tandpasta
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Prevent dichloromethane/water emulsions?
The below story is not really relevant to my question, which is at the end (bolded).
I tried to extract some caffeine from tea leaves, but got a piss poor yield. The primary reason was (I think) that I did not mix the solvent and water
layer well enough because I was afraid of creating an emulsion (which happened after the first addition of dcm and lightly shaking my sep funnel).
Here's what I did, in case you're curious:
-Boil 50g tea leaves in water under reflux for 3 hours, repeat with fresh water
-Flush tea leaves with small amount of water
-Combine all the water
-Boil down the water to a manageable amount
-Add sodium carbonate to prevent the tannins from being extracted
-Add to seperatory funnel with 30ml dcm
-Slightly agitate
-Get emulsion, add brine (helped a bit)
-Drain off dcm, add another 30ml dcm and slightly agitate
-Get a bit more emulsion, tried waiting (didn't really help)
-Drain off dcm, add another 40ml dcm and slightly agitate
-Get a bit more emulsion, tried eating poison
-Drain off dcm and distill to yield solids (white with small amount of green (chlorophyll)
-Add water to solids and recrystallize
My yield was only about 50mg, at least 40 times less than I expected.
I made a couple of stupid errors (this is my first real experiment) which undoubtedly impacted my yield, but I think the major cause of the extremely
low yield is that fact that I only very gently agitated the dcm and aqueous phase the second and third time. Even more gently than the first time
(which was quite gentle). I also didn't do it long enough.
edit: irrelevant update: what I extracted may not even be caffeine, it's only very slightly bitter, very unlike the pure caffeine powder that I
bought.
So, my question now is, is there any good way to prevent dcm/water emulsions? Adding brine only helps a bit, but also contaminates
the final product (a trace amount of NaCl ends up in there, I think). I don't have a centrifuge.
I read about filtering through a cotton ball in a funnel, but this apparently is quite slow and it's a band-aid, not a preventative measure.
Or should I just use hexane next time and just use the dcm for anhydrous experiments?
[Edited on 30-6-2016 by tandpasta]
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Metacelsus
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In my experience, emulsions are exacerbated by the presence of fine particles. Did the tea solution have any suspended solids before you added DCM?
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Tsjerk
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Do you have any means to centrifuge your sample? That always does wonders.
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tandpasta
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Metacelsus, yeah, that was caused by two of my errors:
1. only ordering filter paper impregnated with activated carbon;
2. me poking needle holes in the the coffee filter that I used because it was only letting through <100ml/24 hours
I'm ordering some different types of filter papers for next time. I'm thinking to first filter with #4 (pore size 20-25µm) to get coarser stuff out,
and then with #1.
Or would also using #3 (pore size 6µm) prevent emulsions even better?
Tsjerk, I don't have a centrifuge, unfortunately. I do have a spare vacuum cleaner motor, but I would meet a certain death if I constructed a
centrifuge with it.
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Marvin
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You were expecting 2 grams of caffeine in 50 grams of tea? That seems excessive. The estimates online seem to vary by a factor of 10.
One of the extractions on the internet suggests filtering the emulsion with some glass wool in the stem of the separatory funnel, which the water
phase sticks too.
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Praxichys
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The sodium carbonate probably reacted with fatty acids in the tea under heating and formed minute quantities of soaps, creating the emulsion.
You could try defatting. Macerate the tea leaves in dilute hydrochloric acid. Filter through as fine a filter as you can; a vacuum filtration setup
with filter paper is ideal. Run an extraction with the DCM, which should remove fatty acids and triglycerides. Distill the DCM to recover it. Basify
the HCl/tea solution with sodium carbonate, add the DCM back, and extract the caffeine.
This relies on the fact that the solubility of salts is proportional to the polarity of a solvent. In acid conditions, the fatty acids are free while
the caffeine is its HCl salt; in basic conditions, the caffeine is the free base and moves to the DCM layer, and the tannins are quite protic and stay
with the water.
If the emulsion still gives you a problem, don't bother with brine. Just shake it directly with rock salt. You want a saturated solution. Potassium
carbonate actually works a whole lot better than salt, if you have it. It will even salt MeOH from water. Just be careful - if you add too much you
might begin to precipitate tannins.
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