Natural Deep Eutectic Solvent–Dipotassium Phosphate Aqueous Two-Phase Systems: Physicochemical Characterization, Selective Partitioning of Amino Acids and Glucose, and Functional Insight into Maillard Reaction Applications

Despite the growing interest in natural deep eutectic solvents (NADESs) for green separation, critical aspects of their structural stability in aqueous two-phase systems (ATPS), solute partitioning mechanisms, and potential as reaction media remain poorly understood. This study investigates the development and application of NADES-K₂HPO₄ATPS. Four NADES formulations, namely, betaine-glycerol (Bet:Gly), betaine-propylene glycol (Bet:PG), choline chloride-glycerol (ChCl:Gly), and choline chloride-propylene glycol (ChCl:PG), were synthesized and characterized using¹H NMR and differential scanning calorimetry (DSC). The phase-forming ability of the NADES-K₂HPO₄ATPS was influenced by the hydrophobicity of the NADES; specifically, the Bet:PG formulation required the lowest K₂HPO₄concentration (25.1 wt %) for phase separation. In these systems, the hydrophobic NADES-rich phase preferentially partitioned hydrophobic amino acids (e.g., phenylalanine, K > 100; alanine, K ≈ 10), while glucose was enriched in the K₂HPO₄-rich phase (K ≈ 0.03). DSC analysis confirmed that the NADESs retained their structural integrity within the ATPSs. The Maillard reactions were performed in Bet:PG-K₂HPO₄ATPSs under strongly alkaline conditions (pH 11.65 in the top phase and 11.34 in the bottom phase) at 37 °C. Results demonstrated that Bet:PG enhances the formation and stabilization of the Amadori compounds through hydrogen-bonding and restricted molecular mobility. Overall, this work demonstrates that NADESs retain their supramolecular structure within ATPSs, enabling their dual functionality as both selective extractants and microreactor media. Specifically, the confined microenvironment enhanced the accumulation and stabilization of Amadori compounds. This suggested that NADES-based ATPSs hold promise as tailored platforms for controlling the reaction pathways.