Aqueous solubilization and extraction of curcumin enhanced by imidazolium, quaternary ammonium, and tropine ionic liquids, and insight of ionic liquids-curcumin interaction

The limited water solubility of curcumin becomes an obstacle in its preparation, processing, and application. Ionic liquids (ILs) were wildly applied as "green" solvents possessing design flexibility. Three types of ILs with different cations or anions were employed in the solubilization and extraction of curcumin. It seems that the hydrophobic chain of ILs plays a key role. Superior solubilization and extraction capacity were achieved with surface-active ionic liquids (SAILs), which contain long alkyl chain and a charged hydrophilic head group, resulting in their ability of aggregation and micelle formation. To put an insight into the ILs-curcumin interaction and the solubilization mechanism, UV–vis spectroscopic studies, density functional theory (DFT) calculation, and 1H NMR studies were carried out. At first, curcumin undergoes diketone-enol tautomerism with the binding of ILs monomer. Then, SAILs begin to aggregate and form micelle with the increased concentration, meanwhile, the solubility of curcumin is remarkably enhanced. The solubilized curcumin is located in the palisade of micelle in which a hydrophobic environment is provided.