Abstract:

Over the past decades, stimuli-responsive colloidal materials based on dynamic covalent bonds （especially imine bonds） have aroused significant interest due to the combination of the stability of covalent bonds and the tunability of non-covalent bonds. However, conventional imine-type surfactants are usually switched between active and inactive states. Tuning the symmetry of surfactants using the dynamic imine bonds has barely been reported to date. Herein, a novel imine-type surfactant bearing approximately symmetrical double tails was fabricated utilizing a simple Schiff base reaction by mixing aldehyde-containing surfactant （（N-alkyl-N,N-di-methyl-N-（2-（4-formyl-phenoxy）ethyl） ammonium bromides, HOBAB） and n-octylamine （OA） at a fixed stoichiometric ratio of 1∶1 in water. Both the results of ¹H NMR and FT-IR manifested the formation of dynamic imine bonds, and the conversion rate from aldehyde to imine was as high as 80.75%. With alternatively adding HCl （pH=5.0） and NaOH （pH=10.0）, the surfactant could be reversibly switched between highly asymmetrical HOBAB and approximately symmetrical imine-type double-tail amphiphile because of the breakage and formation of imine bonds. Surface tension experiment showed that the critical micelle concentration of the sample at pH=10.0 was decreased by at least one order of magnitude compared to that at pH=5.0, owing to the presence of imine-type double-tail surfactant. Furthermore, in contrast with HOBAB, the presence of imine-type double-tail amphiphiles remarkably improved the surface/interfacial properties （larger pc₂₀ values and more compact packing at the air/solution interface）. More importantly, this imine-type double-tail surfactant could spontaneously self-assemble into vesicles with radius being approximately 70 nm in aqueous solution, because of the favorable molecular geometry. When the pH value was decreased to 5.0, vesicles had disappeared and spherical micelles with radius being ~2.5 nm re-dominated in the solution, which was mainly ascribed to the change of molecular geometry originated from the breakage of imine bonds. As pH value was increased to 10.0 again, small spherical micelles re-evolved into large vesicles due to the re-formation of imine-type double-tail surfactant. In a word, upon cyclically changing the pH value between 5.0 and 10.0, the aggregates that dominate the solution can be reversibly changed between spherical micelles and vesicles because of the reversible change in molecular geometry between high asymmetry and approximate symmetry. The current research may facilitate the design of new stimuli-responsive soft materials based on dynamic covalent bonds.

Key words: surfactant, dynamic imine bond, vesicle, pH-responsive, controllable transformation