Question

Soap or detergent molecule will dissociate in water to form hydrophobic head and hydrophobic tail. How do they decrease the water tension of water? Does it has anything to do with the hydrophobic tail tat repelled out of the water surface?

Answer 1

no.. i increases the dist betwn 2 water molecules at the surface (at all places for that matter) n since the attraction is a short range one.. it wont be as strong..

Answer 2

An energy perspective is most useful here. Surface tension can be defined as the energy required to create 1 cm^2 of extra surface. The higher the surface tension, the more energy it costs to create additional surface.

Now, you already know that the soap or detergent prefers to go to the surface of the water, right? So the net effect is to replace a water-air surface with two new surfaces, one water-detergent, and one detergent-air.

Why would this happen spontaneously? It's got to either lower the energy, or increase the entropy. It can't possible increase the entropy, because the soap molecules go from being dissolved in the bulk of the water to being concentrated at the surface. Clearly entropy is reduced. But that must mean that the energy is reduced.

So that tells you that the energy of the double surface, water-detergent plus detergent-air, must be lower than the energy of the original single water-air surface. That is, the surface tension has been reduced.

Answer 3

If you want to think about it in terms of intermolecular forces, then consider that the origin of surface tension is that water molecules on the surface are deprived, on one side, of the attractive force they feel from neighboring water molecules. This causes the net remaining forces (from the bulk side) to pull them strongtly towards the bulk.

Now if I instead coat the air side of the water with the polar ends of detergent molecules, I have given all those water molecules at the surface a new attractive interaction on that side. This new attractive interaction partially cancels the attraction from the bulk side, and the net force pulling the water back into the bulk is reduced. Less surface tension.

As for why the hyrophobic tails of the detergent are happy to point out into the water: because they are NOT deprived of attractive interactions with the bulk water, because they don't have any in the first place -- that's what it means to be hyrophobic.