Baumketner A, Stelmakh A, Cai W
Institute for Condensed Matter Physics , NAS of Ukraine , 1 Svientsistsky Street , Lviv 79011 , Ukraine.
Department of Chemistry , Ivan Franko Lviv National University , 6 Kyrylo and Mefodii Street , Lviv 79005 , Ukraine.
J Phys Chem B. 2018 Mar 8;122(9):2669-2682. doi: 10.1021/acs.jpcb.7b11662. Epub 2018 Feb 27.
Cluster crystals are crystalline materials in which each site is occupied by multiple identical particles, atoms, colloids, or polymers. There are two classes of systems that make cluster crystals. One is composed of particles that interact via potentials that are bound at the origin and thus are able to penetrate each other. The other consists of non-interpenetrating particles whose interaction potential diverges at the origin. The goal of this work is to find which systems of the second class can make cluster crystals that are stable at room temperature. First, the general properties of the required potentials are established using an analytical model and Monte Carlo simulations. Next, we ask how such potentials can be constructed by combining hydrophobic attraction and electrostatic repulsion. A colloid model with a hard-sphere core and a repulsive wall is introduced to mimic the hydrophobic interaction. Charge is added to create long-range repulsion. A search in the parameter space of the colloid size, counterion type, and charge configuration uncovers several models for which effective colloid-colloid interaction, determined in explicit solvent as a potential of mean force, has the necessary shape. For the effective potential, cluster crystals are confirmed as low free-energy configurations in replica-exchange molecular dynamics simulations, which also generate the respective transition temperature. The model that exhibits a transition above room temperature is further studied in explicit solvent. Simulations on a 10 ns time scale show that crystalline conformations are stable below the target temperature but disintegrate rapidly above it, supporting the idea that hydrophobic and electrostatic interactions are sufficient to induce an assembly of cluster crystals. Finally, we discuss which physical systems are good candidates for experimental observations of cluster crystals.
簇晶体是一种晶体材料，其中每个位点都被多个相同的粒子、原子、胶体或聚合物占据。有两类系统可以形成簇晶体。一类由通过在原点处有束缚作用且能够相互穿透的势相互作用的粒子组成。另一类由在原点处相互作用势发散的非穿透性粒子组成。这项工作的目标是找出第二类系统中哪些能够形成在室温下稳定的簇晶体。首先，使用解析模型和蒙特卡罗模拟确定所需势的一般性质。接下来，我们研究如何通过结合疏水吸引和静电排斥来构建这样的势。引入一个具有硬球核心和排斥壁的胶体模型来模拟疏水相互作用。添加电荷以产生长程排斥。在胶体大小、抗衡离子类型和电荷构型的参数空间中进行搜索，发现了几个模型，在明确溶剂中作为平均力势确定的有效胶体 - 胶体相互作用具有所需的形状。对于有效势，在副本交换分子动力学模拟中确认簇晶体为低自由能构型，该模拟还生成了各自的转变温度。对在室温以上表现出转变的模型在明确溶剂中进行了进一步研究。在10纳秒时间尺度上的模拟表明，晶体构象在目标温度以下是稳定的，但在温度高于目标温度时会迅速分解，这支持了疏水和静电相互作用足以诱导簇晶体组装的观点。最后，我们讨论哪些物理系统是簇晶体实验观测的良好候选对象。
