Ion Transport Enhancement in Charge-Regulating Hydrogels

Weak polyelectrolyte hydrogels are emerging as a versatile platform for ion transport and separation, desalination, and energy applications. However, a predictive understanding of how dynamic charge regulation at the nanoscopic scale influences ionic mobility in these polymer networks remains elusive. Here, using large-scale molecular simulations, we dissect the coupled mechanisms governing small-ion transport in weak polyelectrolyte hydrogels, and reveal how charge density, competitive binding, and dynamic charge regulation jointly control ion mobility. By systematically isolating these effects, we uncover a pronounced nonmonotonic dependence of ion self-diffusion on hydrogel pKa and salt concentration for multivalent ions. Strikingly, allowing the hydrogel backbone to dynamically regulate its charge eliminates this nonmonotonicity and leads to dramatic enhancements in ion mobility, with diffusion coefficients increasing by more than a factor of 2 for trivalent ions. We attribute this effect to a protonation-assisted hopping mechanism that continuously remodels the local electrostatic landscape, providing an additional molecular-scale pathway for ion unbinding beyond translational escape and accelerating ion-pair kinetics without affecting the fraction of free ions. Our results demonstrate that dynamic nanoscale charge regulation can decouple ion mobility from equilibrium binding strength, offering a powerful design lever unavailable in fixed-charge materials. Building upon these insights, we propose simple molecular design principles for next-generation ion-conducting hydrogels and suggest alternative strategies for nanostructured membranes, energy materials, and biologically inspired charge-adaptive soft matter.