Molecular Dynamics Simulation and Density Functional Theory Calculations of Multiatomic Counterion Binding to Cationic Polyelectrolyte Brushes

Cationic polyelectrolyte brushes are widely employed in nanofluidics, sensing, and biomedical applications; yet their atomistic-level interactions with multiatomic counterions remain poorly understood. In this paper, we investigate the binding of thiocyanate (SCN^-) counterions to cationic poly[2-(methacryloyloxy)ethyl]trimethylammonium (PMETA⁺) brushes through a combination of all-atom molecular dynamics (MD) simulations, electronic structure calculations, and topological analyses of the electron density. Our results reveal that the SCN^- ion exhibits distinct atom-specific binding behavior with the {N(CH₃)₃}⁺ group (of the PMETA⁺ brushes), mediated by both sulfur and nitrogen atoms. These interactions are strongly influenced by the local solvation environment, leading to modulation of hydration shells within the brush interior. Such modulation is represented by the presence of distinct inhomogeneities in the solvation shell structure of the {N(CH₃)₃}⁺ group triggered by the stark differences between the nature of {N(CH₃)₃}⁺-N and {N(CH₃)₃}⁺-S interactions. These differences are further probed by the electron density topology analysis highlighting the anisotropic character of the SCN^- binding, with bond critical points indicating that the S atom interacts with the {N(CH₃)₃}⁺ group through two points (bifurcated ion pairing), while the N atom interacts with the {N(CH₃)₃}⁺ group through a single point (monodentate ion pairing). Overall, this paper serves as a motivation for using combined atomistic-electronic calculations, enabling the capture of the impact of size, shape, multiatomicity, and polarizability of counterions in their interactions with charged macromolecular systems.