1-Butyl-3-methylimidazolium Mandelate: A Multifunctional Ionic Liquid with Enhanced Hydrogen Bonding, Thermal Stability, Antimicrobial Activity, and Extraction Capability

Designing ionic liquids (ILs) where a single functional group orchestrates a suite of enhanced properties remains a key challenge in materials science. Here, we introduce 1-butyl-3-methylimidazolium mandelate, [Bmim][Man], a novel IL where the hydroxyl group on the mandelate anion simultaneously enhances hydrogen bonding, thermal stability, antimicrobial activity, and extraction selectivity. The structure-property relationships of [Bmim][Man] were investigated through measurements of density, viscosity, and conductivity and were compared with analogous ILs. The presence of the hydroxyl group on the mandelate anion resulted in the highest density and viscosity among the series, attributed to strong hydrogen bonding and efficient ion packing. Notably, [Bmim][Man] exhibited a high molar conductivity that decouples from its high viscosity, suggesting an unusual degree of ion dissociation facilitated by the hydroxyl group. Thermogravimetric analysis revealed superior thermal stability. Furthermore, the investigated ionic liquid demonstrated a low critical aggregation concentration (CAC = 0.01982 mol·dm^-3) in water, indicating a strong propensity for self-aggregation. [Bmim][Man] showed synergistic, enhanced antibacterial activity against <i>E. coli</i> and <i>P. aeruginosa</i>. Finally, the functional utility of this designed liquid was demonstrated in separation science, where [Bmim][Man]-based aqueous biphasic systems showed selective extraction capabilities for transition metals, a process driven by the same hydrogen-bonding and coordination interactions that define its bulk properties. These findings establish [Bmim][Man] as a promising multifunctional material where the mandelate anion concurrently dictates liquid microstructure, thermal resilience, antimicrobial performance, and application in extraction.