Department of Mechanical Engineering, Stanford University, Stanford, CA, 94305, USA.
Sci Rep. 2017 Jun 8;7(1):3027. doi: 10.1038/s41598-017-03334-3.
Recent experiments on pure fluids have identified distinct liquid-like and gas-like regimes even under supercritical conditions. The supercritical liquid-gas transition is marked by maxima in response functions that define a line emanating from the critical point, referred to as Widom line. However, the structure of analogous state transitions in mixtures of supercritical fluids has not been determined, and it is not clear whether a Widom line can be identified for binary mixtures. Here, we present first evidence for the existence of multiple Widom lines in binary mixtures from molecular dynamics simulations. By considering mixtures of noble gases, we show that, depending on the phase behavior, mixtures transition from a liquid-like to a gas-like regime via distinctly different pathways, leading to phase relationships of surprising complexity and variety. Specifically, we show that miscible binary mixtures have behavior analogous to a pure fluid and the supercritical state space is characterized by a single liquid-gas transition. In contrast, immiscible binary mixture undergo a phase separation in which the clusters transition separately at different temperatures, resulting in multiple distinct Widom lines. The presence of this unique transition behavior emphasizes the complexity of the supercritical state to be expected in high-order mixtures of practical relevance.
最近对纯流体的实验研究表明，即使在超临界条件下，也存在明显的液态和气态区域。超临界液-气转变由响应函数的最大值标记，这些响应函数定义了一条从临界点发出的线，称为 Widom 线。然而，超临界流体混合物中类似状态转变的结构尚未确定，也不清楚是否可以为二元混合物确定 Widom 线。在这里，我们通过分子动力学模拟首次提供了二元混合物中存在多条 Widom 线的证据。通过考虑稀有气体的混合物，我们表明，根据相行为，混合物通过明显不同的途径从类似液体的区域转变为类似气体的区域，导致具有惊人复杂性和多样性的相关系。具体而言，我们表明，混溶性二元混合物的行为类似于纯流体，超临界状态空间的特征是单一的液-气转变。相比之下，不混溶性二元混合物经历相分离，其中簇在不同温度下分别转变，导致多个不同的 Widom 线。这种独特的转变行为的存在强调了在实际相关的高阶混合物中预期的超临界状态的复杂性。
