Dear all.
I'm wondering what the pKa(s) of cellulose is. Is there a good measurement of this?
I know that the monomer (glucose) has a first pKa of about 12.28 in 25 C but I can't find the corresponding value for the polymer cellulose (or starch either for that matter). This might be connected to the simple fact that cellullose decomposes at alkaline pH and no good equilibrium is measurealbe. Does anybody know or even better have a reference article?
The different acidity constants (of the three OH-groups of cellulose) would also be of great interest(!)
Different methods are reporting different pKa values for cellulose. In my opinion pKa values also depends on type and source of cellulose. For more details please see following articles:
1.http://www.cellulosechemtechnol.ro/pdf/CCT7-8(2013)/p.515-525.pdf
2. Reuveny, S.; Silberstein, L.; Shahar, A.; Freeman, E.; Mizrahi, A. (Feb 1982). "DE-52 and DE-53 Cellulose Microcarriers: I. Growth of Primary and Established Anchorage-Dependent Cells". In Vitro. 18 (2): 92–98. doi:10.2307/20170418.
3. Peterson, Elbert A.; Sober, Herbert A. (1956). "Chromatography of Proteins. I. Cellulose Ion-exchange Adsorbents". Journal of the American Chemical Society. 78 (4): 751–755. doi:10.1021/ja01585a016. ISSN 0002-7863.
I agree with that the source (surface area etc) plays a role, but also what definition is used, in the exact value of the pKa. But I only want to know the approximate pKa value of the (three at each AGU) OH-groups of cellulose.
These papers refers to the pKa of ionizing groups on modified cellulose (e,g, N+-groups or COOH of cellulose derivatives) and not the OH-groups. I was not looking for that. Sorry.
Hej Ola,
Cellulose can be regarded as a polyelectrolye. This means that the deprotonation of the first "acidic" OH Group probably is around 12.2 ( approximately the same as for glucose). Successive deprotonation leading to a higher negative charge, will result in higher pKa values. However, the problem with a determinatrion of these constants is the degradation of cellulose in alkaline solutions. If the kinetics of the degradation is slow in comparison with the deprotonation reactions, maybe it is possible using e.g NMR to get approximate values.
Yes Staffan, I strongly believe that you are right. As you maybe remember I studied the deprotonation of COOH-groups of cellulose in my PhD thesis and we saw then that the surfaces "hold stronger" to the protons as the charge get more anionic charge. This (as you say) give the apparent pKa value a dependence of the charge.
I am a bit curious though and I think that the acid/base reactions in this case is much quicker than the degradation reactions. Maybe a "pKa" (Z=-0.5) around 13 or so is probable. It takes time to set up such an experiment but I may try...
Hello Ola Sundman.
I was arguing with my students about your question you presented here. I found this paper. Take a look:
Article Ionization of Cellobiose in Aqueous Alkali and the Mechanism...
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry