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http://www.rsc.org/suppdata/nj/b3/b303016d/b303016d.pdf

If anyone doesn't know the definition, logP is the the log of the ratio of a molecule in dioxane to water. So a high logP would mean the molecule is mostly in the organic dioxane layer rather than water. E.g a molecule with a logP of 2 would prefer the organic layer 100 times more than water, and would be considered hydrophobic/lipophilic.

Hexane logP=4.15, very hydrophobic and almost all of it will stay in the organic layer.

Cyclohexane logP=3.66, still very hydrophobic, but hexane is almost 4 times more hydrophobic than cyclohexane. Can anyone please try to explain why the cyclic molecule appears to like water slightly more than its straight chain counterpart?

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Think of the hole in the water that the molecules occupies as a bubble. The bigger the bubble the greater the area over which surface tension is acting.

The highest volume to surface ratio is for a sphere. So hexane is like a rod and cyclohexane is like hexagonal pyramid. Guess which has a higher volume to surface ratio?


edit more information...

Not that it is exactly the same, but look at the densities of the liquids. The cyclohexane molecule occupies a small volume in the liquid as compared to hexane, hence it is denser, even though it has 2 hydrogen atoms less per molecule.

Now water has 18 ml/mole so we can figure out roughly how many water molecules are displaced by a molecule of each hydrocarbon.

                                                     # water
                                                     molecules
             density         molecular mass  ml/mole displaced
Cyclohexane  0.7781 g/mL     84.16 g/mol     108.2    6.0
Hexane       0.6548 g/mL     86.18 g/mol     131.6    7.3
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  • $\begingroup$ I'm not fully convinced/haven't grasped this fully so if a bit more depth could be possible that would be brilliant! $\endgroup$ – Laksh Nov 3 '15 at 22:32
  • $\begingroup$ @Aced125 - I added some additional information which might be helpful. $\endgroup$ – MaxW Nov 4 '15 at 1:24
  • $\begingroup$ Ok, I don't seem to understand how a greater amount of water molecules displaced correlates to a higher logP though. $\endgroup$ – Laksh Nov 5 '15 at 18:02
  • $\begingroup$ Neither hexane nor cyclohexane will participate in much hydrogen bonding. So essentially the molecule has to make a "hole" in the hydrogen bonds between the water molecules that would be there otherwise. So it is like the molecules need a "bubble" devoid of anything into which that can slip. Hexane requires a bigger bubble, thus takes more energy and is hence less favorable. $\endgroup$ – MaxW Nov 5 '15 at 18:07

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