Unusual self-assembly of bicephalous surfactants: Nanomorphology and nanostructure from SANS

A novel class of surfactants have been synthesized – which we term bicephalous surfactants, with two quaternary ammonium headgroups linked by a C2 linker and 1 or 2 Cn-acyl tails connected to one of the headgroups. These bicephalous surfactants exhibit remarkable thermoresponsive and rheological behavior. For instance, the single-tailed C18-DQA undergoes reversible thermally-induced gelation-fluidization at ~ the body temperature. It is particularly unusual and perplexing that gelation would occur upon heating, in contrast to other small-molecule gels. Both trial SANS and cryo-TEM results indicated formation elongated aggregates. The zero-shear viscosity of the 800 mM C18-DQA solution would increase from 0.02 Pa s to 100,000 Pa s, by 7-8 orders of magnitude upon gelation. These striking observations from low molecular weight molecules are completely new and could open up significant potential applications. Here, we propose a first SANS study of the nanostructure and nanomorphology of self-assembled bicephalous surfactants, systematically varying the molecular architecture (sing tail and double tail of different lengths) and temperature.

我们合成了一类新型表面活性剂——将其命名为双头基表面活性剂（bicephalous surfactants），这类表面活性剂包含两个季铵盐头基，通过C2连接臂相连，且其中一个头基连接有1条或2条Cn酰基尾链。这类双头基表面活性剂展现出优异的热响应性与流变学行为。例如，单尾型C18-DQA可在接近人体体温的温度下发生可逆的热诱导凝胶-溶胶转变。与其他小分子凝胶不同，加热时发生凝胶化这一现象尤为反常且令人费解。小角中子散射（SANS, Small Angle Neutron Scattering）与冷冻透射电子显微镜（cryo-TEM, Cryogenic Transmission Electron Microscopy）的预实验结果均表明体系形成了伸长型聚集体。浓度为800 mM的C18-DQA溶液在发生凝胶化后，其零剪切黏度可从0.02 Pa·s提升至100000 Pa·s，增幅达7~8个数量级。这类来自低分子量分子的惊人发现前所未有，有望为诸多应用开辟重要潜力。本研究拟首次通过小角中子散射，系统探究分子构筑（不同链长的单尾与双尾结构）与温度对自组装双头基表面活性剂的纳米结构与纳米形貌的影响。