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I know that the hydrophobic effect is usually explained by the entropic effect originating from the disruption of hydrogen bonds between molecules of water and the nonpolar substance. The hydrogen bonds are reoriented to minimize the disruption of the hydrogen bond network of water molecules.

What I don't understand here is how this explains the tendency of non-polar substances to aggregate in water? Supposedly they disrupt the hydrogen bond network, but what forces them to clump together?

Then how does the above explain the tendency of water to decrease its surface area toward the nonpolar substance? Yes, the molecules of water around the nonpolar surface have lower entropy, but how does a decrease in entropy (in sense of a higher structural order) force them to minimize their exposure towards the nonpolar substance?

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    $\begingroup$ Nothing "forces" them. If molecules didn't have enough energy of themselves, they wouldn't do anything. It's just that while they randomly bump around, they get to state of lower Gibbs' enthalpy. $\endgroup$ – Mithoron Jun 11 at 1:13
  • $\begingroup$ chemistry.stackexchange.com/questions/10210/… $\endgroup$ – Mithoron Jun 11 at 1:16
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    $\begingroup$ Does this answer your question? Basis for the hydrophobic effect? $\endgroup$ – Mithoron Jun 11 at 1:17
  • $\begingroup$ I've seen the other question and the respective answers, but they don't really answer what I'm asking. What you're saying, does this imply that water molecules around non-polar substance have a higher Gibbs free energy? Is this increase in energy (compared to other water molecules) induced by the introduction of the nonpolar substance? $\endgroup$ – Gljiva Jun 11 at 14:11
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Whenever a non-polar substance is put into water, water molecules will organise themselves around it in a cage-like formation. The molecules exposed to the non-polar substance at any given time will orient themselves to form as many hydrogen bonds with the rest of the solution as possible.

(Image from here)

'Cage' effect in water

The way I understand it intuitively is that these water molecules are restricted in the orientations they can assume, becoming more ordered to maximise hydrogen bonding, so have an overall lower entropy than if they were free to tumble like in the bulk solution. Similarly, the aggregation of non-polar molecules also allows more free movement within the non-polar 'blob'. If they were distributed evenly, there would be a higher surface area and therefore amount of 'cage' water molecules and non-polar solutes would be more restricted in their movement (trapped in a polar environment). This is the entropic factor that drives the hydrophobic effect.

It is also favourable enthalpically; hydrogen-bonding interactions and non-polar interactions are maximised respectively.

These two approaches demonstrate why it is thermodynamically favourable for non-polar substances to aggregate, not disperse. To clarify, it is still thermodynamically unfavourable to mix the two in the first place, that's why they separate; this is just the better option if they were to mix.

It is also worth noting that oil 'blobs' in water tend to be very round, due to the fact that a sphere is the shape with the smallest surface area to volume ratio. This supports the above reasoning, minimising the number of 'cage' water molecules whilst maximising movement of both water and the hydrophobe.

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  • $\begingroup$ Very nice qualitative explanation but it also opens a can of worms. Entropy and order :-( $\endgroup$ – M. Farooq Jun 11 at 0:14
  • $\begingroup$ @M.Farooq How do you mean? (Not in a stand-offish way I'm genuinely curious) $\endgroup$ – Jabbamanga Jun 11 at 0:52
  • $\begingroup$ Frank Lambert has written a lot on entropy and the associated myths with disorder. He passed away some time ago. A lot of gen. chem. (which I consider as a useless course) corrected their notions including Atkins. Thinking deeply, how entropy and so-called disorder are connected? What do we mean by disorder? I am still searching :-) $\endgroup$ – M. Farooq Jun 11 at 2:19
  • $\begingroup$ Does this from a statistical viewpoint of entropy then mean that when nonpolar molecules are added to water they momentarily decrease the total number of possible microscopic configurations of the system? And afterwards the molecules by randomly moving around at the same time also arrange themselves to a state where the entropy is the highest? And then going back to a state of lower entropy among all possible states is of such low probability that of all configurations we mostly observe the other states? $\endgroup$ – Gljiva Jun 11 at 14:36
  • $\begingroup$ @Gljiva A system will always tend to a state where its Gibb's energy is lowest (or entropy in the universe is highest) by random movement, in accordance with the laws of thermodynamics. It is a case of probability, the more disordered state is much more likely than any other state, so none others are really observed. $\endgroup$ – Jabbamanga Jun 11 at 15:08

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