Behavior of Halide-ion-Screened Cationic Polyelectrolyte Brushes Probed Using All-Atom Molecular Dynamics Simulations

Understanding the response of Polyelectrolyte (PE) brushes to the variation in the nature of screening counterions have attracted significant attention over the years. Here we employ all-atom molecular dynamics (MD) simulations to study the halide-ion-specific effects in the behavior and properties of cationic PMETA-X brushes (PMETA: [Poly(2-(methacryloyloxy)ethyl trimethylammonium]; X: F^-, Cl^-, Br^-, I^-), i.e., PMETA brushes screened with halide ions. The extent of counterion condensation on the PMETA-X brushes varies as I^->Br^->Cl^->F^-, i.e., it shows an opposite trend to the halide ion charge density (F^- >Cl^-> Br^->I^-). This results in the PMETA-X brush height varying as F^->Cl^-≥Br^->I^-,: such a trend matches with existing experiments. Also, highly non-intuitively, water structural order around the {N(CH₃)₃}⁺ moiety of the PMETA-X brushes varies as F^->Cl^-> Br^->I^-(despite the fact that the halide ion charge density and hence the extent of water distortion around the halide ion varies as F^->Cl^-> Br^->I^-): this, however, justifies the extent of counterion condensation varying as I^->Br^->Cl^->F^-, stemming from a greater ease of the counterions to condense on PMETA-X brushes hydrated with water molecules with lesser structural order.