In order to remove free acid and raise pH After acid hydrolysis of MCC with H2SO4, I centrifuged the samples at 10000 rpm (about 17000 g) and then I observed a layer of brown material at the bottom of the falcons which I guess is the oxidized cellulose. I separated it from the samples and centrifuged the rest white sediment. but again after the second centrifugation the brown layer formed. Can centrifugation degrade cellulose nanocrystals due to high force, raised temperature, high acid concentration, or ....?
I didn't noticed any sensible temperature rise after centrifugation, and the pH was below 1.5
Thanks in advance
Dear Catherine
I actually do not know what happen with your sample. but I think you should first stop the reaction by adding immediately ten fold, your solution, cold water or ice. this is done before centrifuging and I think it will be enough to stop the hydrolysis. and I think this brown ppt is due to different constituent as lignin. did you analysis it in your MCC sample first?
Dear Mona
Thank you for your helping and answer. I did stop the reaction by the way you mentioned (ten fold cold water and putting in ice bath). I think if the material was lignin, it would't formed in the second centrifugation, would it?
I think perhaps it is the oxidized cellulose which has formed during acid hydrolysis and is more stable than cellulose in water due to polar functional groups, so I can't see it in the dispersion. but after centrifugation the heavier oxidized cellulose is deposited at the bottom of the falcon and can be seen, but I'm not sure.
Sorry Cathterine, I can not provide explanation with your problem, here I send your paper maybe help what your need.
Thanks
Dear Catherine,
You can collect the samples before and after the centrifugation, then freezed-dry them and check these powders by ATR-FTIR. If there is any change in the structure because of the oxidation process then the spectra should tell you.
Hope you can find something interesting in your phenomenon!
Le Quynh Hoa
If your hydrolysis conditions were too harsh you will form yellowish to brown impurities (could be the oxidized cellulose you are talking about). It could also be that your raw materials has residual lignin. In the centrifuge your impurities concentrate at the bottom, it can still show up the second time in lower amounts. Try using less acid, or lower temperature for the hydrolysis, but make sure your product still has the expected crystallinity. You still need enough acid to get rid of all the disordered cellulose.
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