Piperine solubility enhancement via DES formation: Elucidation of intermolecular interactions and impact of counterpart structure via computational and spectroscopic approaches

The development of new forms of existing APIs with enhanced physicochemical properties is critical for improving their therapeutic potential. In this context, ionic liquids (ILs) and deep eutectic solvents (DESs) have gained significant attention in recent years due to their unique properties and potential for solubility enhancement. In this study, we explore the role of different counterparts in the formation of IL/DESs with piperine (PI), a poorly water-soluble drug. After screening a library of fourteen counterpart molecules, ten liquid PI-counterpart systems were developed and investigated. Thermal analysis confirmed the formation of IL/DES, while computational and spectroscopic studies revealed that hydrogen bonding played a crucial role in the interaction between PI and the counterparts, confirming DES formation. The solubility enhancement of PI in these systems ranged from ∼ 36 % to 294 %, with PI-Oxalic acid (OA) exhibiting the highest saturation solubility (49.71 μg/mL) and PI-Ibuprofen (IB) the lowest (17.23 μg/mL). The presence of hydrogen bonding groups in counterparts was key to successful DES formation. A negative correlation was observed between solubility and logP (r =  − 0.75, p* = 0.0129), while a positive correlation was found between solubility and normalized polar surface area (PSA) (r = 0.68, p* = 0.029). PI-OA and PI-IB were located at the extreme ends of these regression lines, further validating the relationship between these properties and solubility enhancement. These findings highlight essential aspects of rational IL/DES design, optimizing their properties for broader applications.