Electrostatic and Hydrogen-Bond Interactions in End-Functionalized Polymer Brushes: Molecular-Scale Insights from Simulations

Electrostatic interactions among charged polymer ligands offer a powerful means of directing the self-assembly of macromolecular interfaces. Polyethylene glycol (PEG) brushes terminated with carboxylic (–COO^-) or amine (–NH₃⁺) groups exhibit pH- and salt-dependent charging, enabling tunable attractions or repulsions that govern nanoscale organization in solution. Here, large-scale molecular dynamics simulations are used to investigate two parallel gold plates grafted with PEG brushes terminated in –COO^-, –NH₃⁺, or neutral –CH₃ groups, immersed in explicit water and NaCl electrolytes. By varying plate separation and salt concentration, we quantify how counterion binding, hydrogen bonding, hydration structure, and electrostatic correlations determine interfacial behavior.

At large separations, brush interactions follow a Stern-layer picture dominated by counterion binding and weak diffuse screening. At small separations, Poisson–Boltzmann models fail; short-range correlations and charge-mediated hydrogen bonds dominate. Oppositely charged brushes form stable –COO^-–NH₃⁺ complexes with cooperative hydrogen-bond stabilization approaching 10 kBT, whereas like-charged brushes exhibit incomplete screening and persistent repulsion. In addition, water molecules also modulate these forces, forming bound and free populations that reorganize as confinement increases.

These findings reveal how end-group chemistry and ionic conditions tune the balance between attraction and repulsion in polymer solutions. The results provide molecular-level insight into hydration-mediated electrostatics relevant to nanoparticle assembly, biomolecular adsorption, and the design of adaptive soft materials.