A Comparison of Bead-Spring and Site-Binding Models for Weak Polyelectrolytes

Abstract

Understanding the ionization behavior of weak polyelectrolytes in aqueous solutions with added salt is crucial for designing advanced materials. Predicting the ionization states of weak polyelectrolyte is challenging due to the interplay between long-range Coulomb interactions, conformational flexibility, and chemical equilibria. Bead-spring models with explicit ion treatment provide accurate results but are computationally expensive. In contrast, Ising-like site-binding models are computationally efficient but neglect conformational flexibility and use an implicit salt description. To assess the validity of these approximations, a site-binding model is compared with bead-spring models that include implicit and explicit ion treatments. These results show that under strong electrostatic coupling, explicit ion treatment is critical for accurately modeling ionization behavior. Both the site-binding and implicit bead-spring models overestimate monomer correlations in this regime, leading to significant deviations from the explicit bead-spring model. Under weak coupling, typical of aqueous environments with monovalent salts, all models give reasonable ionization curves, with slight differences. The implicit bead-spring model shows slightly stronger suppression of ionization, while the site-binding model aligns more closely with the explicit bead-spring model due to compensating errors in ion treatment and flexibility. In conclusion, while all models perform well under weak coupling, explicit ion treatment is essential for accurate ionization under strong coupling.