Modeling Multivalent-Induced Structural Inhomogeneities in Polyelectrolyte Brushes

The impact of multivalent salt ions on the morphologies of polyelectrolyte brushes is examined using coarse-grained molecular dynamics simulations, enhanced with free-energy sampling methods. Information derived from simulations is utilized to disentangle the interplay of multivalent and solvophobic effects present in collapsed Na(PSS) polyelectrolyte brushes. Simulations and experiments taken together in both the planar and surface forces apparatus geometries provide evidence for a distinction between the mechanisms of multivalency and solvophobicity in the collapsed brush morphologies, and emphasize the critical role of multivalent ion bridging interactions. The strongly heterogeneous collapsed surface structures observed experimentally are hypothesized to result from the interplay of multivalent and solvophobic effects.