Question
Asked 8th Feb, 2022

How does pH (pH 3-6) affect the viscosity of a xanthan gum solution (manipulated by acetic buffer)? Does the helix-coil transition play a role here?

I'm currently looking at the rheological properties of the polymer Xanthan Gum. focusing on its dynamic viscosity to be more specific. I'm assessing the effects of pH (ranging from 3.6 to 5.6, 0.4 increment, total of 6 pH's) on the dynamic viscosity of xanthan gum solution (dissolving xanthan gum powder into acetic buffer with equal ionic strength, concentration is kept at 0.04%).
Firstly, my viscosity data collected shows that, as pH increases from 3.6 to 4.0 then 4.4, the viscosity increases; but as I bring up the pH from 4.4 to 4.8, 4.8 to 5.2, then lastly 5.2 to 5.6, the increasing viscosity trend plateaus and the increase in viscosity is less significant compared to the 3.6-4.4 jump. At this range, does pH has an effect on the viscosity of xanthan gum based on its molecular configuration? Though some sources states that xanthan gum's viscosity remains stable and unchanged within the range of pH 3-12 at a high concentration like 1% not 0.04%, yet some suggest pH still plays an effect, though I'm not sure how on the chemical and molecular aspect.
A possible conjecture I can think of is the xanthan gum's order-disorder and helix-coil transition is affected by protonation. In figure 2, it demonstrates how electrolytes affect the structure of the polymer; in figure 3, it shows how at a state of a helical rod and no longer a random coil, it is capable to hydrogen bonds among each other. Hence, I'm wondering of pH plays an effect on it's structural transition, such that the increased intermolecular forces at the form of a helical rod would make it more viscous in solution.
Here are the resources I have used so far:
Brunchi, CE., Bercea, M., Morariu, S. et al. Some properties of xanthan gum in aqueous solutions: effect of temperature and pH. J Polym Res 23, 123 (2016). https://doi.org/10.1007/s10965-016-1015-4

Most recent answer

9th Feb, 2022
Abdelkader BOUAZIZ
SOUGUEUR Division - Chemistry (POLYMERS), Université Ibn Khaldoun Tiaret
Dear Ryan Lo, you may find various stufies on this topic. Please have a look at the following free access RG fille. My Regards
9 Recommendations

Popular answers (1)

9th Feb, 2022
Abdelkader BOUAZIZ
SOUGUEUR Division - Chemistry (POLYMERS), Université Ibn Khaldoun Tiaret
Dear Ryan Lo, you may find various stufies on this topic. Please have a look at the following free access RG fille. My Regards
9 Recommendations

All Answers (5)

8th Feb, 2022
Abdelkader BOUAZIZ
SOUGUEUR Division - Chemistry (POLYMERS), Université Ibn Khaldoun Tiaret
Dear Ryan Lo, xanthan is a polyelectrolyte with wear carboxylic acid functional groups, i.e., their dissociation is pH dependent. With increasing pH, the dissociation becomes important. This lead to both intera- and inter- molecular repulsions between carboxylate groups, which imparts a more or less extended chains, even the helix arrengement is affected. When you reduce back the pH, you are just reducing chains extension because part of the carboxylate groups retun back to their acid form. Other parameter with minor contribution to the viscosity is the possible formation of H-bonding of the carboxylic acid groups. My Regards
7 Recommendations
8th Feb, 2022
Dj Flesher
Independent Researcher
May I ask a Question or two:
1) What methods are you using to measuring viscosity? Different ways will often produce different results.
2) Are you measuring the entire batch of materiel or a sample?
3) If a sample, how are you sampling/
1 Recommendation
9th Feb, 2022
Ryan Lo
Canadian International School of Hong Kong
1. Using a Pin’s glass capillary viscometer
2 & 3. A sample only; they share the same concentration Of 0.04%, yet different pH of acetic buffers were used to make the xanthan gum powder up into solution. The concentration is extremely low, I’m not taking a sample from anywhere but I’m making up the xanthan gum solution myself
Attachment below is my rough outline of the method I’ve used.
9th Feb, 2022
Dj Flesher
Independent Researcher
Ah-ha! I may be able to help... a little at least.
I had a problem years ago where the type of viscometer that you are using and a dynamic or spinning viscometer (see: https://www.youtube.com/watch?v=FRtqhVPRmOY) gave completely different results.
The product we were testing was clear and, under testing was a 'solution'. It was a mixture of liquid components. However, we later discovered that the "solution" was actually two dynamically changing emulsions. One emulsion was oil-in-water, the other water-in-oil. The micelles formed between the two were in a constant state of inversion, invisible to the human eye. I coined the term 'Poly Colloidal Epitropic Emulsion'. When we tested the emulsion with a particle size detector we found even more (strange but revealing) phenomena.
How does this apply to your experience? You will need to decide. But the key information is that even in a clear solution yo can have invisible micelles that are large, perhaps large enough to block or inhibit the flow through the neck of the capillary viscometer. That gave us High Viscoscity readings on the capillary viscometer and low readings on the dynamic viscometer.
1) One suggestion would be to test your fluids with a dynamic viscometer and/or a Particle size analyzer (Anton Paar brand might work, see: https://www.anton-paar.com/us-en/products/group/particle-size-analyzers/ ). That could give you better information about the "true" (micro or macro) physical nature (size) of possible 'clusters' of xanthan that appear to be a solution (or a slurry).
2) The pH that you are trying to adjust could be changing the cluster (micelle - like) particle size, and thus the real or apparent (micro or macro) viscosity.
3) And that viscosity change might also be related to the chemistry of the pH control choices. (E.g, acetic vs hydrochloric acids... etc.)( I don't know what you are using.)
Good Luck, hope this helps.
9th Feb, 2022
Abdelkader BOUAZIZ
SOUGUEUR Division - Chemistry (POLYMERS), Université Ibn Khaldoun Tiaret
Dear Ryan Lo, you may find various stufies on this topic. Please have a look at the following free access RG fille. My Regards
9 Recommendations

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