Specific ion effects on the build-up and permeability of poly(diallyldimethylammonium) chloride/poly(sodium 4-styrenesulfonate) polyelectrolyte multilayers

Polyelectrolyte multilayers (PEMs) are very promising systems in the field of material science, intensively developed and broadly examined with constantly increasing interest in applications such as sensors, coatings, membranes, and biomedical interfaces. While the impact of ionic strength on PEM formation is well established, the influence of specific counterions remains underexplored. In this study, we systematically examine the effect of six monovalent cations—NH₄⁺, Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺—on the build-up, morphology, permeability, and surface wettability of PDADMAC/PSS multilayers assembled via the layer-by-layer technique. Using a combination of quartz crystal microbalance with dissipation (QCM-D), atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements, we demonstrate that the physicochemical properties of PEMs are strongly influenced by cation identity. Larger, chaotropic cations (Rb⁺, Cs⁺) produce thicker, smoother, and more compact films with reduced permeability, consistent with enhanced charge screening and interchain packing. In contrast, smaller, kosmotropic cations (Li⁺, Na⁺) lead to thinner, rougher, and more permeable films, suggesting weaker chain interactions and looser film structures. K⁺ induces exceptionally high surface hydrophobicity and blocking efficiency, while NH₄⁺ shows distinct behavior likely due to its hydrogen-bonding capabilities. A clear odd–even alternation in contact angle was observed, driven by the chemical nature of the terminating layer (PDADMAC vs. PSS), highlighting the importance of surface composition in wetting behavior. These findings demonstrate that even subtle differences in ion type can modulate film growth kinetics, surface morphology, and functional performance. The results offer new insights into the design of PEMs with tailored permeability, roughness, and surface energy, with direct implications for their optimization in electrochemical sensors, antifouling coatings, and controlled-release systems.