Tailoring the Microstructure and Functional Properties of Plasticized Alginate Films via Orange Essential Oil Incorporation and Ca ²⁺ Mediated Cross-Linking

Abstract

The development of high-performance biopolymer films is frequently hindered by the inherent tradeoff between mechanical robustness and flexibility. This study addresses this challenge by engineering a multifunctional sodium alginate (SA) system modified through a synergistic strategy of 75% (w/w) glycerol plasticization, 25% (w/w) orange essential oil (OEO) incorporation, and Ca2+ ionic cross-linking. Structural and chemical mapping (FTIR/SEM) elucidated that OEO acts as a dual-functional structural modulator: acting as a molecular lubricant that enhances chain mobility while simultaneously mitigating microfissure formation. Mechanical characterization revealed a highly tunable material regime; while the glycerol–OEO synergy facilitates a remarkable ductile transition achieving an elongation at break of 60.8 ± 3.1% subsequent ionotropic gelation via the “egg-box” model restores structural integrity, yielding a high-strength network (TS: 66.4 ± 2.7 MPa; E: 2473 ± 242 MPa). Nanoscale topographical analysis via AFM demonstrated that ionic contraction triggers an oil-phase segregation mechanism (the “squeezing effect”), increasing surface roughness (Rq from 11.3 to 18.2 nm) and enhancing surface hydrophobicity (contact angle: 80.2° ± 14°). Analysis of barrier properties showed that while OEO acts as a hydrophobic barrier in the plasticized state, Ca2+ cross-linking stabilizes the water vapor transmission rate (WVTR: 851 g·m–2·day–1), providing a “breathable” profile ideal for fresh produce. Furthermore, optical analysis unveiled a “biodoping” effect, where OEO constituents introduce localized electronic states that narrow the optical band gap (Eg) from 5.566 to 4.919 eV, significantly enhancing UV-shielding efficiency. By correlating nanoscale structural refinement with macroscopic functional performance, this work provides a comprehensive mechanistic framework for the rational design of multifunctional, bioactive films tailored for sustainable active packaging and advanced biomedical substrates.