[Greenspin]

Hi everyone,

Just joined, and have a silly question about the relative solubility of alkanes in a homologous series:

So, the very slight solubility of alkanes in water decreases as the molecules get bigger, and so causes the alkanes to form an oily layer separate to the liquid, right? But is it a result of Van der Waals' forces acting between the alkanes by attracting them to each other more strongly as the molecules get bigger, and so resulting in a very insoluble substance? So meaning smaller molecules, that experience weaker Van der Waals' forces, are more soluble (relative to large ones) as it is easier to separate them from themselves?

I know hydrocarbons are classed as insoluble, so if I've explained that wrong, I'd be happily corrected.

I know it's a ludicrously simple question, and I've probably used all the wrong words, haha, but for some reason, it's completely over my head and my googling skills have failed me.

Any help is appreciated, thanks.

[Greenspin]

Just noticed it says that I posted my original post at 6:41 a.m. I'm posting from England, and it's 6:46 p.m. for me now. Will be interesting to see how often I am online at the same time as the majority.

[sjb]

You can change your profile so that the time is accurate for you, the majority of the time this won't need changing except for around the start and end of BST as the forum software changes on a different date

[fledarmus]

Actually, I think you need to consider that the intermolecular bonds between the water molecules are much stronger than the van der waal's bonds between the alkanes. How many water-water interactions do you need to disrupt in order to get a larger molecule in solution than a smaller molecule?

In most cases, the separation of water and oil layers has more to do with the forces in the water layer rather than the forces in the oil layer, because in general the intermolecular forces in aqueous solutions are stronger than those in non-aqueous solutions.

[Babcock_Hall]

The mail driving force for hexane, C₆H₁₄, forming an oily layer apart from water is entropic, and it probably derives from water molecules' becoming more disordered as a molecule of hexane leaves the aqueous layer and enters the organic layer.  IIRC the enthalpy of forming two layers is slightly unfavorable.  It might be interesting to look into how much of each member of the series actually is dissolved in water.

[curiouscat]

It might be interesting to look into how much of each member of the series actually is dissolved in water.

At 20 °C mg L-1

C4H10  61
C5H12  40
C6H14  9.5
C8H18  7
C9H20  0.22
C15H32      2.866×10^-3


Fairly consistent trend?

[Babcock_Hall]

Interesting.  There is a table (7-4) of entropies and enthalpies for transferring methane, ethane, toluene, and other hydrocarbons from water to certain organic solvents in Voet and Voet, Biochemistry, 3rd ed., p. 174.  The transfers are all endothermic or athermic.  Zubay's Biochemistry textbook mentions that hexane has a small, favorable enthalpy of dissolution into water, meaning that it is slightly endothermic to move hexane out of water, and into the hexane phase.

[curiouscat]

Interesting.  There is a table (7-4) of entropies and enthalpies for transferring methane, ethane, toluene, and other hydrocarbons from water to certain organic solvents in Voet and Voet, Biochemistry, 3rd ed., p. 174.  The transfers are all endothermic or athermic.

In my experience these things are very hard to reason about. Differences are small and long range ordering in the Liq. phase is notoriously hard to predict even with state of the art simulations.

[Babcock_Hall]

The party line is that the hydrophobic effect is an entropy effect and that it is based upon the need of water to order itself as it forms a cage around a hydrophobic molecule (the cage maximizes H-bonds).  That certainly appears to be the case for benzene or toluene dissolved in water versus in their own phase.  There is no change in enthalpy, but the entropy is quite favorable.  However, someone who knows more thermodynamics than I do pointed out to me that the situation changes quite a bit on going from 25 °C to 100 °C.

[Greenspin]

Thanks for all the reply's!