Recent progress in theoretical and simulation studies of structure of supercritical fluids are reviewed. First, studies of magnetic transition by Ising spin model are revisited in order to deepen our general understanding of static properties of supercritical fluids, because there are a number of common features of physics between these systems near their critical points. Second, an analysis of the structure in terms of clusters is presented, being in line with that of percolation theory. Finally, very recent development of computer simulations for supercritical water and its solutions is described, which is one of the most promising tool for the microscopic study of the structure and dynamics of the fluids.

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Supercritical fluids have attracted much interest as promising solvent of a new type. The characteristic properties of supercritical fluids as solvents are said to be mainly due to the ability of clustering. Several studies on clustering of supercritical fluids by neutron and X-ray diffraction methods are presented. The paging of molecules in the clusters are determined by wide-angle diffraction experiments. The studies on water and carbon dioxide are presented. The infonnation on the cluster size and inhomogeneity are obtained by small-angle diffraction experiments. The density fluctuation and the correlation length are the measures of inhomogeneity of the systems and cluster size, respectively. The temperature and pressure dependence of these parameters is shown for supercritical carbon dioxide.

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Studies on binary diffusion coefficients D₁₂ of organic compounds in supercritical (SC) carbon dioxide were reviewed, and the correlations for predicting D₁₂ were tested. As a result, the Schmidt number correlation, and the correlation with the solvent viscosity were found to be effective for binary diffusion coefficients not only in SC CO₂ but also in liquid organic solvents. The Dymond's free volume equation was applicable only in the limited pressure range, whereas many investigators correlate their measured D₁₂ values with this equation.

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The viscosity dependence of chemical reactions in solution reveals various aspects of solute-solvent interactions. This paper is concerned with the dynamic solvent effects on isomerizations in solution. The isomerization accompanying large amplitude motion due to bulky atomic groups serves as a prototype for this purpose. The isomerization of 2-vinylanthracene in S₁-excited state has been studied over a wide solvent viscosity range from supercritical fluids to compressed liquids. The “Kramers turnover” behavior has been observed . The turnover region was simulated by a single curve of an additive formula. The dynamic solvent effect on the isomerization accompanied with intramolecular charge-transfer is described by the shift of reaction path, which is called “pressure tuning” effect.

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Supercritical fluids have been used to form various organic solids. There are two types of formation methods: precipitation by rapid expansion of supercritical solutions (RESS) and precipitation by adding supercritical fluids as a gas antisolvent (GAS). RESS method is based on the reduction of solvent power of supercritical fluids by lowering pressure level and can produce very fine particles by controlling crystallization conditions. GAS method is a recrystallization method and exploits the ability of supercritical fluids to lower the solvent power of the liquid for the compounds in solution. As a separation method of solid mixtures the supercritical fluid crystallization has been investigated for the naphtalene-phenanthrene mixture by use of supercritical carbon dioxide.

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A high pressure optical cell is designed for measurement of the volume phase transition of a polymer gel (Nisopropyl-acrylamide gel) induced by hydrostatic pressure. The volume of the gel is measured by a microscope, and the image during the phase transition is taken by a camera or a CCD camera.

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