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I distributed drops of chloroform (with a lipid in them) on water (in a Langmuir-Blodgett trough). The chloroform drops form a layer on the water (so that the lipids are distributed on the water surface) and I need to explain why. (Afterwards, the chloroform evaporates, leaving back a lipid monolayer.)

I was told that the water pulls each chloroform drop apart and that this has to do with the fact that the surface tension of water to air is greater than the sum of the surface tension of chloroform to air and surface tension of chloroform to water.

Can someone explain how this works in more detail?

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As you pointed out, the surface tension of water-air interface is greater than the surface tension of chloroform-air interface. At $25~^\circ\mathrm{C}$, the surface tension of water-air interface is $71.99\pm0.05~\mathrm{mNm^{-1}}$ and that of chloroform-air interface is $26.67~\mathrm{mNm^{-1}}$ (source).

Now, let's understand why a drop of chloroform placed over water gets puller apart forming a layer. Surface tension is the force per unit length. More the surface tension, more is the force per unit length and vice versa. Surface tension is similar to tension in a stretched rubber sheet. As you place a drop of chloroform on water, the system looks as shown in the following diagram:

Initial state diagram

As we seen earlier, surface tension of water-air interface is greater than that of chloroform-air interface. So in the boundary separating chloroform and water there exists an imbalance of force. The force vectors are the red arrows in the diagram above and their strength is denoted by their length and weight (thickness).

Due to this, the drop of chloroform expands to form a layer on top of water. This can also be looked in the nature's tendency to lower energy. The surface energy of chloroform-air interface is lesser than that of water-air interface. So, a lower energy configuration is preferred.

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  • $\begingroup$ Maybe you should also try to explain chemically why the surface tension of the water-air interface would be expected to be higher than the chloroform-air interface, using the idea that surface tension is dependant on intermolecular forces. Rest of the answer pretty much covers everything $\endgroup$
    – Yusuf Hasan
    Jul 4, 2020 at 10:56
  • $\begingroup$ Could you please explain why the surface energy of chloroform-air interface is lesser than that of water-air interface? $\endgroup$
    – Filippo
    Jul 4, 2020 at 11:38
  • $\begingroup$ I understand that water has a tendency to reduce its surface, but your answer implies that the chloroform is bonded to the water, right? If yes, how? $\endgroup$
    – Filippo
    Jul 4, 2020 at 11:40
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    $\begingroup$ @Filippo: Surface energy $U$, area $A$ and surface tension $S$ are related by $S=U/A$. The chloroform-air interface has a lesser surface energy than that of water-air for the same reason it has a lesser surface tension. Now an obvious question is why does the former have a lesser surface tension than the latter? This depends on the magnitude of the intermolecular forces. Generally, the stronger these bonds are more is the surface tension. This is also supported by the lower boiling point of chloroform compared to that of water. $\endgroup$
    – Vishnu
    Jul 4, 2020 at 12:53
  • $\begingroup$ @GuruVishnu Thank you very much :) Good to know that you can also regard this process as a minimization of (surface) energy. $\endgroup$
    – Filippo
    Jul 4, 2020 at 13:59

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