Mhanna Ramona, Lee Jonghun, Narayanan Suresh, Reich Daniel H, Leheny Robert L
Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA.
Nanoscale. 2019 Apr 23;11(16):7875-7884. doi: 10.1039/c8nr10440a.
Small angle X-ray scattering with in situ shear was employed to study the assembly and ordering of dispersions of gold nanorods within wormlike micelle solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSal). Above a threshold CPyCl concentration but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented domains with hexagonal order. Under steady shear at rates between 0.5 and 7.5 s-1, the nanorod assemblies acquired macroscopic orientational order in which the hexagonal planes were coincident with the flow-vorticity plane. The nanorods could be re-dispersed by strong shear but re-assembled following cessation of the shear. In the nematic phase of the micelles at higher surfactant concentration, the nanorods did not acquire hexagonal order but instead formed smectic-like layers in the gradient-vorticity plane under shear. Finally, at still higher surfactant concentration, where the micelles form a hexagonal phase, the nanorods showed no translational ordering but did acquire nematic-like order under shear due to alignment in the flow. Depletion forces mediated by the wormlike micelles are identified as the driving mechanism for this sequence of nanorod ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solution.
采用原位剪切小角X射线散射技术研究了由表面活性剂十六烷基吡啶氯化物（CPyCl）和抗衡离子水杨酸钠（NaSal）形成的蠕虫状胶束溶液中金纳米棒分散体的组装和有序排列。在高于CPyCl阈值浓度但低于胶束的各向同性向向列相转变浓度时，纳米棒在静态条件下自组装成具有六方有序的各向同性取向区域。在0.5至7.5 s-1的稳定剪切速率下，纳米棒组装体获得了宏观取向有序，其中六方平面与流动涡度平面重合。纳米棒可以通过强剪切重新分散，但在剪切停止后会重新组装。在较高表面活性剂浓度下胶束的向列相中，纳米棒没有获得六方有序，而是在剪切作用下在梯度涡度平面中形成了类似近晶的层。最后，在更高的表面活性剂浓度下，胶束形成六方相，纳米棒没有表现出平移有序，但由于在流动中排列，在剪切作用下确实获得了类似向列相的有序。蠕虫状胶束介导的耗尽力被确定为这种纳米棒有序行为序列的驱动机制，这表明了一种在溶液中可控、可重构的纳米粒子组装的新机制。
