Departamento de Estructura de la Materia, Física Térmica y Electrónica, Facultad de Ciencias Fisicas, Universidad Complutense de Madrid, Madrid 28040, Spain.
Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.
J Chem Phys. 2018 Sep 7;149(9):094503. doi: 10.1063/1.5042209.
Among the numerous anomalies of water, the acceleration of dynamics under pressure is particularly puzzling. Whereas the diffusivity anomaly observed in experiments has been reproduced in several computer studies, the parallel viscosity anomaly has received less attention. Here we simulate viscosity and the self-diffusion coefficient of the TIP4P/2005 water model over a broad temperature and pressure range. We reproduce the experimental behavior and find additional anomalies at negative pressure. The anomalous effect of pressure on dynamic properties becomes more pronounced upon cooling, reaching two orders of magnitude for viscosity at 220 K. We analyze our results with a dynamic extension of a thermodynamic two-state model, an approach which has proved successful in describing experimental data. Water is regarded as a mixture of interconverting species with contrasting dynamic behaviors, one being strong (Arrhenius) and the other fragile (non-Arrhenius). The dynamic parameters of the two-state models are remarkably close between experiment and simulations. The larger pressure range accessible to simulations suggests a modification of the dynamic two-state model, which in turn also improves the agreement with experimental data. Furthermore, our simulations demonstrate the decoupling between viscosity and self-diffusion coefficient as a function of temperature . The Stokes-Einstein relation, which predicts a constant /, is violated when is lowered, in connection with the Widom line defined by an equal fraction of the two interconverting species. These results provide a unifying picture of thermodynamics and dynamics in water and call for experiments at negative pressure.
在众多水的异常现象中，压力下动力学的加速尤其令人困惑。虽然实验中观察到的扩散异常已经在几项计算机研究中得到了重现，但平行粘度异常受到的关注较少。在这里，我们模拟了 TIP4P/2005 水模型在宽温度和压力范围内的粘度和自扩散系数。我们重现了实验行为，并在负压下发现了其他异常。在冷却时，压力对动态特性的异常影响变得更加明显，在 220 K 时粘度达到两个数量级。我们用热力学两态模型的动态扩展来分析我们的结果，该方法已被证明成功描述了实验数据。水被认为是具有相反动态行为的相互转化物种的混合物，一种是强（阿仑尼乌斯），另一种是脆弱（非阿仑尼乌斯）。实验和模拟之间的两态模型的动态参数非常接近。模拟可以访问更大的压力范围，这表明需要对动态两态模型进行修改，这反过来也提高了与实验数据的一致性。此外，我们的模拟演示了粘度和自扩散系数随温度的解耦。当降低时，预测恒定的斯特克斯-爱因斯坦关系被违反，与由两种相互转化的物种等分数定义的威德姆线有关。这些结果为水的热力学和动力学提供了一个统一的图景，并呼吁在负压下进行实验。
