Formation and Interfacial Behavior of Chitosan–Alginate Interpolyelectrolyte Complexes: From Bulk Dispersions to Layer-by-Layer Films

This study investigates the formation, physicochemical properties, and interfacial behavior of interpolyelectrolyte complexes (IPECs) composed of chitosan (CS) and sodium alginate (ALG) in aqueous media at pH 4.5. Using a combination of turbidity, ζ-potential, conductivity, and interfacial tension measurements, we explore how mixing protocols and solution composition influence complex formation and stability. The results reveal that while ζ-potential remains largely unaffected by polymer concentration, turbidity and interfacial tension exhibit strong dependence, particularly near the stoichiometric charge equivalence point (<i>Z</i> ≈ 1). These findings suggest that neutral complexes formed at <i>Z</i> ≈ 1 display enhanced aggregation and surface activity, especially when ALG is in excess. Additionally, we extend the study to layer-by-layer (LbL) films assembled from CS and ALG, monitored via Quartz Crystal Microbalance with dissipation (QCM-D). The films exhibit quasi-linear growth and increasing elastic modulus with layer number, indicating uniform deposition and strong interlayer interactions. The viscoelastic properties of the multilayers further confirm the structural integrity and potential applicability of these systems in surface engineering and encapsulation technologies. Overall, this work provides a comprehensive understanding of CS-ALG complexation from bulk to interfacial assemblies.