Regensburg 2022 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 12: Poster 1

CPP 12.10: Poster

Montag, 5. September 2022, 18:00–20:00, P1

Huge pKa-Shifts in Weak Polyelectrolyte Brushes Explained by Coarse-Grained Simulations — •David Beyer¹, Christian Holm¹, and Peter Košovan² — ¹Institute for Computational Physics, University of Stuttgart, D-70569 Stuttgart, Germany — ²Department of Physical and Macromolecular Chemistry, Charles University, Prague, Czechia

Following recent experiments, we study the titration behaviour of weak (pH-responsive) polyelectrolyte brushes at different salt concentrations using coarse-grained computer simulations. To account for charge regulation and the exchange of small ions with the bulk solution, we make use of the Grand-Reaction Monte-Carlo method (G-RxMC). Our simulations yield ionization curves which strongly deviate from the ideal result. Furthermore, we observe a strong dependence of the deviation on the bulk salt concentration, amounting to a shift of approximately one unit of pH as the salt concentration decreases by one order of magnitude. We theoretically explain the observed titration behaviour as a consequence of the Donnan partitioning between the brush and the bulk solution. To confirm our theoretical explanation we measure the pH inside the brush, which eliminates the Donnan contribution. Our results show that the Donnan effect can account for a shift in pH between the bulk and the brush of more than 4 units in extreme cases. We finally plot the degree of ionization as a function of the pH inside the brush, thus eliminating the Donnan contribution. Up to a small shift due to the electrostatic interactions, the resulting curves almost coincide with the ideal result, thus confirming our hypothesis.