Department of Physics, Kyushu University , 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Langmuir. 2017 Jan 31;33(4):1030-1036. doi: 10.1021/acs.langmuir.6b03723. Epub 2017 Jan 19.
Amphiphilic Janus particles (AJP), composed of hydrophilic and hydrophobic hemispheres, are one of the simplest anisotropic colloids, and they exhibit higher surface activities than particles with homogeneous surface properties. Consequently, a ternary system of AJP, water, and oil can form extremely stable Pickering emulsions, with internal structures that depend on the Janus structure of the particles and the system composition. However, the detail of these structures has not been fully explored, especially for the composition range where the amount of the minority liquid phase and AJP are comparable, where one would expect the Janus characteristics to be directly reflected. In this study, we varied the volume ratio of the particles and the minority liquid phase, water, by 2 orders of magnitude around the comparable composition range, and observed the resultant structures at the resolution of the individual particle dimensions by optical microscopy. When the volume ratio of water is smaller than that of the Janus particles, capillary interactions between the hydrophilic hemispheres of the particles induce micelle-like clusters in which the hydrophilic sides of the particles face inward. With increasing water content, these clusters grow into a rodlike morphology. When the water volume exceeds that of the particles, the structure transforms into an emulsion state composed of spherical droplets, colloidosomes, because of the surface activity of particles at the liquid-liquid interface. Thus, we found that a change in volume fraction alters the mechanism of structure formation in the ternary system, and large resulting morphological changes in the self-assembled structures reflect the anisotropy of the particles. The self-assembly shows essential commonalities with that in microemulsions of surfactant molecules, however the AJP system is stabilized only kinetically. Analysis of the dependence of the emulsion droplet size on composition shows that almost all the particles are adsorbed at the water-oil interface; i.e., the particles show ideal surface activity.
两亲性 Janus 粒子（AJP）由亲水和疏水半球组成，是最简单的各向异性胶体之一，其表面活性高于具有均匀表面性质的粒子。因此，AJP、水和油的三元体系可以形成极其稳定的 Pickering 乳液，其内部结构取决于粒子的 Janus 结构和体系组成。然而，这些结构的细节尚未得到充分探索，特别是在少量液相和 AJP 量相当的组成范围内，人们期望 Janus 特性能够直接反映出来。在这项研究中，我们在可比组成范围内将粒子和少量液相（水）的体积比变化了两个数量级，并通过光学显微镜观察了在单个粒子尺寸分辨率下的结果结构。当水的体积比小于 Janus 粒子时，粒子亲水半球之间的毛细相互作用会导致胶束状聚集体，其中粒子的亲水侧朝向内部。随着水含量的增加，这些聚集体会生长成棒状形态。当水的体积超过粒子的体积时，由于粒子在液-液界面的表面活性，结构会转变为由球形液滴、胶体囊组成的乳液状态。因此，我们发现体积分数的变化改变了三元体系中结构形成的机制，自组装结构的大形态变化反映了粒子的各向异性。自组装与表面活性剂分子微乳液的自组装表现出基本的共同性，然而，AJP 体系仅通过动力学稳定。对乳液液滴尺寸对组成的依赖性的分析表明，几乎所有的粒子都被吸附在油水界面上，即粒子表现出理想的表面活性。
