Kringle Loni, Thornley Wyatt A, Kay Bruce D, Kimmel Greg A
Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352.
Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352
Proc Natl Acad Sci U S A. 2021 Apr 6;118(14). doi: 10.1073/pnas.2022884118.
The origin of water's anomalous properties has been debated for decades. Resolution of the problem is hindered by a lack of experimental data in a crucial region of temperatures, , and pressures where supercooled water rapidly crystallizes-a region often referred to as "no man's land." A recently developed technique where water is heated and cooled at rates greater than 10 K/s now enables experiments in this region. Here, it is used to investigate the structural relaxation and crystallization of deeply supercooled water for 170 K < < 260 K. Water's relaxation toward a new equilibrium structure depends on its initial structure with hyperquenched glassy water (HQW) typically relaxing more quickly than low-density amorphous solid water (LDA). For HQW and > 230 K, simple exponential relaxation kinetics is observed. For HQW at lower temperatures, increasingly nonexponential relaxation is observed, which is consistent with the dynamics expected on a rough potential energy landscape. For LDA, approximately exponential relaxation is observed for > 230 K and < 200 K, with nonexponential relaxation only at intermediate temperatures. At all temperatures, water's structure can be reproduced by a linear combination of two, local structural motifs, and we show that a simple model accounts for the complex kinetics within this context. The relaxation time, , is always shorter than the crystallization time, For HQW, the ratio, / , goes through a minimum at ∼198 K where the ratio is about 60.
水的异常性质的起源已经争论了几十年。由于在一个关键的温度、压力区域缺乏实验数据，这个问题的解决受到了阻碍，在这个区域过冷水会迅速结晶——这个区域通常被称为“无人区”。最近开发的一种技术，即水以大于10 K/s的速率加热和冷却，现在能够在这个区域进行实验。在这里，它被用于研究170 K < T < 260 K时深度过冷水的结构弛豫和结晶。水向新的平衡结构的弛豫取决于其初始结构，超淬火玻璃态水（HQW）通常比低密度无定形固态水（LDA）弛豫得更快。对于HQW且T > 230 K，观察到简单的指数弛豫动力学。对于较低温度下的HQW，观察到越来越多的非指数弛豫，这与在粗糙势能面上预期的动力学一致。对于LDA，在T > 230 K和T < 200 K时观察到近似指数弛豫，仅在中间温度下观察到非指数弛豫。在所有温度下，水的结构都可以由两种局部结构基序的线性组合来再现，并且我们表明一个简单的模型可以解释这种情况下的复杂动力学。弛豫时间τ总是比结晶时间τc短。对于HQW，比率τ/τc在约198 K处经历一个最小值，此时该比率约为60。
