Anion- and Cation-Specific Response of the Aqueous Conformation of Strong and Weak Polyanionic Brushes

Polyelectrolyte brushes (PEBs) are surface-tethered polymer chains bearing ionizable groups, whose structure and behavior are highly sensitive to environmental stimuli. When in solution, their response to ions makes them attractive for a variety of applications, including drug delivery, sensing, and smart coatings. In this study, we systematically investigate the specific effect of a wide range of divalent cations and counterions on the swelling behavior of strong and weak anionic PEBs, specifically poly(3-sulfopropyl methacrylate) (PSPMA) and poly(acrylic acid) (PAA) brushes. The brushes were synthesized via surface-initiated atom transfer radical polymerization (Si-ATRP) and investigated using in situ ellipsometry and X-ray photoelectron spectroscopy (XPS) to assess swelling behavior and ion retention. Various parameters, including brush thickness, electrolyte chemistry, ionic strength, and pH, were examined. The brushes exhibited significantly different swelling responses depending on the specific ion introduced in solution. Correlations between the observed swelling behavior and key ionic parameters, including hydration enthalpy, ionic radius, pH, and ionic strength, were established. Collectively, these findings offer new insights into ion-specific responsiveness in PEBs and inform the design and optimization of advanced responsive materials for applications in selective ion binding, tunable hydration, and surface functionalization.