I prepared a 1 WT% solution of sodium alginate and diluted it to 0.25 WT% with ionized water. I first tested the particle size of this 0.25 WT% sodium alginate solution and kept it stirred at room temperature for one week after the test. After one week, I found that this 0.25 WT% sodium alginate solution produced a lot of small white precipitates. I would like to know why the 0.25 WT% sodium alginate solution, which was dissolved before, has precipitated after continuous stirring.
In fact, I prepared another 0.25WT% sodium alginate solution in the same way, and this solution was left on the lab bench without stirring after I measured its particle size, and a week later I saw some white precipitates, but not as much as the stirred one. I also wonder if the polymer molecules in the solution have gathered together over time? Why did they clump together?
I would appreciate it if you could answer my question.
Maybe there happen nucleophilic attack over time then cross links form between some polymer chains or aggregation may occur for polymer chains because of hydrogen bonds.
Sina Safavi
Thanks for your answer!!
can I ask one more question?now I understand why the polymer chain will gather together over time. But how the precipitates created? How do I link the gathering of polymer chain with the precipitates?
thank you so much.
Hi Weinire Tuerxun,
Alginate can precipitate if the charges that keep it solubilized are interacting with an oppositely charged ion (a cation). But biopolymer solutions are also prone to become hazy due to microbiological infections. So, first step is to find out if the precipitate is indeed alginate and not bacteria, yeast or molds that are feasting on your solution. Isolate the precipitate and run FT-IR on it, FT-IR requires a minimum of sample so this should be feasible.
After you have established the precipitate is indeed alginate, I would check the pH of the solution. In solutions with low salt content the pKa of alginate climbs to values above pH= 4.5. If you are close to the pKa the charged groups on the alginate become neutralized, aggregate and cause precipitation.
If it is not pH, then check for bivalent metal ions, as these will crosslink the alginate. You can prepare alginate solutions in tap water (with some Calcium ions in it) and you will obtain a clear solution as the calcium ions are initially spread over many different alginate molecules, however in time they will migrate and collect on the smaller alginate chains and cause precipitation. Alginate solutions should be kept in clear PE/PP/PET bottles and any contact with metal surfaces (stirrers, vessels, metallic covers etc.) is to be avoided. Also some glasses may leach bivalent metal ions, so I never stored my alginate solutions in glass.
Anyway, I do not believe in nucleophilic attacks leading to covalent bonds being formed, these are non-covalent interactions causing the neutralization of the charged groups leading to precipitation.
Kind regards, Leo
Dear Weinire Tuerxun there is some parameter that precipitates formation depends on them like zeta potential or particle weight. Some physical properties will change When chains aggregate. hence for example aggregation and cross links increase particles weight and size then precipitates form because gravity take them down or if chains or particles charges change and cause reduction of zeta potential then chain or particles will not stable in solution then precipitates form.
The sodium alginate salt is prone to react with atmospheric CO2. After agitation, moreover if accompanied by swirling, the salt can become carbonated, what would also be favoured by prolonged contact with the laboratory atmosphere. The pH of the solution tends to be lowered.
Alginate gelling tends to be favoured at lower pH, while higher pH tends to favour the sol colloidal phase. Once gel flocs start to form, such recently formed gel flocs may loose water by syneresis while they age, rather than to coalesce with adjacent gel flocs to extend the swollen hydrogel network structure. Eventually, the density gradient between the aged gel flocs and the suspending liquid solution may tend to favour flocculation, over gelation. Agitation also tends to oppose gelation, being this effect prolonged for some time after being stopped.
Leo Vleugels
Dear Leo Vleugels,
Thanks for your very detailed answer. I really appreciate your help.
After one week stirring, the 0.25wt% sodium alginate solution give me pH value around 5. But unfortunately, I don't know the pH value before I stirring it.
(And I just roughly checked the pH value by using pH test paper, I don't know is it a correct way or not?)
Actually I am a little bit confused about what you talked about the pH value. Could you please explain it a little bit more for me? (really sorry for that.)
About bivalent metal ions, when I diluted it, I used deionized water, the calcium content of this deionized water is lower than 1ppm. (I only measured the calcium content once before.) But yeah I think maybe the stirrers and glass bottle it self leach bivalent metal ions, and it affect my solution.
What I am thinking now is, I think the electrostatic repulsion between the polymer chains becomes weaker with time, then the alginate chain get close to each other and form very stable hydrogen bounds, this large amount of stable bonding makes it insoluble in water and become precipitate. (The second sample which I left on the lab bench without stirring, also formed precipitate, but not so much , so I think the stirring process accelerates the proximity between polymer chains and increases the point of polymer interaction (formation of hydrogen bonds).
How do you think?
Sorry for asking too much and thanks again for your answering.
Sina Safavi
Dear Sina Safavi, thank you very much for explaining it for me. Now I understand your meaning. And I will try to figure out is it because of aggregate of polymer or because of the crosslinking happened between alginate and bivalent metal ions. Thank you so much.
Carlos Araújo Queiroz
Dear Carlos ,
Thank you for your explanation, it helps me a lot!!
I think that Carlos Araújo Queiroz has the most plausible answer. Indeed drawing in carbon dioxide is stimulated by stirring and it is prone to lower pH to a point where most charges on the alginate chains are protonated (and thus uncharged and prone to aggregation). If you left an open beaker while stirring, this may be the cause. Well, here's a thing to try (if not already suggested) add a tiny bit of a buffer to keep the pH at > 7. There are ample recipes of buffers, select one that matches with the purpose and requirements of your experiment. Next, keep bottles always closed and in case of doubt, measure pH with a calibrated pH-meter, pH indicator paper is too inaccurate.
Leo Vleugels
Thanks for your replying, actually the bottles always closed during the stirring process. Okay I will measure pH with a calibrated pH-meter.
Thanks a lot.
As the researchers said (thanks for this helpful information)
just make sure:
Filtration the final solution
Check the pH
Deionized water as a solvent
Use airtight container
Store it in 4 C
Weinire Tuerxun
Seems no one has asked the obvious question. Why stir for 1 week? Why not 1 month or 1 year?
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