Why do surfactants lower the surface tension Everybody knows that detergents, or generally amphiphilic substances, lower the surface tension. Of water, usually.
I wonder if that's true for any polar solvent (most likely), i.e. will the same detergents work in, say, ethanol or liquid ammonia?
What happens in a nonpolar solvent? The detergents aggregate on the surface there too, but now with the polar end sticking out. Does the surface tension go up, or down?
And what exactly is the reason for the effect on water anyway, quantitatively? Imagine a surface of water which is covered by exactly a single layer of surfactant molecules, e.g. in a Langmuir trough. Now go to half or double that amount. Is there a discontinuity in the change of surface tension? If yes or no, why?
(sorry for the jumble of questions, I'm not quite sure from which angle to tackle this. The effects seems quite obvious, but I'm not sure it is. My real question is how to explain the effect on a molecular level.)
We usually say that at an interface, molecules will orient themselves at a "preferred" position, but we usually ignore that such a "preferred position" can actually be their "least unfavorable" position.

Energetically speaking, the interaction of a lipid chain with a gas is less unfavorable than interaction of a polar group with gas, because interactions from a lipid chain is mainly based on van der Waals interactions which are quite weak compared to polar (i.e. electromagnetic) interactions with a polar group. Also, they cannot make stabilizing bonds with the medium, i.e. water usually.

It must also be noted that the surface tension is linked to the interactions of molecules at the interface.

So "why do surfactants lower the surface tension"?

If you add some "surfactant", aka "tensioactive" agents, in water, its "least unfavorable" position would be at the interface between water and the other medium (usually gas). It will bring down the cohesive energy at the surface, compared to water alone, because it relies on van der Walls interaction. And that is why it lowers surface tension.