Monte Carlo study of weak polyelectrolyte and polyampholyte microgels: an investigation of the pH-dependent interplay of conformation and ionization
Year: 2026
Type: dissertation
Abstract: In this thesis, Metropolis Monte Carlo simulations were employed to investigate the pH-dependent ionization and conformational behavior of weak polyelectrolyte and polyampholyte microgels under salt-free conditions and under explicit consideration of counterions. The methodological approach explicitly accounts for the chemical protonation and deprotonation equilibria of the acidic and basic groups as well as their electrostatic interactions, thus allowing for a detailed insight into the subtle interplay of polymer conformations and the ionization of acidic and/or basic groups. For weak acidic microgels, general system properties of the considered model and the influence of (i) microgel concentration, (ii) the fraction of acidic groups, and (iii) the crosslinking density were systematically analyzed. The simulations reveal a pronounced suppression of ionization compared to the ideal titration behavior, as can be seen by a shift of the titration curve to higher values of pH due to the local proximity of acidic groups within the polymer network. In contrast to macroscopic gels, counterions are able to leave the microgel, resulting in an effective network charge that hinders further ionization as compared to the ideal case. This effect becomes more pronounced at lower concentrations, for which counterion release is enhanced. The degree of swelling was observed to be mainly governed by the degree of ionization, with all investigated systems exhibiting a distinct transition from deswollen, uncharged networks to highly swollen, charged networks. Furthermore, the ionization was found to be inhomogeneously distributed, both radially across the microgel and along the individual polymer chains. For weak polyampholyte microgels, the impact of varying acid and base dissociation constants on ionization and conformation was studied. All systems displayed a pH-dependent charge reversal from positively to negatively charged states, with overall charge neutrality at the isoelectric point. This charge reversal was accompanied by a U-shaped swelling transition with a minimum of the microgel size at the point of charge neutrality. The width of the swelling transition was found to depend on the chosen relative acid and base dissociation constants, which control the strength and therefore influence of the intramolecular electrostatic interactions between acidic and basic groups. Stronger interactions broadened the transition, an effect further amplified by counterion release and the associated gain in translational entropy. The analysis of the radial ionization profiles revealed a variety of charge patterns that depend on the overall microgel charge. Taken together, these results provide a detailed microscopic picture of the pH-responsive ionization and swelling behavior of weak polyelectrolyte and polyampholyte microgels. The findings highlight the role of network conformation, chemical composition, and electrostatic interactions in determining the equilibrium properties of these model systems and thereby advance the understanding of pH-responsive polymer networks.
Source: RWTH Publications (RWTH Aachen)
Author: Hofzumahaus, Cornelius
Institution: RWTH Aachen University
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204279923
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