Studies on the self-aggregation, interfacial and thermodynamic properties of a surface active imidazolium-based ionic liquid in aqueous solution: Effects of salt and temperature

Abstract

The influence of four sodium salts (NaCl, NaBr, Na2SO4, and Na3PO4) on the self-aggregation, interfacial, and thermodynamic properties of a surface active ionic liquid (1-hexadecyl-3-methylimidazolium chloride, C16MImCl) has been explored in aqueous solutions by conductometry, tensiometry, spectrofluorimetry, isothermal titration calorimetry and dynamic light scattering (DLS). Analyses of the critical micellar concentration (cmc) values indicate that the anions of the added salts promote the self-aggregation of C16MImCl in the order: Cl− < Br− < PO43−< SO42−. Dehydration of imidazolium head groups, in general, governs the process of micellization of aqueous C16MImCl in presence of the investigated salts within the investigated temperature range (298.15–318.15 K), while the melting of iceberg takes the leading role below 303.15 K for the C16MImCl-Na3PO4 system. The results indicate that addition of salt leads to a greater spontaneity of micellization, and that exothermicity prevails in these systems. Differential effect of the salts on the interfacial properties of C16MeImCl has been interpreted on the basis of the coupled influence of the electrostatic charge neutralization of surfactants at the interface, and the van der Wa-als repulsion of surfactant tails and electrostatic repulsion of surfactant head groups. C16MeImCl has been predicted to form spherical micelles in presence of varying amounts of NaCl, Na2SO4 and Na3PO4, while there occurs probably a transition in the micellar geometry from spherical to non-spherical shape when added NaBr concentration exceeds 0.01 mol.kg−1. Fluorescence studies demonstrate that a combined quenching mechanism is operative for the quenching of pyrene fluorescence in the investigated C16MImCl-salt systems. Micellar aggregation numbers obtained from Steady State Fluorescence Quenching method have always been found be somewhat smaller than those estimated from Time Resolved Fluorescence Quenching method. The order of instability of the C16MImCl-micelles ascertained from Zeta potential measurements conform to what has been inferred from the cmc values. The hydrodynamic diameters of C16MImCl-micelles, obtained from DLS studies, have been found to increase with increasing salinity of the solutions.