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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 17: Charged Soft Matter, Polyelectrolytes and Ionic Liquids I
CPP 17.3: Vortrag
Dienstag, 19. März 2024, 10:15–10:30, H 0107
Explaining Giant Apparent pKa Shifts in Weak Polyelectrolyte Brushes — •David Beyer1, Peter Košovan2, and Christian Holm1 — 1Institute for Computational Physics, University of Stuttgart, D-70569 Stuttgart, Germany — 2Department of Physical and Macromolecular Chemistry, Charles University, 128 00 Prague 2, Czechia
Recent experiments on weak polyelectrolyte brushes found marked shifts in the effective pKa that are linear in the logarithm of the salt concentration. Comparing explicit-particle simulations with mean-field calculations we show that for high grafting densities the salt concentration effect can be explained using the ideal Donnan theory, but for low grafting densities the full shift is due to a combination of the Donnan effect and the polyelectrolyte effect. The latter originates from electrostatic correlations that are neglected in the Donnan picture and that are only approximately included in the mean-field theory. Moreover, we demonstrate that the magnitude of the polyelectrolyte effect is almost invariant with respect to salt concentration but depends on the grafting density of the brush. This invariance is due to a complex cancellation of multiple effects. Based on our results, we show how the experimentally determined pKa shifts may be used to infer the grafting density of brushes, a parameter that is difficult to measure.
Keywords: Weak Polyelectrolyte; Polymer Brush; Monte Carlo; Molecular Dynamics; pH-response