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Sadly the current top answer is quite wrong.

Foaming behaviour and micelle formation are not linked in any meaningful way. I won't address why, I'll just answer your question, by rephrasing it just a tiny bit:

>Why do some surfactants form stable foams while others don't?

Because if you blow air through a suitably concentrated solution of almost every surfactant you can generate 'bubbles'. The question is really whether or not they are stable.

The stability of a foam depends on the interfacial tension of the hydrophobic/air interface and on the ability of the hydrophilic side to trap water.

The foam is basically air/detergent/water/detergent/air. They are quite similar to lamellar sheets that many detergents form, which are water/detergent/detergent/water. I don't know if forming lamellar phases in solution is a predictor of forming stable foams or not.

If the interfacial tension is low, the bubbles are extremely metastable, they'll pop and merge and generally fall apart fairly quickly. Conceptually the interfacial tension (if you can hold everything else constant) is covarient with how densely the hydrophobic tails pack, like if you could make a model system where you could arbitrarily vary the density of the hydrophobic bit, the denser one would also have higher interfacial tension.

But this can lead you to the false conclusion that less curved foams must be more stable, which is indeed a false conclusion. Because of course it's actually a free energy property so there's a series of enthalpic and entropic terms to consider.

If the hydrophilic side can't trap water sufficiently well then they will 'drain'. That is to say, the water in the detergent sandwich will be pulled back by gravity into the bulk solution.

The size of the air bubble in the foam is determined by the aspect ratio of the detergent which is not particularly correlated with either of the other two properties (this is why I included the spurious conclusion about less curved foams being more stable) and the initial size of the bubble is also affected by how you prepare the foam which is a confounding variable.

There are additives you can add to detergents to stabilize or destabilize a foam.

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I've always wondered about this too, and it was never addressed during my undergrad chemistry degree. Thanks for the great explanation! I didn't get the see the answer you are referring to before it was removed, but the molecular organization you are describing (the air/surfactant/water/surfactant/air organization) certainly sounds a lot like a micelle structure to me? What am I missing (again I didn't read the other comment).

The very best foaming products contain at least two surfactants.

The classic foaming surfactants are anionic such as sodium dodecyl sulfate (SDS) and sodium laureth sulfate (SLES). They form great foam heights but the bubbles collapse really quickly.

Foam stabilizer chemicals are added to consumer products, such as laureth-3 or 4, cocamide DEA. These usually don't make foam by themselves, but what they do is like iron rebar in concrete, they reinforce the bubble that has formed.

Baby shampoo is a nice example. It's usually the mixture of SLES and cocamido propyl betaine (an amphoteric surfactant). The previous stabilizer mentioned is fantastic, but it is a skin irritant and burns your eyes. The amphoteric cocamido is very mild but less effective, hence, you need a lot more of it (higher cost) or you get a lower foam height / short stability time.

The difference between two-product formulas and solid bar soap, is the bar soap is usually a single surfactant maybe with some glycerol added. The bar soap needs a shitload of mixing energy to make a stable foam. You need to use a foaming brush and lots of stirring to input enough energy so that the molecules align correctly and get the best ratio of air/water/surfactant. Even after all that energy is put in, it still doesn't last as long as a two-surfactant mixture.

It's a fairly niche area, I'm not surprised that you didn't cover it in Undergrad.

I had a class on Interfacial Thermodynamics that covered the theory relevant to this (that I remember almost nothing about other than it having 80 or so 'essential' equations to comprehend in a 10 lecture course), surfactants weren't discussed explicitly in the class but thermo is thermo and applies to all systems equally.

I came across this particular area of knowledge doing some post-doctoral work for an excellent physical chemist who specialised in surfactants.

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The answer that was removed kind of boiled down to 'if it forms micelles it forms foam' with some very vague statements in support. It was poorly written, didn't use scientific language, mixed up cause and effect. Most of the facts weren't wrong, but they weren't factors in foaming behaviour. It actually made it pretty difficult to dispute, which is why I didn't bother to explicitly address it. Didn't want to get drawn into a potential pedantry showdown!

The bit I remember was a dubious claim that seemed to suggest that the air partitions into the core of the micelle, which is completely untrue (but was hard to understand exactly what he meant because of the lack of technical language). FYI when you bubble air into a surfactant solution you are in fact creating a new area of air/water interface along which the surfactant forms a typical monolayer.

Anyhoo! Not a problem anymore.