Metastable liquid-liquid transition in a molecular model of water
Metastable liquid-liquid transition in a molecular model of water
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DOI：
10.1038/nature13405
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发表时间：
2014-06-19
期刊：
影响因子：
64.8
通讯作者：
Debenedetti, Pablo G.
中科院分区：
文献类型：
--
作者：
Palmer, Jeremy C.;Martelli, Fausto;Debenedetti, Pablo G.
液态水的等温压缩系数（1）和等压热容（2），以及其热膨胀系数（3）的大小，在冷却到平衡凝固点以下时急剧增加。许多实验（4-8），理论（9-11）和计算（12，13）研究试图了解这种异常行为的分子起源和影响。在提出的不同理论方案（9，14，15）中，一种假设存在一级相变，该相变涉及两种形式的液态水，并终止于位于深度过冷条件下的临界点（9，12）。一些实验证据与这一假设相一致（4，16），但迄今为止还没有获得水的液-液转变的确切证据：快速的冰结晶迄今为止阻止了对深度过冷水的决定性测量，尽管最近已经克服了这一挑战（16）。因此，计算机模拟对于探索水的结构和行为在这一制度是至关重要的，并已表明（13，17 -21），一些水模型表现出液-液转变，而其他人没有。然而，最近的工作（22，23）认为，液-液转变被错误地解释，实际上是所有水的原子模型中的液晶转变。在这里，我们表明，通过使用六种先进的采样方法来计算自由能表面，研究水的ST 2模型中的液-液转变（24），在相同的深度过冷热力学条件下存在两个亚稳液相和一个稳定的晶相，并且两种液体之间的转变满足一级转变的热力学标准（25）。我们遵循水的配位壳和拓扑环结构沿着从低密度液体到立方冰的化学可逆路径的重排（26）。我们还表明，该系统之间的两个液相，而不是结晶自由波动。这些发现为水的ST 2模型中的液-液转变提供了明确的证据，并指出结晶和弛豫之间的时间尺度分离对于实现它至关重要。
Liquid water's isothermal compressibility(1) and isobaric heat capacity(2), and the magnitude of its thermal expansion coefficient(3), increase sharply on cooling below the equilibrium freezing point. Many experimental(4-8), theoretical(9-11) and computational(12,13) studies have sought to understand the molecular origin and implications of this anomalous behaviour. Of the different theoretical scenarios(9,14,15) put forward, one posits the existence of a first-order phase transition that involves two forms of liquid water and terminates at a critical point located at deeply supercooled conditions(9,12). Some experimental evidence is consistent with this hypothesis(4,16), but no definitive proof of a liquid-liquid transition in water has been obtained to date: rapid ice crystallization has so far prevented decisive measurements on deeply supercooled water, although this challenge has been overcome recently(16). Computer simulations are therefore crucial for exploring water's structure and behaviour in this regime, and have shown(13,17-21) that some water models exhibit liquid-liquid transitions and others do not. However, recent work(22,23) has argued that the liquid-liquid transition has been mistakenly interpreted, and is in fact a liquid-crystal transition in all atomistic models of water. Here we show, by studying the liquid-liquid transition in the ST2 model of water(24) with the use of six advanced sampling methods to compute the free-energy surface, that two metastable liquid phases and a stable crystal phase exist at the same deeply supercooled thermodynamic condition, and that the transition between the two liquids satisfies the thermodynamic criteria of a first-order transition(25). We follow the rearrangement of water's coordination shell and topological ring structure along a thermodynamically reversible path from the low-density liquid to cubic ice(26). We also show that the system fluctuates freely between the two liquid phases rather than crystallizing. These findings provide unambiguous evidence for a liquid-liquid transition in the ST2 model of water, and point to the separation of time scales between crystallization and relaxation as being crucial for enabling it.