Tuning Permeability and Transport in Polyelectrolyte Membranes: The Role of Countercations.

Polyelectrolyte multilayers (PEMs) are widely utilized in membrane technologies, biosensing, and drug delivery, where precise control over permeability, which refers to the ease of transport through the multilayer, is essential. While the influence of anions on PEMs is well-documented, the role of countercations in regulating transport properties through films remains underexplored. Here, we investigate the effects of sodium (Na) and potassium (K) countercations on the formation, structure, permeability, and transport properties of PAH/PSS and PDADMAC/PSS multilayers. Using a quartz crystal microbalance with dissipation (QCM-D), atomic force microscopy (AFM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy, we demonstrate that K-assembled films exhibit higher mass, denser packing, and significantly reduced permeability compared to Na-assembled films. Extended characterizations reveal selected permeability toward ionic probes and frequency-dependent impedance behavior in K, underscoring the potential of the films as tunable barriers. We further demonstrate their application in a model drug release system, highlighting controlled release profiles influenced by countercation choice. These findings provide insights into cation-mediated tuning of PEM properties, offering a robust strategy for designing advanced materials for separation, sensing, and biomedical applications.