Bahri Kheiri, Eslami Hossein, Müller-Plathe Florian
Department of Chemistry, College of Sciences, Persian Gulf University, Boushehr 75168, Iran.
Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, Darmstadt 64287, Germany.
J Chem Theory Comput. 2022 Mar 8;18(3):1870-1882. doi: 10.1021/acs.jctc.1c01116. Epub 2022 Feb 14.
A simplified two-dimensional effective-solvent model of triblock Janus particles, consisting of three interaction sites in a linear configuration, a core particle, and two particles modeling the attractive patches at the poles, is developed to study the mechanism of nucleation and self-assembly in triblock Janus particles. The potential energy parameters are tuned against phase transition temperatures and free energy barriers to the nucleation of crystalline phases, calculated from our previous detailed model of Janus particles. Vapor-liquid equilibria and critical temperatures are calculated by grand-canonical molecular dynamics simulations for particles of different patch widths. With metadynamics, phase equilibria, mechanism of nucleation, and free energy barriers to nucleation are investigated. The minimum free energy path to nucleation indicates two steps. The first step, with a higher free energy increase, consists of the densification of the fluid into a disordered cluster. In the second step, of a lower free energy barrier, the inner particles of the disordered cluster reorient to form a crystalline nucleus. This two-step mechanism of nucleation of a kagome lattice is in complete agreement with the experiment and with our previous simulations using a detailed model of Janus particles. Large systems at a slight supersaturation generate multiple crystalline domains, which are misaligned at the grain boundaries. In complete agreement with the experiment and with previous simulation results, we observe a two-step mechanism for crystal growth: melting of the smaller (less stable) crystallites to a fluid followed by recrystallization at the surface of neighboring bigger (more stable) crystallites. A comparison of the present softer modeling of a Janus particle with harder models in the literature for self-assembly of Janus particles indicates that softer potentials stabilize open lattices (e.g., kagome) more than dense lattices (e.g., hexagonal). Also, experimental locations of phase transition points and barrier heights to nucleation are better reproduced by the present model than by the existing simple models.
为了研究三嵌段Janus颗粒的成核和自组装机制，我们建立了一个简化的二维有效溶剂模型，该模型由线性配置的三个相互作用位点、一个核心颗粒以及两个模拟极点处吸引斑的颗粒组成。根据我们之前关于Janus颗粒的详细模型计算出的结晶相的相变温度和自由能垒，对势能参数进行了调整。通过巨正则分子动力学模拟，计算了不同斑宽度颗粒的气液平衡和临界温度。利用元动力学方法，研究了相平衡、成核机制和成核自由能垒。成核的最小自由能路径表明有两个步骤。第一步，自由能增加较高，包括流体致密化为无序团簇。第二步，自由能垒较低，无序团簇的内部颗粒重新取向形成晶核。这种 kagome 晶格的两步成核机制与实验以及我们之前使用Janus颗粒详细模型的模拟结果完全一致。在轻微过饱和状态下的大系统会产生多个晶域，这些晶域在晶界处错位。与实验和先前的模拟结果完全一致，我们观察到晶体生长的两步机制：较小（较不稳定）的微晶熔化为流体，随后在相邻较大（较稳定）微晶的表面再结晶。将本文中Janus颗粒较软的建模与文献中用于Janus颗粒自组装的较硬模型进行比较表明，较软的势场比较密的晶格（如六方晶格）更能稳定开放晶格（如kagome晶格）。此外，与现有的简单模型相比，本文模型能更好地再现相变点的实验位置和成核的势垒高度。
