By Miriam I. Otero Rivera
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.