Modulation of micellization behavior of imidazolium based surface active ionic liquids by aromatic anions in aqueous medium

Abstract

Surface active ionic liquids (SAILs) represent a class of ionic surfactants that many times display better surface-active properties as compared to conventional ionic surfactants. The micellization of SAILs can be further tailored by the addition of salts, and aromatic salts comprising hydrotropic anions have never been explored for altering the micellization of SAILs in low concentration range. Considering this, we have investigated the effect of addition of three aromatic salts i.e. Sodium 4-hydroxy benzenesulfonate, sodium benzenesulfonate, and sodium p-toluenesulfonate on the micellization of 1-dodecyl-3-methylimidazolium chloride, [C12mim][Cl] and its amide functionalized counterpart, 3-(2-(dodecylamino)−2-oxoethyl)−1-methyl-1H-imidazol-3-ium chloride, [C12Amim][Cl]. The surface-active behavior of SAILs has been investigated by tensiometry, whereas micellization in bulk has been probed through conductivity, steady-state fluorescence, dynamic light scattering (DLS) and zeta-potential measurements. The possible location of aromatic anions adsorbed on the micellar surface has been probed using 1H NMR measurements. Isothermal titration calorimetry has been utilized to gain insights into the thermodynamic aspects of micellization. The inferences gained from different techniques have been correlated and found to be in good agreement with each other. It has been observed that the functionalization of alkyl chain length of SAIL along with relative hydrophobicity and bulkiness of the aromatic anions control the characteristic properties of micellization.