Madivala Basavaraj, Fransaer Jan, Vermant Jan
Department of Chemical Engineering, K.U. Leuven, W. de Croylaan 46, B-3001, Leuven, Belgium.
Langmuir. 2009 Mar 3;25(5):2718-28. doi: 10.1021/la803554u.
Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties.
受限在液体界面的胶体颗粒有重要应用，例如在乳液和泡沫的稳定化方面。此外，颗粒在界面处的自组装为新型应用和结构化颗粒膜提供了潜力。由于界面处胶体颗粒的胶体相互作用与本体中的不同，因此可以在界面处实现本体中无法产生的胶体微观结构。在本工作中，改变了颗粒形状、表面电荷和润湿性，并研究了颗粒在流体界面处的自组装情况。通过机械拉伸法制备了单分散微米级椭球形模型颗粒。选择这些颗粒非常适合用光学显微镜进行研究。当沉积在两种流体之间的界面时，形状诱导的毛细相互作用与静电排斥相互竞争。改变表面电荷和在界面处的位置可用于操纵实验观察到的自组装过程。癸烷 - 水界面处带电椭球体的初始微观结构由单个椭球体与椭球体线性链共存组成，这些线性链在其尖端相连。通过光学显微镜结合定量图像分析研究了这些单分子层中的聚集行为，并观察到了显著的尖端 - 尖端聚集。通过计算对分布函数和取向分布函数作为时间的函数来量化微观结构信息。与油水界面处的颗粒相比，具有相同表面化学性质和电荷的空气 - 水界面处的颗粒似乎具有较弱的静电相互作用，并且它们在界面处也有不同的平衡位置。后者导致毛细力的差异。静电和毛细力平衡的后续变化产生了非常致密的网络，其典型构建单元是在尖端以三角形或花状构型相连的椭球体。这些网络非常稳定，不会随时间演变。所得的单分子层具有弹性，在压缩下会发生屈曲。此外，通过表面剪切流变学测量的这些单分子层的力学性能表明，即使在低表面覆盖率下，椭球体单分子层也表现出相当大的表面模量，并且与具有相同尺寸和表面性质的球体聚集网络相比，可用于创建更具弹性的单分子层。
