Surface active fatty acid ILs: Influence of the hydrophobic tail and/or the imidazolium hydroxyl functionalization on aggregates formation

Nine structurally-related fatty acid ionic liquids have been prepared and their thermal behavior as well as their ability to self-assemble in water has been investigated. The thermal properties were studied by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), while the aggregation behavior was analyzed by tensiometry, isothermal titration calorimetry (ITC), conductometry, dynamic light scattering (DLS), and electrophoretic light scattering (ELS). Structural variations concerned both the imidazolium cation (presence or not of hydroxyl groups) and the anion (length of the fatty add), and affected in different ways the ILs' properties. With regards to the thermal stability, the presence of the hydroxyl group on the cation had a beneficial effect on stability regardless of the anion's length. This latter played instead a major role in allowing the solubility of the ILs in water (C18-stearate ILs were not soluble in water) and in determining the critical micelle concentration (CMC) values, as the reduction of the anion length in moving from C16-palmitate to C14-myristate anions disfavors aggregates formation. Several other parameters characterizing the adsorption of fatty acid ILs at the air/water interface, some thermodynamic parameters of the aggregation process as well as size distribution and stability of the aggregates were also determined. (C) 2019 Elsevier B.V. All rights reserved.