Protein–Polysaccharide Bilayer Films: Influence of Protein and Cross-Linker on Interfacial and Functional Properties

Self-supporting bilayer films were produced by combining a protein (casein or gelatin) and a polysaccharide (carboxymethylcellulose, CMC). Effective interfacial interactions, achieved either physically (primarily via electrostatic interactions) or chemically (with citric acid and 1,2,3,4-butanetetracarboxylic acid (BTCA) as cross-linkers), granted improved mechanical and functional performance. Gelatin layers exhibited strong adhesion across different pH values (3, 4.5, and 8), indicating a minimal role of electrostatic forces in interlayer interactions. In contrast, casein required the incorporation of tannic acid (TA) into the CMC layer as a compatibilizing agent to achieve effective adhesion. Polysaccharide cross-linkers were evaluated in casein-CMC bilayers, where citric acid reduced the water absorption, while BTCA improved water vapor barrier properties but decreased mechanical resistance. Understanding interfacial interactions enables the design of biobased materials with tailored properties, boosting competitiveness and functionality within the circular bioeconomy.