(104c) The Structure of Polyelectrolyte Brushes in the Presence of Multivalent Counterions

Polyelectrolyte brushes are of great importance to a wide range of fields, ranging from colloidal stabilization to responsive and tunable materials to lubrication. We synthesized high-density polystyrene sulfonate (PSS) brushes using surface initiated atom-transfer radical polymerization, and performed neutron reflectivity (NR) and surface force measurements using a surface forces apparatus (SFA) to investigate the effect of monovalent Na⁺, divalent Ca²⁺, Mg²⁺, and Ba²⁺ ions, to the structure of the PSS brushes. NR and SFA results demonstrate that in monovalent salt solution, the behavior PSS brush agrees with scaling theory well, exhibiting two distinct regimes, the osmotic and salted brush regimes. Divalent cations, Mg²⁺, Ca²⁺, and Ba²⁺, while all significantly affecting the structure of PSS brushes, show strong ion specific effects that are related to the specific interactions between the divalent cations and the sulfonate groups. Our results demonstrate that the presence of multivalent counterions, even at relatively low concentrations, can strongly affect the structure of polyelectrolyte brushes. The results also highlight the importance of ion specificity to the structure of polyelectrolyte brushes in solution.