期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：1
页码：E6-E14
DOI：10.1073/pnas.1421192111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceNucleation from solution is a ubiquitous process that plays important roles in physics, chemistry, engineering, and material science. Despite its importance, nucleation is far from being completely understood. In this work, we combine advanced molecular-dynamics simulation techniques and theory to provide a description of urea nucleation from aqueous solution. In particular, our analysis shows that a two-step nucleation mechanism is favorable and that two polymorphs are seen to compete in the early stages of the nucleation process. In our analysis, we have derived and validated a theoretical correction to finite-size effects to compute free-energy profiles in the limit of a macroscopic system at constant supersaturation. 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 free-energy 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.
