Nanosized objects in equilibrium supercritical fluids

Abstract

Supercritical fluids (SCF) now attract a large attention of both practitioners and researchers. They open new possibilities for dry cleaning, processing of chemical and nuclear wastes, residuum oil supercritical extraction and selective extraction of useful substances from biomasses and mineral ores [1-3]. From a scientific point of view they present a new state of matter, intermediate between gases and liquids. The growing utilization of SCF in modern chemical technologies is based on the remarkable mixture of solvent power, characteristic to liquids, and transport properties, specific for gases. This mixture of features specific for liquids and gases in SCF is due to their high heterogeneity resulting from the plurality of nanosized clusters in the gas-like zone of the fluid and nanosized pores in the liquid-like zone. Supercritical fluids near their critical density possess huge density fluctuations. The well known and seen by eyes critical opalescence results from heterogeneities in fluids near the critical point with dimensions measured in microns [4]. But the visible transparency of the supercritical fluid over the critical point does not mean that the heterogeneities have disappeared. Their dimensions simply moved from the microto nanoscale [5]. Dr. K. Nishikawa and her group by X-rays scattering have discovered the peak line of the density fluctuations in SCF [6-8]. They named the peak line on the (T, P) diagram as the ridge. This line may be traced also by peaks of some other thermophysical properties, especially, the constant pressure heat capacity [9, 10]. In many works this peak line is named as the Widom line [11, 12]. This line divides the supercritical zone into two regions that “can be identified by different dynamical regimes: gas-like and liquid-like” [12]. The ridge or the Widom line looks like the extension of the saturation curve in the supercritical zone. The aim of this report is the investigation of the nanosized clusters and their characteristics near the ridge line in the gas-like SCF. The investigation is performed by the computer aided analysis of precise equilibrium thermophysical data for several pure van der Waals and polar fluids. The source of data is the NIST database [13], generalizing experimental results of many scientific groups from all over the World. The knowledge of the clusters’ characteristics may be useful for practical applications and better understanding of the supercritical fluids’ nature.