Foam is formed when pockets of gas are trapped in a solid or liquid. We see it all around us all the time: from the foam of the waves on the beach to that of the beer we are about to drink, from the mattress we use for camping to the sponge for washing ourselves.
The incorporation of gas occurs mainly by mechanical action, that is by violently shaking the liquid in the presence of the gas. There are substances which can facilitate this mechanism by lowering the surface tension of the liquid: our belovedsurfactants.
So, surfactants help forming foam AND surfactants clean. It's no surprise that the cleaning power of a detergent has always been perceived as related to the foaming one!
In reality the connection is not so straightforward. Without going int too much detail about the forces at play at the molecular level [1,2], we can simplify by saying that, while the ability to create foam has to do with the way in which the surfactant interact with molecules on the surface of the liquid, the cleaning power is determined by how the surfactant molecules interact with the dirt molecules.
Cleaning and foaming power are therefore not directly related and it is only our habit that makes us perceive them as such.
Proof of this is the fact that there are a whole class of surfactants that clean extremely well while creating very little foam and which are therefore suitable in all those cases in which having a lot of foam would be a problem, such as dishwashers.
The paradox is that, over time, the consumer association has become so strong that now many brands choose to put certain surfactant in the mix JUST to make foam. Shocking!
[1] "Surfactants and Interfacial Phenomena", Milton J. Rosen, Joy T. Kunjappu, John Wiley & Sons, Mar 6, 2012
[2] "Chemical water technology", Dmitry Yu. Murzin, Walter de Gruyter GmbH & Co KG, Apr 9, 2018
We have chosen our surfactants for their cleaning power, leaving aside their foaming profile. Now that you know how it works, let's see if you can win the force of habit.
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