Adhesion and Interfacial Interactions Promoted by Tannic Acid and 1,2,3,4-Butanetetracarboxylic Acid in Casein/Carboxymethylcellulose Bilayer Films

The design of functional multilayer materials relies on interfacial phenomena that govern component compatibility, structural integrity, and material performance. Self-supporting films were produced by combining casein and carboxymethylcellulose (CMC) bilayer. Here, we incorporated tannic acid (TA) and 1,2,3,4-butanetetracarboxylic acid (BTCA) into CMC layers as protein- and polysaccharide-crosslinking additives at varying ratios to tailor interfacial energy. The incorporation of TA led to interlayer adhesion, as evidenced by the increased force required for bilayer delamination. Additionally, the reduced moisture adsorption capacity upon TA addition supports the occurrence of interfacial crosslinking with the casein layer. ATR–FTIR measurements in the delaminated faces suggested a structural rearrangement that exposed nonpolar groups at the interfacial region. The vOCG approach predicted the contributions of secondary interactions, revealing that BTCA enhances electrostatic interactions, while TA contributes to nonpolar interactions. The theoretical work of adhesion was modulated according to the proportions of both crosslinkers. Understanding interfacial interactions helps design biobased materials with tailored properties and suitable performance as sustainable alternatives to plastics in the circular bioeconomy.