Abstract
Despite the technological importance of supercritical fluids, controversy remains about the details of their microscopic dynamics. In this work, we study four supercritical fluid systems—water, Si, Te, and Lennard-Jones fluid—via classical molecular dynamics simulations. A universal two-component behavior is observed in the intermolecular dynamics of these systems, and the changing ratio between the two components leads to a crossover from liquidlike to gaslike dynamics, most rapidly around the Widom line. We find evidence to connect the liquidlike component dominating at lower temperatures with intermolecular bonding and the component prominent at higher temperatures with free-particle, gaslike dynamics. The ratio between the components can be used to describe important properties of the fluid, such as its self-diffusion coefficient, in the transition region. Our results provide an insight into the fundamental mechanism controlling the dynamics of supercritical fluids and highlight the role of spatiotemporally inhomogeneous dynamics even in thermodynamic states where no large-scale fluctuations exist in the fluid.
Highlights
In the past few decades, supercritical fluids have attracted renewed interest due to their applications in a wide range of chemical and materials processing industries.[1]
Contrary to previous approaches,[6,8] we found that the intermolecular dynamics at a given P,T state cannot be consistently described using models developed for liquids, but instead can be decomposed into two components a high-frequency component associated with the stretching mode between hydrogen-bonded molecules and a low-frequency component representing free-particle motions
The molecular dynamics of fluids is usually described by the dynamic structure factor, S(Q, ω), which measures the correlation of density fluctuations in wavenumber (Q) and frequency (ω) space.[19]
Summary
In the past few decades, supercritical fluids have attracted renewed interest due to their applications in a wide range of chemical and materials processing industries.[1]. It is important to understand these properties and their dependence on the thermodynamic state. Thanks to many years of research, the thermodynamics of supercritical fluids, which is based on their macroscopic properties, has become well understood. The concept of the Widom line has been introduced to refer to the line of maxima of a given response function, such as the isobaric heat capacity, CP.[3] not a rigorous separatrix between liquid and gas states,[4] the Widom line indicates rapid changes in the thermodynamic properties of supercritical fluids, especially in the near-critical region. Around the Widom line, a crossover between liquidlike and gaslike properties is expected for the fluid.[5]
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