make some soapy water and try to keep dirt, grease, dog hair, dust out of it and see how long it lasts.
Colleen

#### chris

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #9 on: 18/02/2007 12:53:24 »
Assuming the washing up isn't left in the bowl (clearly not the case in my house), soap loses its bubble potential because it reacts with calcium ions to produce "scum".

Soaps are usually sodium salts, which are soluble. But when you add them to water they associate with any calcium ions present, producing insoluble precipitates. These are no longer able to contribute to the bubble-making potential of the solution, so fewer bubbles all round!

The reason for this is that calcium forms Ca2+ salts, so it associates with 2 soap molecules, one on either side of the ion. Calcium is small and highly charged and hence clings onto its new soapy portions more strongly than sodium, which makes Na+ salts. Hence one sodium ion only associates with one "soap" molecule, and it's easier for these to dissociate.

When you add soap to your washing up, if there is an excess of calcium (i.e. the water is very hard), the reaction occurs rapidly. But if there's less calcium (you've got softer water) then it takes longer to go to completion.

Chris

My question is, why does adding a piece of soap to a washing machine or dishwasher prevent the situation that unfolded in the "elegantlywasted" kitchen recently? (You can also stop bubble-bath foaming the same way)? What's going on?
« Last Edit: 18/02/2007 12:57:14 by chris »

#### rosy

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #10 on: 18/02/2007 13:12:05 »
Ah, of course.
I was going to remark that we make up diluted washing up liquid in the lab with <50% water (over what was in the bottle already) to make it easier to use and it doesn't seem to affect its behaviour over weeks/months. But I rather think we use the lab de-ionised water supply to do it, so no Ca2+ problem.

#### elegantlywasted

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #11 on: 18/02/2007 15:23:40 »
Okay, so is there a way to measure how many bubbles a certain amount of soap will create, with a certain amount of agitation? (ie. bubble potential)

#### WylieE

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #12 on: 18/02/2007 16:53:08 »
Thanks,
I understand now, and I won't have to try to find a clean bowl to do an experiment with (phew).

So if you add soap (regular bar soap?) to a dishwasher you won't get bubbling over?

Colleen

#### elegantlywasted

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #13 on: 18/02/2007 17:11:35 »
Well I don't have a dishwasher to experiment with, but I would think that the bar soap wouldn't have as much of a lather or bubbles as a liquid would.

#### WylieE

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #14 on: 18/02/2007 19:55:05 »
So I was thinking that Dr. Chris meant if you add liquid dishsoap but throw in a bar of soap too this would stop the bubbling over like you saw.  Is this true?

Yeah, no dishwasher here either (explaining the stack of dirty dishes in the kitchen).  Unfortunately, no jacuzzi either which would be much more fun to test that idea with.

Colleen

#### chris

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #15 on: 18/02/2007 23:36:59 »
It's definitely true. If you have only non-automatic washing powder (for hand washing), and you want to use it in a machine, chucking in half a cake of soap will stop your kitchen turning into a giant bubble bath. I'm just not sure why. Perhaps someone can help us out?

Chris

#### jolly

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #16 on: 24/02/2007 20:52:03 »
deleted as inapproprate
« Last Edit: 06/03/2007 00:51:57 by jolly »

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##### Is there a limit to how many bubbles a certain soap can make?
« Reply #16 on: 24/02/2007 20:52:03 »