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：2016
卷号：113
期号：48
页码：13618-13623
DOI：10.1073/pnas.1604938113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceWe successfully achieve unprecedentedly deep levels of supersaturation (S [~] 4.1) with KH2PO4 (KDP) solutions by using a newly developed device that combines electrostatic levitation with Raman and X-ray scattering. Our study reveals two interesting phenomena. One is an existence of two different solution states, that is, low-concentration KDP solution (LCS) and high-concentration KDP solution (HCS). The other is an emergence of different crystallization paths that depend on the degree of supersaturation: (i) LCS to stable KDP crystal (tetrahedral structure), (ii) LCS to HCS to a metastable KDP crystal (monoclinic structure) to stable KDP crystal. This is a direct in situ observation of multiple pathways of nucleation in aqueous solution. Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH2PO4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.
关键词：multipath nucleation ; liquid-droplet levitation ; supersaturation ; in situ X-ray diffraction ; in situ micro-Raman spectroscopy
