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.

译文

即使在超临界条件下,最近对纯流体的实验也确定了不同的液态和气态状态。超临界液-气转变以响应函数的最大值为标志,该函数定义了从临界点发出的线,称为Widom线。但是,尚未确定超临界流体混合物中类似状态转变的结构,并且尚不清楚是否可以为二元混合物确定Widom线。在这里,我们从分子动力学模拟中提供了二元混合物中存在多个Widom线的第一个证据。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状态过渡到类似气体的状态,从而导致惊人的复杂性和多样性的相关系。具体来说,我们表明可混溶的二元混合物具有类似于纯流体的行为,并且超临界状态空间的特征是单个液-气转变。相反,不混溶的二元混合物经历相分离,其中团簇在不同温度下分别跃迁,从而产生多个不同的Widom线。这种独特的过渡行为的存在强调了在具有实际相关性的高阶混合物中预期的超临界状态的复杂性。

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