Molecular-dynamics simulations of urea nucleation from aqueous solution
Molecular-dynamics simulations of urea nucleation from aqueous solution
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DOI：
10.1073/pnas.1421192111
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发表时间：
2015-01-06
影响因子：
11.1
通讯作者：
Parrinello, Michele
中科院分区：
文献类型：
--
作者：
Salvalaglio, Matteo;Perego, Claudio;Parrinello, Michele
尽管其在科学和工程的许多分支中具有普遍的特征和相关性，但溶液成核仍然难以捉摸。在这个框架中，分子模拟是一种强大的工具，可以深入了解分子尺度的成核作用。在这项工作中，我们结合理论和分子模拟来描述水溶液中的尿素成核。利用经过调和的元动力学，我们计算与相变相关的自由能变化。我们发现这种自由能分布的特点是显着的有限尺寸效应，但是可以解释这一点。我们的分析中对成核过程的描述与经典成核理论不同。事实上，晶体状簇的成核之前存在较大的浓度波动，这表明主要的两步过程，即胚胎晶核从致密、无序的尿素簇中出现。此外，在成核的早期阶段，两种不同的多晶型物被发现相互竞争。
Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the freeenergy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete.