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

We employ all-atom MD simulation framework to unravel water microstructure and ion properties for cationic PMETAC or [Poly(2-(methacryloyloxy)ethyl trimethylammonium chloride] brushes with chloride ions as counterions. First, we identify locally separate water regions (with different structural properties) each around the {N(CH3)3}+ and the C=O functional groups of the PMETAC chain and one around the Cl- ion. These water regions or domains, which are effectively the 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/groups around which they are located. For example, the N atom is present as the {N(CH3)3}+ group in the PMETAC chain and the presence of the methyl groups make the {N(CH3)3}+ group trigger apolar hydration as evidenced by the corresponding orientation of the dipole of the water molecules within the water domain formed around the {N(CH3)3}+ moiety. Further, we report that the water molecules constituting the apolar hydration layer around the N(CH3)3}+ group have enhanced tetrahedrality as 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: the stability of this water layer prevents the Cl- ion from coming very close to the {N(CH3)3}+ group, despite the strong electrostatic attraction between these oppositely charged species. 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 PE chain, due to the partial negative charge on the oxygen atom and the specific structure of the system (namely the fact that the C=O group is a part of the PMETAC chains in brush-like configuration and are significantly away from other influencing moieties), 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, apolar hydration at another site, and ensures large mobility of the supported Cl- counterions.