Trying to understand surface tension

Who has an understanding of the effect of washing-up liquid on surface tension on a molecular level? Please help!

I’ve recently shown a lot of experiments on the topic of surface tension. And while it is a helpful analogy to think of a thin membrane on top of the water that lets water striders or paper clips sit on the water and not sink, and that rips open when washing-up liquid is introduced – this is really not satisfactory to me. But I am having a hard time understanding surface tension on a molecular level.

So let’s go back to the basics. Water molecules have a polar structure that allows each water molecule to form up to four hydrogen bonds to neighboring water molecules. A water molecule in midst of other water molecules will hence experience strong cohesive forces in all directions, which vanish in sum.

A water molecule at the very water surface will only experience strong cohesive forces from water molecules underneath it or from water molecules sitting at the surface right next to it, since there are no (or hardly any) water molecules above it, and adhesion with air molecules is much less strong. A water molecule at the surface will hence not easily leave the surface, and the surface itself will try to minimize its area, since that’s the best configuration energetically. If small weights are put on the water surface, the water surface will be deformed slightly, but not break, and this behavior will indeed look similar to a membrane spread over the water surface.

So far, so good. But why does this “membrane” rip when we put washing-up liquid on it?

That is because the molecules in dish soap attach to the uppermost water molecules with their hydrophil end, hence the resulting force on the uppermost layer of water molecules isn’t downward any more. The hydrophob ends of the soap molecules make sure the soap stays at the surface and prohibit formation of a new “membrane”.

Do you hate these graphics, too? My excuse: no computer allowed on this vacation and the ipad doesn’t have a better app. Do you have suggestions for ipad graphics apps that can deal with typed text well? I’m all ears!


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