The most intriguing hypothesis explaining many water anomalies is a metastable liquid–liquid phase transition (LLPT) at high pressure and low temperatures, experimentally hidden by homogeneous nucleation. Recent infrared spectroscopic experiments showed that upon addition of hydrazinium trifluoroacetate to water, the supercooled ionic solution undergoes a sharp, reversible LLPT at ambient pressure, possible offspring of that in pure water. Here, we calculate the temperature-dependent signature of the OH-stretching band, reporting on the low/high density phase of water, in neat water and in the same experimentally investigated ionic solution. The comparison between the infrared signature of the pure liquid and that of the ionic solution can be achieved only computationally, providing insight into the nature of the experimentally observed phase transition and allowing us to investigate the effects of ionic compounds on the high to low density supercooled liquid water transition. We show that the experimentally observed crossover behavior in the ionic solution can be reproduced only if the phase transition between the low- and high-density liquid states of water is coupled to a mixing–unmixing transition between the water component and the ions: at low temperatures, water and ions are separated and the water component is a low density liquid. At high temperatures, water and ions get mixed and the water component is a high-density liquid. The separation at low temperatures into ion-rich and ion-poor regions allows unveiling the polyamorphic nature of liquid water, leading to a crossover behavior resembling that observed in supercooled neat water under high pressure.

中文翻译：

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纳米级偏析与过冷离子液体水溶液中液态水多晶型相耦合

解释许多水异常的最有趣的假设是高压和低温下的亚稳态液-液相转变（LLPT），实验上被均匀成核隐藏。最近的红外光谱实验表明，将三氟乙酸肼加入水中后，过冷的离子溶液在环境压力下会发生尖锐的、可逆的 LLPT，这可能是纯水中的产物。在这里，我们计算了 OH 拉伸带的温度相关特征，报告了水的低/高密度相，在纯水中和在相同的实验研究离子溶液中。纯液体和离子溶液的红外特征之间的比较只能通过计算来实现，深入了解实验观察到的相变的性质，并允许我们研究离子化合物对高密度到低密度过冷液态水转变的影响。我们表明，只有当水的低密度和高密度液态之间的相变与水组分和离子之间的混合-非混合转变耦合时，才能重现离子溶液中实验观察到的交叉行为：在低温度升高，水和离子分离，水成分是低密度液体。在高温下，水和离子混合，水成分是高密度液体。在低温下分离成富含离子和缺乏离子的区域可以揭示液态水的多晶性质，