Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093.
Institut für Physikalische Chemie, Universität Göttingen, D-37077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2019 Dec 3;116(49):24413-24419. doi: 10.1073/pnas.1914254116. Epub 2019 Nov 4.
The appearance of ice I in the smallest possible clusters and the nature of its phase coexistence with liquid water could not thus far be unraveled. The experimental and theoretical infrared spectroscopic and free-energy results of this work show the emergence of the characteristic hydrogen-bonding pattern of ice I in clusters containing only around 90 water molecules. The onset of crystallization is accompanied by an increase of surface oscillator intensity with decreasing surface-to-volume ratio, a spectral indicator of nanoscale crystallinity of water. In the size range from 90 to 150 water molecules, we observe mixtures of largely crystalline and purely amorphous clusters. Our analysis suggests that the liquid-ice I transition in clusters loses its sharp 1st-order character at the end of the crystalline-size regime and occurs over a range of temperatures through heterophasic oscillations in time, a process without analog in bulk water.
如此一来，最小冰晶簇的形态以及冰Ⅰ与液态水的共存状态依然无法被破解。这项工作的实验和理论红外光谱和自由能结果表明，在仅包含约 90 个水分子的冰晶簇中出现了冰Ⅰ的特征氢键模式。结晶的开始伴随着表面振动强度随比表面积的减小而增加，这是水纳米晶度的光谱指标。在 90 到 150 个水分子的范围内，我们观察到了主要为结晶和纯无定形的混合物。我们的分析表明，在结晶尺寸范围内，液-冰Ⅰ的转变失去了其尖锐的 1 级特征，并通过异相时间振荡在一定温度范围内发生，这一过程在块状水中没有类似的情况。
