解开超冷水之谜
解开“超冷水”之谜
2011/12/03 20:28:26
水可以在远低于零度的情况下以液体形式存在，尤其是在所谓的“超冷水”状态下。 原因是，如果你想让液态水结冰，你需要一个冰核——一个结晶成为冰核，其他结晶围绕冰核凝结。 但是，在非常纯净的水中，没有可以围绕形成冰晶的污染物或微粒，由于水独特的热力学，结晶难以实现。 直到现在，超冷水的测量温度为零下41摄氏度，不过科学家早就怀疑，这一温度可以更低。 他们无法获得肯定的结论，因为在这一温度下水结晶的速度太快了，无法准确测量没有结晶的液体的特性。
美国犹他大学的化学家瓦莱里亚尼·莫利内罗和埃米莉·穆尔利用电脑来模拟超冷水的凝结行为。 他们模拟了当32768个水分子冷却时将会发生的情况。在电脑上经过数千小时的实验，结论得出来了。 水一定会结冰的温度是零下48摄氏度。当水接近这一温度时，它就变得密度不那么大，开始变得更容易压缩，其结构开始改变。 结果，每一个水分子松散地与另外四个水分子相连，形成了金字塔形状。研究人员称之为“中间冰”，即快要结冰还没有完全凝结成冰的状态。 这一研究结果发表在了周三出版的英国《自然》周刊上。 研究全球变暖的气候学家想要知道水凝固并结晶成冰的温度和速度。他们发现，在云中水温可以低至零下40摄氏度。
莫利内罗说：“你需要这个来预测，大气中有多少水以液态形式或结晶形式存在。这对于预测全球气候至关重要。”
Structural transformation in supercooled water controls the crystallization rate of ice
Moore EB, Molinero V
Nature, 2011, 479(7374): 506-508
One of water’s unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature, TH ≈ 232 K, yet the mechanism of ice crystallization—including the size and structure of critical nuclei—has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase on moving into the supercooled region, according to power laws that would diverge (that is, approach infinity) at ~225 K (refs 1, 2), so there may be a link between water’s thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below TH. And although atomistic studies have captured water crystallization3, high computational costs have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model4 in the supercooled regime around TH which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanisms of ice formation. The results of the simulations and classical nucleation theory using experimental data suggest that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below TH and well above its glass transition temperature, 136 K. By establishing a relationship between the structural transformation in liquid water and its anomalous thermodynamics and crystallization rate, our findings also provide mechanistic insight into the observed5 dependence of homogeneous ice nucleation rates on the thermodynamics of water.