3

Surfactants are known to adhere to the interface. What's the physical mechanism for the transport of surfactants from the bulk to the interface?

Wiz123
  • 235
  • 1
  • 6

1 Answers1

5

In a liquid the molecules are randomly moving around due to thermal motion. This applies to both the molecules of the solvent and also to the surfactant molecules dissolved in the solvent. This means that near the surface you have surfactant molecules just below the interface moving towards the interface, and also surfactant molecules at the interface moving away from the interface back into the bulk. The concentration of the surfactant at the interface is due to a balance between these two motions.

The reason we get an excess of surfactant at the interface is because the energy of the surfactant molecules is lower at the interface that it is in the bulk. That means the thermal motion of the surfactant molecules away from the interface into the bulk is slightly slower than the motion from the bulk towards the interface. Thus results in a "pile up" of the surfactant at the interface.

John Rennie
  • 355,118
  • This seems like a good explanation - but can you comment on why 'the energy of the surfactant molecules is lower at the interface that it is in the bulk'? Presumably the same is not true of the liquid molecules... – Martin C. Nov 14 '22 at 07:59
  • 1
    @MartinC. that is a surprising complicated subject and would be better addressed in its own question. – John Rennie Nov 14 '22 at 08:16
  • here you go: https://physics.stackexchange.com/questions/736604/why-is-the-energy-of-surfactant-molecules-lower-at-the-liquid-air-interface-than – Martin C. Nov 14 '22 at 09:44