Miriam’s comment on: Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

By Miriam I. Otero Rivera
MIO blog image 18-05
In this article by Bergbreiter’s group, present the effect of a LCST event on the solution pH of a polymer like poly(N isopropylacrylamide) better known as PNIPAM. They present different experiments were they change pendant groups on PNIPAM and all of these resulting polymer derivatives had lower critical solution temperature (LCST) properties. In the experiments they show how after heating the solution to its LCST the polymer experience a dehydration and the polymer phase separates from solution. In several experiments, the presence of a cationic or anionic pendant groups, that have low or high pH values respectively, its stabilization is dependent of its solvation, and after the LCST event, the species becomes neutral since it is more stable in the dehydrated state resulting from the LCST phenomenon. These experiments show that changes in the environment of the polymers results in great changes in solution acidity and basicity. Also they show that these process of hydration and dehydration is reversible by cooling the solution they obtain the polymer back in solution and the pH was revert to its initial value, this process of heating and cooling was reversible for up to 100 cycles.
Similarly, the article studied the effect of the concentration of the polymer, and the mole % loading of the pendant group (carboxylic acid) in changing the bulk solution pH as a function of temperature. First they saw that if the pH of the solution is significantly different from the pKa of the carboxylic acid (4.76) there is no change in pH. They also saw that at lower mole % loading of carboxylic acid the cloud point curve is most narrow and that at lower concentration of the polymer the clouding curves are broader. Also, studies with solutions containing added salts (LiCl or LiBr) show that the solution’s ionic strength does not significantly affect the ΔpH, although the clouding curve’s onset changes. Finally, they performed experiments with kosmotropic salts, which lower the LCST below room temperature due to the Hofmeister effect, as a result the addition of these salts to solutions lead to the same behavior of changing the pH of the solution, and also the process was reversible. This experiment was visualized using the pH-indicator phenolphtalein. In this experiment they also used visible spectroscopy to show how  reaching  the LCST temperature produce an inflection, before a drastic change in absorbance.  This article show how properties in the pendant group of a polymers change pH of solution as a result of changes in solubility because of changes in temperature, showing the thermoresponsiveness of polymers. This article was one of relevance to our laboratory because we work with LCST events on supramolecular G-quadruplexes. This article can be used as a reference to know the resulting effects on solubility and pH of an LCST event on the systems with which we work in our lab.


2 thoughts on “Miriam’s comment on: Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

  1. Rating (synopsis): 3/5
    Rating (figure): 3/5

    What is a pendant group? As expressed in the article they have a purpose, but I think this is missing from this comment. While Miriam focuses on the results showing them in a simple language; the point of the paper is missing in her narrative. As in other web entries, without context of the rational of why, it is a hard to follow while read. This is not only missing for the general knowledge of the paper, but also for experiments. Summarizing experiments does not exactly tells us why the following ones where done. One can infer of course but it is not very clear by her post. That being said, as a summary per experiment of results it is very clear. One has to appreciate that. As the image goes, same as the comment, it splits the results with no real connection to the story.

  2. Rating (Synopsis): 3/5
    Rating (Figure): 2/5

    Miriam’s synopsis would have benefitted from a little more proofreading and editing, as it has numerous grammatical errors and misuses of commas. However, the information is there and is understandable after reading sentences a couple of times. To expand on Miriam’s synopsis, pendant groups are side-groups that are attached to polymer chains (in this case, PNIPAM). She did not mention that in the synopsis and neither did the authors in their article, but that information is only a quick Google search away, so I don’t think it’s something to get hung-up on. This synopsis attempts to summarize the article’s main findings, but, in my opinion, it includes too much experimental data that is presented with little to no connection to each other, and this makes the synopsis hard to follow.

    I recognize that Miriam attempted to include information about the authors’ work with kosmotropic salts in her TOC figure, given that the authors did not do so in the TOC figure they published. However, the placement of this information in the figure seems disconnected from the rest of it. I liked the original TOC figure and how Miriam modified it by adding a PNIPAM monomer, but I don’t think the graphs are necessary to make the figure better, even if I am impressed by the reversibility of the heating-cooling process of the polymer.

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