Synergistic regulation of polyelectrolyte brush conformations by solvent quality and trivalent ions

Surface polyelectrolyte brush materials responsive to solvent quality and added ions have widespread applications in interfacial materials. The interplay between solvent quality and ion valency plays a pivotal role in determining the conformation of polyelectrolyte brushes, yet its mechanisms remain underexplored. In this study, we systematically investigate these coupling effects on sodium poly(styrene sulfonate) (PSS) brushes through a combination of theoretical modeling, all-atom molecular dynamics (MD) simulations, and atomic force microscopy (AFM) experiments. By tuning the water-to-isopropyl alcohol (IPA) ratio in binary solvents, we reveal that solvent quality drives a gradual decrease in brush height, culminating in a rapid collapse at higher IPA volume fractions (${\varphi }_{\text{IPA}}\approx 0.8$). Theoretically, we extend our unified framework for ion-valency effects to incorporate Flory–Huggins interaction parameters derived from solvent solubility parameters, yielding predictions consistent with experimental and simulation results. Our findings highlight that the solvent-polymer interactions govern brush height more significantly than dielectric constants in mixed solvents. Solvent-induced brush collapse occurs uniformly, whereas multivalent ions induce localized adsorption, leading to chain aggregation and non-homogeneous collapse. The constructed brush height landscape further demonstrates that solvent quality predominates for short chains, while both solvent quality and ion valency exhibit synergistic and nonlinear effects on longer chains, with pronounced collapse transitions observed under specific conditions. This study provides a comprehensive understanding of the coupled effects of solvent quality and ion valency on polyelectrolyte brushes, offering valuable insights for designing stimuli-responsive surfaces. These findings are particularly relevant for applications in vapor sensing, gas separation, and advanced surface engineering technologies, where precise control over brush height and morphology is crucial.