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Author: Subject: How does activated charcoal selectively decolorize oils without just adsorbing the desired oils?
Electra
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[*] posted on 31-1-2018 at 19:44
How does activated charcoal selectively decolorize oils without just adsorbing the desired oils?


Activated Charcoal is frequently used to decolorize water or oil solutions. I understand how, in that colored compounds are usually non polar, large, and easily adsorbed. What I don't understand is how such a technique can be used on an oil, and not just absorb a bunch of the desired oil instead.

The only thing I can think of is that the adsorption process may be reversible, with the bulkier compounds being the ones that ultimately get stuck, displacing the other nonpolar compounds... however I was under the impression that adsorption onto activated charcoal was not an easily reversible process that would take place passively in a solvent. Is it?


If this were the case, could activated charcoal be used to remove side products in your reaction that are much bulkier than your desired product? Imagine a nonpolar solution where 95% of it is a liquid with a molar mass of 150g (random), and the impurity is double or triple that. Typically distillation would be the easiest way to get purify this, but could activated charcoal be used in some regard?
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[*] posted on 1-2-2018 at 05:45


Well I know that a lot of coloured organic compounds contain double bonds and other reactive groups, such as beta carotene - despite being a hydrocarbon, it is deep orange, and also azo dyes which have the -N=N- system. Activated carbon is used as a heterogenous catalyst, so that makes me think that it catalyses the decomposition to non-coloured molecules. I don’t believe that the focus of using it to decolourise solutions is removal of the responsible species as such since smaller molecules adsorb more easily, like many gases for instance, but on the other hand I do believe it depends on the specific mixture and how the carbon is able to catalyse some reaction.



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[*] posted on 1-2-2018 at 06:13


Quote: Originally posted by Electra  
... could activated charcoal be used to remove side products in your reaction that are much bulkier than your desired product? ...

i don't know either.

If you can find a fairly simple reaction that produces enough suitable side-products that could be easily detectable by an amateur (e.g. colour) then this sounds like a do-able experiment.




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[*] posted on 1-2-2018 at 12:34


I had come across a page that said adsorption onto virgin carbon is non-reversible, whereas adsorption onto solvent regenerated carbon is. From what I can gather, solvent regenerated carbon is simply carbon that is... being mixed with a solvent. So it seems to be suggested that within a solvent, the adsorption is a reversible process.

I have an experiment in mind that I can test this with I think.
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[*] posted on 1-2-2018 at 13:07


Please share.

Quite often "I have an experiment in mind " ends up undone, for various reasons (life etc)

Share, and it might get done.




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[*] posted on 1-2-2018 at 15:03


Most molecules should have a hard time finding a good reason to bond to C in some way, especially if they're small.



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[*] posted on 1-2-2018 at 15:15


So dyes can be removed with activated carbon for example if food coloring was in water the activated charcoal removes it
Damn lying britta filter I was wondering how they say it removes dissolved solids

[Edited on 1-2-2018 by symboom]




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[*] posted on 1-2-2018 at 15:49


Great question, and one I've wondered about myself. I wonder if the affinity of activated carbon for larger non-polar substance is related to the electrical conductivity of graphite, giving its electrons greater mobility, and thus, a greater capacity to move around to better 'fit' the distribution of charge across the surface of a given molecule. That might explain why many compounds adsorb fairly tightly onto activated carbon, effectively keeping the adsorbed material from showing a vapor pressure/escaping from the surface.

Saturation with solvent might reduce the activity of the carbon by interacting with the electrons of the graphite, giving them less freedom to move and electrically conform to the surface of the adsorbed material (hence, a less tight, more reversible adsorption.)

There's probably an issue of the attraction between the adsorbed molecule and the solvent. The adsorbed molecules aren't going to have any significant interaction with air molecules, but solvent molecules will contact the adsorbed molecule much more intimately and create more forces on it that could remove it from the graphite.

As far as why it might selectively remove a discoloring contaminant instead of just saturating with your main product, I'd be curious to know how general the trend is. If it's relatively limited (some cases it works, some it doesn't) I would suspect it's something particular to those chemicals. But if it's a broader phenomenon (where the activated carbon shows a preference generally for adsorbing the minor component), I might wonder if might be something similar to crystallization, where the main product is being drawn back into solution (pulled off the carbon) because it conforms easily to the surface of other readily available product molecules, while the minor product (contaminant) is less likely to match back up with another highly conforming molecule and get pulled back into solution.

If that is the case, the activated carbon adsorbs both materials readily, but the trace product loses the equilibrium footrace due to few 'allies' (matching molecules) left in solution to help it get back off the carbon.

I truly don't know the answer. Fun to think about, though. :-)
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[*] posted on 1-2-2018 at 16:06


Activated carbon is often used in chromatography, to allow some products to elute, and others to be detained, like any such substrate.

I have yet to hear of it being used in situ to remove reaction products as they form, but it could be a novel synthetic technique!




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Electra
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[*] posted on 1-2-2018 at 22:36


Quote: Originally posted by PirateDocBrown  
Activated carbon is often used in chromatography, to allow some products to elute, and others to be detained, like any such substrate.


I've never been exposed to this before. When you say it's used in chromatography, do you mean in a way similar to silica gel, where it slows the movement of certain types of compounds, or when you say "detain", do you mean it is used to permanently retain some compounds during chromatography?

The latter would be more akin to a simple filtration I would think. People filter through carbon all the time to remove trace nonpolar impurities in water, or dyes in either oil or water.

Quote: Originally posted by aga  
Please share.

Quite often "I have an experiment in mind " ends up undone, for various reasons (life etc)

Share, and it might get done.


Nothing too serious in mind yet, just a few rough ideas that I need to think on more.

[Edited on 2-2-2018 by Electra]
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