Aggregation and Morphological Aptitude of Drug-Based Ionic Liquids in Aqueous Solution

Here, we present how replacing the usual inorganic counter ion with a pharmaceutically active aromatic one can greatly affect the interfacial as well as bulk properties of ionic liquids (ILs). We have synthesized a series of novel drug-based ILs, namely, 1-alkyl-3-methylimidazolium diclofenate ([C mim][DF]; n = 6, 8, 10, 12, and 14) abbreviated as DF-ILs, wherein DF^- is a well-recognized analgesic and nonsteroidal anti-inflammatory drug. We show strong synergistic interactions between C mim⁺ and aromatic DF^- attributed to reduced electrostatic repulsions and increased hydrophobicity from their incorporation, reflecting a 300-fold smaller critical aggregation concentration than that of their Cl^- analogue [C mim][Cl]. Interfacial properties for such strongly associating systems are discussed and clearly established to have remarkably improved properties than those of their Cl^- analogues. The decreasing polarity of the cybotactic region of pyrene with increase in the chain length "n" indicates an increased extent of packing of cationic head groups in the Stern layer. DF^- ion seems to play a vital role in the formation of the resulting aggregates, as probed by small angle neutron scattering and transmission electron microscopy. The thermodynamical insights of the aggregation process have been studied using isothermal titration calorimetry and temperature-dependent conductivity experiments. Unilamellar vesicles are formed at extremely low concentration, and also it is the first report that puts into picture the formation of vesicles for [C₆mim][DF] with such a short chain.