Hydrophilic and Apolar Hydration in Densely Grafted Cationic Brushes and Counterions with Large Mobilities

We employ an all-atom molecular dynamics (MD) simulation framework to unravel water microstructure and ion properties for cationic [poly(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) brushes with chloride ions as counterions. First, we identify locally separate water domains (or first hydration shells) each around {N(CH₃)₃}⁺ and the C═O functional groups of the PMETAC chain and one around the Cl^- ion. These first hydration shells around the respective moieties overlap, and the extent of the overlap depends on the nature of the species triggering it. Second, despite the overlap, the water molecules in these domains demonstrate disparate properties dictated by the properties of the atoms and groups around which they are located. For example, the presence of the methyl groups makes the {N(CH₃)₃}⁺ group trigger apolar hydration as evidenced by the corresponding orientation of the dipole of the water molecules around the {N(CH₃)₃}⁺ moiety. These water molecules around the {N(CH₃)₃}⁺ group also have enhanced tetrahedrality compared to the water molecules constituting the hydration layer around the C═O group and the Cl^- counterion. Our simulations also identify that there is an intervening water layer between the Cl^- ion and {N(CH₃)₃}⁺ group: this layer prevents the Cl^- ion from coming very close to the {N(CH₃)₃}⁺ group. As a consequence, there is a significantly large mobility of the Cl^- ions inside the PMETAC brush layer. Furthermore, the C═O group of the polyelectrolyte (PE) chain, due to the partial negative charge on the oxygen atom and the specific structure of the PMETAC brush system, demonstrates strongly hydrophilic behavior and enforces a specific dipole response of water molecules analogous to that experienced by water around anionic species of high charge density. In summary, our findings confirm that PMETAC brushes undergo hydrophilic hydration at one site and apolar hydration at another site and ensure large mobility of the supported Cl^- counterions.