基于动态亚胺键构筑的pH响应型囊泡

pH-responsive vesicles based on dynamic covalent bond

基于动态亚胺键构筑p H响应型表面活性剂及自组装体已成为刺激响应型胶体体系的研究热点之一。然而,传统的亚胺型表面活性剂只能在表面活性剂与非表面活性剂的前体之间可逆开关,无法实现表面活性剂结构对称度的调控。本文利用含醛基的表面活性剂N-十六烷基-N,N-二甲基-N-(2-(4-甲酰基-苯氧基)乙基)溴化铵(HOBAB)与廉价的正辛胺(OA),通过简单的席夫碱反应,原位合成了一种结构近似对称的单头双尾表面活性剂。该表面活性剂可自发形成半径约为70 nm的囊泡状聚集体,且表现出明显的pH响应特性。当pH值低于8.0后,亚胺键断裂,原位产生的动态双尾表面活性剂结构瓦解,HOBAB重新生成,体系中的囊泡随之解离,形成了球形胶束。随着体系pH值在酸性(pH=5.0)与碱性(pH=10.0)之间的循环调控,溶液中的主表面活性剂结构可在高度不对称与近似对称之间可逆切换,同时伴随着聚集体在球形胶束与囊泡之间的可逆转变。

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-formylphenoxy)ethyl)ammonium bromides,HOBAB)and n-octylamine(OA)at a fixed stoichiometric ratio of 1∶1 in water.Both the results of ^(1)H 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 Na OH(p H=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 doubletail 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 p H 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 p H 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 p H 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.