pH triggered self-assembly structural transition of ionic liquids in aqueous solutions: smart use of pH-responsive additives

Abstract

The creation of smart self-assembling fluids that undergo a morphological transition in response to a specific pH value can allow for the enhanced accumulation of drug delivery agents. In this work, we developed a series of pH-responsive fluids composed of 1-alkyl-3-methylimidazolium bromide [C_nmim]Br (n = 12, 14) and one of the pH-responsive hydrotropes of potassium hydrogen phthalate ([C₆H₄COOKCOOH]), sodium sulfosalicylate ([C₆H₃OHCOOHSO₃Na]), or m-carboxylbenzenesulfonate sodium ([C₆H₄COOHSO₃Na]). The self-assembled structures of these ILs in aqueous hydrotrope solutions were investigated by surface tension, dynamic light scattering, cryogenic-transmission electron microscopy, small-angle X-ray scattering, polarized optical microscopy, and nuclear magnetic resonance spectroscopy. It was found that the ionic liquids, [C_nmim]Br (n = 12, 14), could self-assemble into vesicles with the addition of the hydrotrope, and a reversible transition between spherical micelles and vesicles was observed with the change of solution pH value. The transition in the self-assembled structures of the ILs is suggested to be driven by the change in the molecular structure and hydrophilicity/hydrophobicity of the hydrotrope.