Simulations Explain the Swelling Behavior of Hydrogels with Alternating Neutral and Weakly Acidic Blocks

We used computer simulations to study the equilibrium swelling of weak (pH-responsive) polyelectrolyte hydrogels as a function pH and salt concentration in supernatant solution. Our were designed represent recently synthesized gels composed tetrapoly(acrylic acid) tetrapoly(ethylene glycol) stars. To model ionization acid groups exchange small ions with reservoir, we applied developed grand-reaction Monte Carlo method. showed that these significantly deviates from ideal Henderson–Hasselbalch equation due two main contributions: (1) electrostatic interactions (2) Donnan partitioning ions. The first contribution dominates alternating neutral acidic blocks, contrasting our previous observations both contributions are comparably strong homogeneously distributed groups. also critically examined counterion condensation argument, previously invoked explain why experimentally observed was lower than predicted by theory. Thus, detailed analysis allowed us understand which above effects different systems why, thereby allowing identify correct physical origin deviations swelling. Such an understanding is important not only for correctly interpreting experimental measurements but designing tailored exhibit specific response concentration.