Ren Fude, Wang Xiaolei, Zhang Qing, Wang Xiaojun, Chang Lingling, Zhang Zhiteng
School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, China.
Gansu Yinguang Chemical Industry Group, Baiyin 730900, China.
Molecules. 2024 Mar 5;29(5):1159. doi: 10.3390/molecules29051159.
External electric fields are an effective tool to induce phase transformations. The crystallization of ionic crystals from solution is a common phase transformation. However, understanding of mechanisms is poor at the molecular level. In this work, we carried out an experimental and theoretical investigation of the external electric-field-induced crystallization of TKX-50 from saturated formic acid solution by finite-temperature string (FTS) with order parameters (OPs) as collective variables for ionic crystals. The minimum-free-energy path was sketched by the string method in collective variables. The results show that the K-means clustering algorithm based on Euclidean distance and density weights can be used for enhanced sampling of the OPs in external electric-field-induced crystallization of ionic crystal from solution, which improves the conventional FTS. The crystallization from solution is a process of surface-mediated nucleation. The external electric field can accelerate the evolution of the string and decrease the difference in the potential of mean forces between the crystal and the transition state. Due to the significant change in OPs induced by the external electric field in nucleation, the crystalline quality was enhanced, which explains the experimental results that the external electric field enhanced the density, detonation velocity, and detonation pressure of TKX-50. This work provides an effective way to explore the crystallization of ionic crystals from solution at the molecular level, and it is useful for improving the properties of ionic crystal explosives by using external electric fields.
外部电场是诱导相变的有效工具。离子晶体从溶液中结晶是一种常见的相变。然而，在分子水平上对其机制的理解尚不完善。在这项工作中，我们通过有限温度弦（FTS）方法，以序参量（OPs）作为离子晶体的集体变量，对外部电场诱导TKX - 50从饱和甲酸溶液中结晶进行了实验和理论研究。在集体变量中通过弦方法勾勒出最小自由能路径。结果表明，基于欧几里得距离和密度权重的K均值聚类算法可用于增强离子晶体从溶液中外部电场诱导结晶过程中序参量的采样，这改进了传统的FTS方法。从溶液中结晶是一个表面介导的成核过程。外部电场可以加速弦的演化，并减小晶体与过渡态之间平均力势的差异。由于成核过程中外部电场引起序参量的显著变化，晶体质量得到提高，这解释了外部电场提高TKX - 50密度、爆速和爆压的实验结果。这项工作提供了一种在分子水平上探索离子晶体从溶液中结晶的有效方法，对于利用外部电场改善离子晶体炸药的性能具有重要意义。
