ABSTRACT
Harry W.Rollins Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho
Ya-Ping Sun Clemson University, Clemson, South Carolina
I.INTRODUCTION
Supercritical fluids* have been studied extensively for the past two decades in attempts to gain accurate and detailed knowledge of their fundamental properties. Such knowledge is essential to the utilization and optimization of supercritical fluid technology in materials preparation and processing. Among the most important properties of a supercritical fluid are the low and tunable densities that can be varied between those of a gas and a normal liquid and the local density effects observed in supercritical fluid solutions (most strongly associated with near-critical conditions). A supercritical fluid may be considered macroscopically homogeneous but microscopically inhomogeneous, consisting of clusters of solvent molecules and free volumes. That a supercritical fluid is macroscopically homogeneous is obvious-the fluid at a temperature above the critical temperature exists as a single phase regardless of pressure. As a consequence,
extremely wide variations in the solvent properties may be achieved. The microscopic inhomogeneity of a supercritical fluid is a more complex issue and is probably dependent on the density of the fluid. The microscopic properties and their effects on and links to the macroscopic properties have been the focus of numerous experimental investigations, many of which employed molecular spectroscopic techniques. The main issues have been the existence and extent of local density augmentation (or solute-solvent clustering) and solvent-facilitated solute concentration augmentation (or solute-solute clustering) in supercritical fluid solutions.
