[Homeslice]

In this thread:

https://www.chemicalforums.com/index.php?topic=109440.0


I create from soap a mix of fatty acids.  Let's say I want to separate those fatty acids.  For example, oleic and linoleic acids - they have slightly (or more) different melting points and boiling points. 

Any reason I could not separate them out by using distiller tube and heating them to a temperature that is the boiling point for one, keeping it at that temperature for a good while until that is all boiled into the collecting flask, then changing the flask, increasing the temperature to the next one, rinse and repeat over and over?

Or getting a laboratory freezer and turning it down to just below the melting point of one, waiting until that one crystalizes, then pouring what's left into another flask and removing the first one, then turning down the temperature further down to the next one on the list, rinse and repeat?

Any reason those types of things would not work?

Thanks!

[billnotgatez]

I did a GOOGLE

heating organic compounds

and got several interesting results
to include safety issues

[rolnor]

Its not a simple thing to separate things with distillation, the bp need to be different by some degrees. Fatty acids like this have very high bp and they are not so different, you get a mixture. You can use a column but still its not easy. Separate by mp is even harder, in this case impossible.

[Homeslice]

Thank you bill and rolnor!  I'll check out your link bill!

rolnor, understood completely about the boiling - their boil points are not THAT far apart so you will always get some kind of mix.

On the melting point/freezing route though, why do you say it is basically impossible?  Take linolenic acid for example, its melting point is -5 degrees Celsius.  Compare that to oleic acid, that looks like it has a melting point of around +14 degrees Celsius.  That is almost a 20 degree separation.  If one had a laboratory freezer that could hold accurate temps, and set it to 10 degrees Celsius, wouldn't that cause the oleic acid the crystalize, then you could pour out the liquid, leaving the solid oleic acid behind, and you would know what you poured out was essentially oleic acid free?  I understand for some fatty acids that have melting points VERY close this would not work nearly as well.

Thanks!!!

[Homeslice]

*Ignore me, I am impatient* for rolnor! 

[rolnor]

If you mix alcohol and water and cool this, will the water crystalize out at 0°C and leave pure alcohol as liquid?

[Homeslice]

rolnor, i thought if you mixed ethanol and water, the water WOULD freeze out, leaving a much more pure ethanol.  Maybe not right at zero, but at some point. 

At this link:

https://en.wikipedia.org/wiki/Fractional_freezing

It says:

Ethanol and liquid water are completely miscible, but ethanol is practically insoluble in water ice. That means almost pure water ice can be precipitated from a lean ethanol-water mixture by cooling it sufficiently. The precipitation of water ice from the mixture enriches ethanol in the remaining liquid phase. The two phases can then be separated by filtration or decanting.


But will that approach not work here?  Maybe not, because unlike ethanol being practically insoluble in water ice maybe the fatty acids are fully soluble the the solid crystal form of one another?

Thanks!!

[rolnor]

Maybe I took a bad example then! It also says "cool sufficiently" you probably needs very low temp. What I mean is that your acid will not crystallize out, you just get a mixture like syrup or semi-crystaline mass upon cooling. The acids are so similar that they work as solvent for each other. You do have a problem with separation, maybe if your mixture contains very little impurities you can just crystalize. I dont know how crystalline your acid is, this is a long-chain molecule, I think it is hard to get good crystals. Distillation needs low vacuum and is not trivial if it works at all.

[Homeslice]

Yes rolnor, I figured what you were getting at.  Water will freeze out of a water/ethanol mix because when that water hits freezing point and turns to ice, ethanol is completely insoluble (if that is the right word) in the ice, so ice freezes out, and you are left with the ethanol. 

But with those two fatty acids, they are so close physically that they are completely soluble (or miscible maybe is the right word?), so the mix of them just tend to freeze together and it does you no good to separate them. 

I guess I am going to be looking at vacuum distillation units soon to give it a try. 

Thanks so much!!!