Structural transformation in supercooled water controls the crystallization rate of ice

摘要

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, T_H≈ 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 T_H. 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 T_H 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 T_H 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.%水的各种异常性质让科学家困惑了几十年,人rn们也提出很多假设来解释它们的起源.其中的rn一个谜题是,是什么决定水在结冰之前可以被rn冷到的最低温度这样一个问题.低温时的快速rn结晶妨碍了实验研究,而模拟工作就在电脑上rn所花的计算时间来说通常成本又太高.现在,rnEmily Moone和Valeria Molinero利用一个允许进rn行严格计算的简单水模型发现,超冷液态水中rn"四配位"分子所占比例控制冰的形成速度和rn机制.该结构变化还导致结晶速度在225K日寸一.rn出现一个峰值:在这个温度以下,冰核的形成rn速度快于液态水的平衡速度.这一发现可解释rn所观察到的热力学异常,也可解释为什么均匀rn的冰成核速度取决于水的热力学.