Charge Regulation in Crowded Solutions of a Weak Polyelectrolyte: Steric Interactions

Weak polyelectrolytes (WPE) are used to prepare responsive materials in a variety ofapplications. When dissolved in highly crowded environments, of biorelevant media or synthetic complex fluids, that contain substantial concentrations of macromolecules, molecular assemblies, or nanoparticles, WPE exhibit deviations from the acid–base equilibrium observed in single‐component solutions. In this mini review, studies of charge regulation (CR) are described in simple yet relevant model systems comprising poly (acrylic acid) (PAA) and high concentrations of non‐charged micelles (Pluronic) or nonadsorbing, short chains of poly (vinyl alcohol) (PVA). Titration experiments reveal in the first mixture significant reduction in the degree of ionization of the PAA. In PAA–PVA mixtures, an unexpected increase in the degree of ionization of PAA is observed. Structural characterization of the solutions, via transmission electron microscopy and small‐angle X‐ray scattering, reveals that the local organization is affected by the interaction between PAA chains and the additives, leading to crowding of the micelles in one case, and crowding of the PAA chains in the other. These systems provide insights into the molecular mechanisms leading to the responsiveness of WPE and the effect of crowding on CR.