Synergistic Therapeutic Potential of New Metal-Free Deep Eutectic Solvents: Synthesis, Physicochemical, Biological, and <i>Ab Initio</i> Studies

A new library of deep eutectic solvents (DESs) was synthesized from the binary combinations of diethylammonium chloride (DEAC), choline chloride (ChCl), glycerol, malonic acid, maleic acid, urea, and niacin across diverse molar proportions by employing either of the thermal- or microwave-assisted routes. Reaction optimization demonstrated that microwave irradiation (30-40 °C, 10-20 s) yielded stable, clear liquids for most of the DEAC:glycerol systems (1:2-1:5), whereas some thermally synthesized maleic acid-urea and malonic acid-DEAC mixtures resulted in rapid postsynthesis solidification, reflecting strong composition-dependent stability constraints. FTIR characterization confirmed DES formation through marked hydrogen bonding, peak broadening, and shifts in the signal positions for more polar O-H, N-H, and C═O bonds. Moreover, a systematic pH profiling across concentration and temperature exhibited well-defined acidity trends, such as malonic-acid- and maleic-acid-based DESs, resulting in strongly acidic aqueous solutions (pH 1.3-2.2), while polyol-based DES solutions in the DEAC:glycerol series remained weakly acidic (pH 4.0-5.3), indicating distinct hydrogen-bond donor/acceptor environments and their sensitivity to aqueous media. The DESs DEAC:glycerol (1:3-1:5) and niacin:urea (1:2) were proved to be the most potent scavengers, exhibiting activity comparable to the reference standard, in antioxidant evaluation. Further, the eutectic combinations DEAC:glycerol (1:3-1:5) (80-100% inhibition at 100 mg/mL; IC₅₀ ≈ 2.0-2.5 mg/mL) demonstrated greater phytotoxicity among the screened DESs, matching the activity of benzofuran, while the DEAC:glycerol (1:2) combination was nontoxic. In antibacterial screening, the DESs DEAC:glycerol (1:3), malonic acid:DEAC (1:1), niacin:urea (1:2), and maleic acid:urea (1:1) resulted in greater activity with inhibition zones of 20-23 mm against <i>E. coli</i> and 20-22 mm against <i>B. subtilis</i> and the IC₅₀ values ranging 0.23-0.50 mg/mL which were comparable to the control, indicating strong intrinsic bioactivity arose from synergistic component interactions. Furthermore, the DFT analyses B3LYP-D3/6-31+G(d,p) corroborated experimental stability trends, thus revealing deep, cooperative hydrogen-bond networks along with low-energy gradient surfaces, which were consistent with persistent noncovalent interactions in the most stable DESs. Collectively, these outcomes establish clear structure-property-bioactivity relationships and provide mechanistic insights into how compositional tuning governs acidity, stability, and biological potency in DES systems.