The anomalies and criticality of liquid water
The anomalies and criticality of liquid water
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DOI：
10.1073/pnas.2008426117
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发表时间：
2020-10
期刊：
影响因子：
--
通讯作者：
Rui Shi;Hajime Tanaka
中科院分区：
文献类型：
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作者：
Rui Shi;Hajime Tanaka
水是地球上最重要但又最神秘的液体。一个基本问题是液态水是否存在液-液临界点（LLCP）。然而，在LLCP可能位于的区域中不可避免的冰结晶使得实验搜索异常具有挑战性。在这里，我们建议检测LLCP的热力学和动力学波动最大值线的相图之间的交叉点。这种独特的方法表明，真实的水的LLCP位于184 K和173 MPa左右。然而，与广泛传播的信念相反，我们发现临界性对实验可获得的液态水的异常性质影响不大，因为它离LLCP太远了。水的异常现象的起源一直是一个长期争论的问题。一个两态模型，可以追溯到伦琴，依赖于两种类型的局部结构的动力学共存-局部有利的四面体结构（LFTS）和无序的正常液体结构（DNLS）-在液态水中。唯象地，这个模型不仅解释了水的热力学异常，而且可以合理化的存在的液-液临界点（LLCP），如果有一个合作的LFTS的形成。最近，我们发现了直接的证据LFTS和DNLS共存的实验结构因子的液态水。然而，LLCP的存在及其对水的性质的影响仍然难以捉摸，使水的异常的起源不清楚。在这里，我们提出了一个独特的策略来定位液态水的LLCP。首先，我们对大量的实验结构，热力学和动力学数据进行了全面的分析，基于我们的分层两态模型。该模型预测，两个热力学和动力学波动最大值线应交叉LLCP，如果它存在，我们证实了百微秒模拟模型沃茨。根据最近的实验结果的压缩性和扩散率测量在无人区，我们发现，这两条线交叉约184 K和173 MPa的真实的水，这表明存在的LLCP周围。然而，我们发现，临界性几乎可以忽略不计，在实验可达区域的液态水，因为它是太远的LLCP。我们的发现将为解决长期争论提供线索。
Significance Water is the most essential yet mysterious liquid on our planet. One fundamental question is whether a liquid–liquid critical point (LLCP) exists in liquid water. However, the inevitable crystallization of ice in the region where LLCP might be located makes the experimental search exceptionally challenging. Here we propose to detect the LLCP as the intersection between thermodynamic and dynamical fluctuation maxima lines in the phase diagram. This unique approach suggests that the LLCP of real water is located around 184 K and 173 MPa. However, contrary to the widely spread belief, we find that the criticality has little influence on the anomalous properties of the experimentally accessible liquid state of water because it is too far from the LLCP. The origin of water’s anomalies has been a matter of long-standing debate. A two-state model, dating back to Röntgen, relies on the dynamical coexistence of two types of local structures—locally favored tetrahedral structure (LFTS) and disordered normal-liquid structure (DNLS)—in liquid water. Phenomenologically, this model not only explains water’s thermodynamic anomalies but also can rationalize the existence of a liquid–liquid critical point (LLCP) if there is a cooperative formation of LFTS. We recently found direct evidence for the coexistence of LFTS and DNLS in the experimental structure factor of liquid water. However, the existence of the LLCP and its impact on water’s properties has remained elusive, leaving the origin of water’s anomalies unclear. Here we propose a unique strategy to locate the LLCP of liquid water. First, we make a comprehensive analysis of a large set of experimental structural, thermodynamic, and dynamic data based on our hierarchical two-state model. This model predicts that the two thermodynamic and dynamical fluctuation maxima lines should cross at the LLCP if it exists, which we confirm by hundred-microsecond simulations for model waters. Based on recent experimental results of the compressibility and diffusivity measurements in the no man’s land, we reveal that the two lines cross around 184 K and 173 MPa for real water, suggesting the presence of the LLCP around there. Nevertheless, we find that the criticality is almost negligible in the experimentally accessible region of liquid water because it is too far from the LLCP. Our findings would provide a clue to settle the long-standing debate.