Institute for Physical Science and Technology and Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, USA.
J Chem Phys. 2013 May 7;138(17):174501. doi: 10.1063/1.4802992.
The thermodynamic properties of the supercooled liquid state of the mW model of water show anomalous behavior. Like in real water, the heat capacity and compressibility sharply increase upon supercooling. One of the possible explanations of these anomalies, the existence of a second (liquid-liquid) critical point, is not supported by simulations for this model. In this work, we reproduce the anomalies of the mW model with two thermodynamic scenarios: one based on a non-ideal "mixture" with two different types of local order of the water molecules, and one based on weak crystallization theory. We show that both descriptions accurately reproduce the model's basic thermodynamic properties. However, the coupling constant required for the power laws implied by weak crystallization theory is too large relative to the regular backgrounds, contradicting assumptions of weak crystallization theory. Fluctuation corrections outside the scope of this work would be necessary to fit the forms predicted by weak crystallization theory. For the two-state approach, the direct computation of the low-density fraction of molecules in the mW model is in agreement with the prediction of the phenomenological equation of state. The non-ideality of the "mixture" of the two states never becomes strong enough to cause liquid-liquid phase separation, also in agreement with simulation results.
mW 模型水的过冷液体状态的热力学性质表现出异常行为。与真实水一样，热容和压缩率在过冷时急剧增加。这些异常现象的一种可能解释是存在第二个（液-液）临界点，但这种模型的模拟并不支持这一解释。在这项工作中，我们通过两种热力学情景再现了 mW 模型的异常现象：一种基于两种不同类型水分子局部有序的非理想“混合物”，另一种基于弱结晶理论。我们表明，这两种描述都准确地再现了模型的基本热力学性质。然而，弱结晶理论所暗示的幂律所需的耦合常数相对于正则背景太大，与弱结晶理论的假设相矛盾。在这项工作的范围之外，需要波动修正来拟合弱结晶理论所预测的形式。对于两态方法，mW 模型中低密度分子分数的直接计算与唯象状态方程的预测一致。两种状态的“混合物”的非理想性从未强到足以导致液-液分相，这也与模拟结果一致。
