Drug anion based surface active ionic liquids: Molecular interactions, surface activity and micellization behavior in aqueous solutions

Abstract The design of surface active ionic liquids (SAILs) requires thorough understanding of interactions (in bulk as well as at molecular level) between the cation - anion pairs and their interaction with water molecules. These interactions determine the chemical & thermal stability, surface activity and micelle formation. In this work we examine the interactions in SAILs of type, 1-hexadecyl-1-methylpiperidinium or pyridinium acetylsalicylates (AcSa), [C16mpip or py][AcSa] and their precursors based on chloride and hydroxide anions by ATR–FTIR measurements and B3LYP level density functional calculations. The thermo-analytical, surface activity and micellar characteristics were assessed by DSC, dynamic TGA, surface tension, electrical conductivity, ITC, particle size analysis, small angle x-ray scattering, proton NMR and steady state fluorescence measurements. The interactions within the ion pairs are intensified with the increase in the size and complexity of the anion. The thermal stability was dependent on type of the anion and followed the order: AcSa > OH > Cl. AcSa based SAILs are effective and efficient in reducing the surface tension values of water. SAILs formed oblate ellipsoidal shaped micelles, that grow along the semi major axis with the increase in concentration.