Ion partition in polyelectrolyte nanogels*

The partition of monovalent and divalent ions in polyelectrolyte gels plays a critical role in biological function, however, no generally accepted model currently exists that can provide a microscopic description of ion partition in polyelectrolyte gels. The challenge in modeling of such systems is due to the coupling between the polyelectrolyte chain configurations and the spatial distribution of the ionic species in solution. We perform molecular dynamics simulations of a minimal model of a polyelectrolyte nanogel particle in solution with an explicit solvent and ions, where the relative strength of dispersion interactions between the solvent and the charged species defines the solvent quality and the position of the ion along the Hofmeister series. Our findings demonstrate that the architecture of the polyelectrolyte gel and the solvent play a crucial role in gel swelling to the addition of salt and the ion partitioning between the gel and the surrounding solution. Overall, our findings provide a guideline for the development of a more predictive theory of the thermodynamic and transport properties of these complex systems.