I am looking for any article or supportive document which suggest the surface charge behaviour of polyacrylamide in water at different pH.
Hi Anuj!
Well, polyacrylamide is a polymer with no net charge. The only charge comes from the persulfate initiator which brings negative a negative charge to the polymer.as sulfate group. The sulfate group has a pKa around 2. Thus, you will see big differences from pH 0 (1% charges), pH 1 (10% charges), pH 2 (50% charges), pH 3 (90% charges), pH 4 (99% charges), and so on.
Nevertheless, as you mentioned that you have a gel, I assume that you have a crosslinked polymer network. In this case, no matter whether you have or not charges there, the gel (a single huge macromolecule) will never move. That's the reason why SDS-PAGE are prepared from crosslinked polyacrylamide.
If you have crosslinked polyacrylamide nanoparticles, then the most probable is that you will have negative charges in the outer part of the particle, and the number will strongly depend on the number of chains to form the nanoparticle – which depends on the chemical approach and stoichiometry.
SALUT!
Tony
Thanks Tony,
Yes you are right about the material surface charge.
I have acrylamide gel in nanosize, which is actually polymerized by irradiation i.e. without use of APS (so, no sulfate grps) or any othre type of initiator. And further, when I performed the zeta potential analysis, I found the negative zeta at pH 3. and negative value increases with increasing pH (I have tested from pH3 to pH11).
So, my interest is to know that which group is contributing for negative zeta potential from the polymer backbone?
Hi!
I need more information about the chemical composition for the obtaining of the gel. So, irradiation (alpha, beta, gamma, e-beam, X-ray???) generates radical from water and the HO* radical might start the polymerization process. Moreover, the amide bond might be cleaved and you can generate carboxylates with pKa around 4-5. This might explain your results.
Please, irradiation polymerization is wild-west chemistry, and unexpected things might occur.
Give me more details about the mixture before irradiation and I will give you some hints.
SALUT!
Tony
Hi again!
So, what I see is that you only had water aa, and baa... well, gamma irradiation is just creating HO* radicals from water. Moreover, do not forget the equilibrium in amides which leads towards OH groups instead of carbonyls. Thus, the proton from this OH groups might be extracted and form reactive O* radical centers, and so on... wild-west chemistry!
Did you performed FTIR or 13C NMR?
BTW, 1:1 aa/baa should form almost non-swellable materials… I guess that the reaction is not completed and many monomers are left in solution…
SALUT!
Tony
I agree with Antoni. Carboxylate groups can account for the observation (even apart from the preparation process, they can appear via the amide groups hydrolysis upon storage - this is quite a know process, especially in alkaline medium). However, at pH 3 the carboxylate groups should be almost completely protonated, and I would not expect any noticeable negative potential. Likely, some strong acidic groups are present (could be sulfate from APS, but Anuj says it was not applied... What was the starting acrylamide purity? Could some salt be present at some stage to be absorbed for some reason? Is the polymer washed of any low-molecular residues prior to the measurement?
Hi Evgeny,
I have used 99% pure acrylamide from sigma chemicals. And it was dissolved in deionized water with Bis-acrylamide (purity ~ 99%) for synthesis. There was no other reagents were used in the synthesis. However, after synthesis gel were washed with deionized water to remove all unreacted moieties followed by drying at room temperature for further use. Although, in between I repeated the experiment and the same results are obtained, unlikely the fact that acrylamide is an amphophylic polymer.
Thanks for the attention!!
Anuj
Anuj,
I still support the "wild west" chemistry products you obtain by gamma-irradiation. Could you be more explicit and say the something else about the z-potential values of your particles? and how does the value change as function of pH?
SALUT!
Tony
Hi Antoni,
At pH 3 its shows +_1 zeta, PZC around 5-6 pH and then reach to approx -25 at pH 11.
Sorry for the delay Anuj,
It seems to me that you have free carboxylic acids... pKa = 5...
That's all.
SALUT!
Tony
Hi Tony,
Thanks for the follow up.
Yes, carboxylic acid have pKa in between 4-5. But what are the possibilities of generating carboxylic grps during crosslinking in aqueous system by irradiation.
Thanks
Anuj
Hi Anuj,
So, basically the formation of OH radicals is the key point. You generate a huge excess of these OH radicals which react with the carbonyl group. After some rearrangement, you get the free carboxylic acid.
Check the file I attached.
Of course, other mechanisms are possible too.
SALUT!
Tony
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