Unmasking supercooled water transitions The unusual thermodynamic properties of water suggest that when supercooled, a second critical point should exist between two liquid phases that differ in density. Pure water crystallizes before such conditions can be reached. Woutersen et al. studied hydrazinium trifluoroacetate solutions that have similar hydrogen bonding to that of water. They observed a liquid-liquid transition near 190 K by using infrared spectroscopy and calorimetry. The liquid states were analogous to the predicted high- and low-density amorphous phases of water. Science, this issue p. 1127 Supercooled hydrazinium trifluoroacetate solutions exhibit reversible liquid-liquid transitions at ~190 kelvin. Simulations and theory suggest that the thermodynamic anomalies of water may be related to a phase transition between two supercooled liquid states, but so far this phase transition has not been observed experimentally because of preemptive ice crystallization. We used calorimetry, infrared spectroscopy, and molecular dynamics simulations to investigate a water-rich hydrazinium trifluoroacetate solution in which the local hydrogen bond structure surrounding a water molecule resembles that in neat water at elevated pressure, but which does not crystallize upon cooling. Instead, this solution underwent a sharp, reversible phase transition between two homogeneous liquid states. The hydrogen-bond structures of these two states are similar to those established for high- and low-density amorphous (HDA and LDA) water. Such structural similarity supports theories that predict a similar sharp transition in pure water under pressure if ice crystallization could be suppressed.

中文翻译：

---

与 HDA-LDA 转变相关的过冷水溶液中的液-液转变

揭示过冷水的转变 水不寻常的热力学性质表明，当过冷时，两个密度不同的液相之间应该存在第二个临界点。在达到这样的条件之前，纯水就会结晶。沃特森等人。研究了具有与水相似的氢键的肼三氟乙酸盐溶液。他们使用红外光谱和量热法观察到接近 190 K 的液-液转变。液态类似于预测的高密度和低密度无定形水相。科学，这个问题 p。1127 过冷肼三氟乙酸盐溶液在 ~190 开尔文下表现出可逆的液-液转变。模拟和理论表明，水的热力学异常可能与两种过冷液态之间的相变有关，但由于先发制人的冰结晶，到目前为止还没有通过实验观察到这种相变。我们使用量热法、红外光谱和分子动力学模拟来研究富含水的三氟乙酸肼溶液，其中水分子周围的局部氢键结构类似于高压下纯水中的氢键结构，但在冷却时不会结晶。相反，该溶液在两种均质液态之间经历了急剧的可逆相变。这两种状态的氢键结构类似于高密度和低密度无定形（HDA 和 LDA）水的氢键结构。