Anion-Based pH Responsive Ionic Liquids: Design, Synthesis,
and Reversible Self-Assembling Structural Changes in Aqueous Solution
- Publication date
- Publisher
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<sub><i>n</i></sub>mim]<sup>+</sup> (<i>n</i> = 4, 6, 8, 10, 12, 14), and
different pH responsive anions such as potassium phthalic acid ([C<sub>6</sub>H<sub>4</sub>COOKCOO]<sup>−</sup>), sodium sulfosalicylic
acid ([C<sub>6</sub>H<sub>3</sub>OHCOOSO<sub>3</sub>Na]<sup>−</sup>), and sodium <i>m</i>-carboxylbenzenesulfonate ([C<sub>6</sub>H<sub>4</sub>COOSO<sub>3</sub>Na]<sup>−</sup>). 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<sub><i>n</i></sub>mim]X (<i>n</i> = 12 and 14, X = [C<sub>6</sub>H<sub>4</sub>COOKCOO], [C<sub>6</sub>H<sub>3</sub>OHCOOSO<sub>3</sub>Na],
and [C<sub>6</sub>H<sub>4</sub>COOSO<sub>3</sub>Na]) 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