Menthol-based hydrophobic deep eutectic solvents: design, chemical interaction, and performance evaluation for microplastic extraction from salt matrices

This study demonstrates a systematic application of hydrophobic natural deep eutectic solvents (NADESs) as chemically tunable extraction systems for the selective recovery of polyethylene (PE) microplastics from salt matrices. Two menthol-based NADES formulations, composed of decanoic acid and menthol at molar ratios 1:1 and 1:2, were prepared and comprehensively characterised in terms of physicochemical properties, including viscosity, density, pH, and surface interactions with PE. Extraction performance was comparatively evaluated against conventional hydrogen peroxide (H2O2) digestion. H2O2 provided recovery within the accepted analytical range with high reproducibility. The NADES with a 1:2 molar ratio (DES2) exhibited high extraction efficiency, achieving up to 99% recovery after 24 h, with performance comparable to H2O2 and demonstrating improved stability. In contrast, DES1 showed higher apparent recovery accompanied by increased variability, indicating interaction-driven overestimation. Time-dependent extraction revealed a rapid initial stage followed by a diffusion-controlled process, and kinetic modelling indicated that the extraction followed a pseudo-first-order model (R2 = 0.964). ATR–FTIR analyses confirmed hydrogen-bonding interactions and the appearance of carbonyl-related functionalities on PE surfaces, more pronounced for DES1 than DES2, while SEM imaging revealed solvent-dependent surface modifications, with DES2 inducing more homogeneous and controlled morphological changes. Overall, the results demonstrate that menthol-based hydrophobic NADESs function as interaction-driven and chemically programmable extraction media, where extraction performance is governed by the balance between solvent–polymer affinity and interaction strength. In particular, DES2 provides a controlled and reproducible extraction profile, establishing a chemically rational basis for the development of NADES-guided strategies for microplastic recovery in salt-based model systems.