Role of clustering in the anomalous properties of supercritical fluids

Abstract

We propose that the anomalous (non-monotonic) behavior of physical properties of supercritical fluids (SCF) in the Widom delta is attributed to the formation of medium-sized clusters. This hypothesis is experimentally verified for carbon dioxide using both experimental methods and molecular dynamics simulations. From a microscopic point of view, the non-monotonic behavior of the nonlinear refractive index, speed of sound, and Raman scattering efficiency is caused by the formation of quasi-linear clusters of medium size (5–200 molecules per cluster). Within the clusters, the molecule concentration is close to that of the liquid phase, while outside the clusters, it resembles the gas phase, leading to experimentally observed high (∼15 %) density fluctuations. Isolated linear clusters exhibit high second-order hyperpolarizability, resulting in an increase in the molecular contribution to the nonlinear refractive index and the intensity of Raman scattering. The appearance of multiple Widom lines on the pressure–temperature (p-T) diagram, each associated with unique physical properties, arises from the combined effects of cluster-specific and density-related factors. This interplay results in the divergence of Widom lines and the formation of the characteristic feature known as the Widom delta.