All-Atom Molecular Dynamics Simulations of Cationic Polyelectrolyte Brushes in the Presence of Halide Counterions

Understanding the response of the charged polyelectrolyte (PE) brushes and brush-supported water and ions to the changes in the nature of screening counterions is significant in developing strategies for utilizing PE brushes in various applications. In this paper, we employ all-atom molecular dynamics (MD) simulations for studying the behavior of cationic [poly(2-(methacryloyloxy)ethyl) trimethylammonium] (PMETA) brushes and brush-supported water and ions in the presence of different halide (X: I–, Br–, Cl–, and F–) screening counterions. We find that despite the F– ion having the largest charge density, the extent of binding of the counterions on the PMETAX brushes varies as I– > Br– > Cl– > F–, leading to PMETAX brush height being least with I– counterions and greatest with F– counterions. This trend in the binding of the halide ions matches the previous experimental result and can be explained by identifying the chaotropic nature of the I– and Br– ions that promote a disruption of water structure and a more favorable binding of the ions to the polymer chains. Such a binding trend also ensures that the order of the water molecules around the PMETAX chains or the counterions as well as the number of water–water hydrogen bonds inside the brush layer increases in the following order of the counterion-specific PMETAX brushes: F– > Cl– > Br– > I–. Furthermore, halide-ion-PMETAX-chain binding takes place via both interchain and intrachain bridging: intrachain bridging dominates for the case of counterions that show enhanced binding (I– and Br–), while interchain bridging is more favored for the case of counterions that show weakened binding (F– and Cl–). Also, a greater degree of intrachain bridging leads to greater compressibility and flexibility of the brush layer. Finally, we show that the mobility of the halide ions follows a nonmonotonic trend with the charge density: the mobility decreases as I– < Br– < Cl– (as their binding to the PMETAX chains varies as I– > Br– > Cl–), but the mobility of F– ions is in between that of I– and Br– ions. We argue that the strongly attached hydration layer and the ensuing friction inside the brush layer lead to such a reduced mobility of the F– ions.