Impact of Charge Correlation, Chain Rigidity, and Chemical Specific Interactions on the Behavior of Weak Polyelectrolytes in Solution

In this work, we performed titration simulations of weak linear polyelectrolytes via the Monte Carlo method and the constant pH ensemble aiming to understand how polyelectrolyte concentration, chain rigidity, and the formation of intra- and interchain charged hydrogen bonds (c-H-bonds) impact on ionization and conformations of polyacidic species, counterions (CIs) distribution, and system Helmholtz energy. Increasing polyelectrolyte concentration resulted in enhanced acidity for all cases investigated due to the increased screening of chain charges by CIs and, when possible, the formation of interchain c-H-bonds. Our simulations also evidenced that polyelectrolytes able to form c-H-bonds can populate simultaneously two conformational states (aggregated and unfolded) in a range of pH, the transition between the two appearing first order-like. To better understand how properties of two polyelectrolytic chains are modified by their relative distance, we performed window sampling (WS) simulations, which highlighted nontrivial features in the ionization and conformational behaviors. As byproducts of WS simulations, we obtained also the potential of mean force between two chains; from this, it emerges that the reversible work needed to reach a specific interchain distance does not always increase with the pH, especially for c-H-bonds forming semirigid chains brought at short distances.