Anion-Based pH Responsive Ionic Liquids: Design, Synthesis, and Reversible Self-Assembling Structural Changes in Aqueous Solution

Abstract

The creation of pH responsive materials that undergo morphological transitions between micelle and vesicle induced by solution pH change is of great importance for their potential application in drug delivery and biochemical engineering. Here, we have developed a series of 18 pH responsive ionic liquids composed of 1-alkyl-3-methylimidazolium cation, [C(n)mim](+) (n = 4, 6, 8, 10, 12, 14), and different pH responsive anions such as potassium phthalic acid ([C6H4COOKCOO](-)), sodium sulfosalicylic acid ([C6H3OHCOOSO3Na](-)), and sodium m-carboxylbenzenesulfonate ([C6H4COOSO3Na](-)). The aggregation behavior and self-assembly structures of the ILs in aqueous solution have been investigated by surface tension, dynamic light scattering, transmission electron microscopy, small-angle X-ray scattering, and nuclear magnetic resonance spectroscopy. It was found for the first time that single tail ionic liquids, [C(n)mim]X (n = 12 and 14, X = [C6H4COOKCOO], [C6H3OHCOOSO3Na], and [C6H4COOSO3Na]) could form vesicles without any additives, and reversible transition was observed between spherical micelles and vesicles with the change of solution pH value. The transition in self-assembly structures is suggested to be driven by the variation in molecular structure and hydrophilicity/hydrophobicity of anions of the ILs.