Multiscale DFT–MD Investigation of Interfacial Structure, Energetics, and Sorption Performance in Chitosan–Mg–Fe Layered Double Hydroxide Nanocomposites

The interfacial structure and adsorption behavior of chitosan–Mg–Fe layered double hydroxide (LDH) nanocomposites were investigated using a combined density functional theory (DFT) and molecular dynamics (MD) approach, supported by experimental characterization. The effect of chitosan (CS) content (10–60 wt.%) on interfacial energetics, structural organization, and sorption-related properties was systematically analyzed. DFT calculations show that adsorption is governed by cooperative interactions, including hydrogen bonding, anion-mediated charge screening, and coordination-like interactions between CS functional groups and LDH metal centers. Adsorption energies range from -0.94 eV for acetylated units to -2.84 eV for intercalated oligomeric configurations, indicating enhanced stabilization under confinement. MD simulations reveal that CS induces significant interlayer expansion from 7.8 Å to ~14.1 Å (consistent with the DFT-optimized spacing of 14.05 Å) and promotes partial exfoliation of LDH layers, accompanied by restructuring of hydrogen-bond networks and reduced water mobility. A pronounced nonlinear dependence of interfacial stabilization on composition is observed, with an optimal CS content of 30–40 wt.%. This regime corresponds to maximum adsorption energy, highest accessibility of amino functional groups (~94%), and optimal interlayer expansion, while preserving LDH structural integrity. At higher loadings, polymer aggregation and steric shielding reduce the accessibility of active sites despite continued expansion. Electronic structure analysis indicates a reduction in the band gap upon CS incorporation, attributed to hybridization of polymer-derived states with the LDH valence band, suggesting improved visiblelight response. Experimental surface area and pore-size distribution measurements confirm the formation of mesoporous structures and show strong agreement with theoretical predictions. These results establish a quantitative structure–composition–property relationship and provide a predictive framework for the rational design of polymer–LDH nanocomposites with enhanced interfacial stability and sorption performance.