Aasen Ailo, Hammer Morten, Reguera David, Wilhelmsen Øivind
Department of Gas Technology, SINTEF Energy Research, NO-7465 Trondheim, Norway.
Porelab, Department of Chemistry, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
J Chem Phys. 2024 Jul 28;161(4). doi: 10.1063/5.0220207.
The description of metastable fluids, those in local but not global equilibrium, remains an important problem of thermodynamics, and it is crucial for many industrial applications and all first order phase transitions. One way to estimate their properties is by extrapolation from nearby stable states. This is often done isothermally, in terms of a virial expansion for gases or a Taylor expansion in density for liquids. This work presents evidence that an isochoric expansion of pressure at a given temperature is superior to an isothermal density expansion. Two different isochoric extrapolation strategies are evaluated, one best suited for vapors and one for liquids. Both are exact for important model systems, including the van der Waals equation of state. Moreover, we present a simple method to evaluate all the coefficients of the isochoric expansion directly from a simulation in the canonical ensemble. Using only the properties of stable states, the isochoric extrapolation methods reproduce simulation results with Lennard-Jones potentials, mostly within their uncertainties. The isochoric extrapolation methods are able to predict deeply metastable pressures accurately even from temperatures well above the critical. Isochoric extrapolation also predicts a mechanical stability limit, i.e., the thermodynamic spinodal. For water, the liquid spinodal pressure is predicted to be monotonically decreasing with decreasing temperature, in contrast to the re-entrant behavior predicted by the direct extension of the reference equation of state.
亚稳态流体，即处于局部而非全局平衡的流体，其描述仍是热力学中的一个重要问题，对许多工业应用和所有一级相变都至关重要。估计它们性质的一种方法是从附近的稳定状态进行外推。这通常是等温进行的，对于气体是根据维里展开，对于液体是根据密度的泰勒展开。这项工作表明，在给定温度下压力的等容膨胀优于等温密度膨胀。评估了两种不同的等容外推策略，一种最适合蒸汽，另一种适合液体。对于包括范德瓦尔斯状态方程在内的重要模型系统，这两种策略都是精确的。此外，我们提出了一种简单的方法，可直接从正则系综中的模拟评估等容膨胀的所有系数。仅使用稳定状态的性质，等容外推方法就能重现具有 Lennard-Jones 势的模拟结果，大多在其不确定性范围内。等容外推方法甚至能从远高于临界温度的温度准确预测深度亚稳态压力。等容外推还能预测机械稳定性极限，即热力学旋节线。对于水，预计液体旋节线压力会随着温度降低而单调下降，这与参考状态方程直接扩展所预测的再入行为相反。
