Contraction and Coagulation of Spherical Polyelectrolyte Brushes in the Presence of Ag⁺, Mg²⁺, and Ca²⁺ Cations

Unlike Na⁺ cations, which interact with fully neutralized poly­(acrylic acid) as purely electrostatic entities, cations like Ag⁺, Mg²⁺, and Ca²⁺ exhibit specific interactions with the COO^– residues of anionic polyacrylates. The present work analyzes the interaction of all four cations with a layer of polyacrylate chains grafted onto spherical polystyrene core as an outer shell. First and foremost the analysis answers the question on how these specific interactions influence the nature of the shell and solution behavior of the spherical polyelectrolyte brushes. It could be unambiguously demonstrated that Ag⁺, Mg²⁺, and Ca²⁺ cations induce a drastic shrinking of the polyacrylate shells at cation concentrations lower by 2–3 orders of magnitude compared to the transition of a fully stretched osmotic brush to a shrunken salted brush accomplished with Na⁺ cations. Ag⁺ induces a particularly abrupt contraction, which comes close to the annealing of such brushes achieved with a neutralization by protons. Finally, the solution behavior of the brushes in the presence of Ag⁺ and Ca²⁺ cations is compared with the respective pattern of molecularly dissolved linear polyacrylate chains in terms of phase diagrams.