We have reviewed our recent neutron scattering works on imidazolium-based ionic liquids. Neutron diffraction and quasielastic neutron scattering measurements were performed to investigate both structural and dynamical properties of the ionic liquids. They have characteristic nano-structures that are similar to the layer structure of the smectic A liquid crystalline phase. The motions of the alkyl-chains, ionic groups, and nano-structures were observed in a wide dynamic range over 1 ps to 1 μs.

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The transient response method, such as the Taylor dispersion and chromatographic impulse response (CIR) methods, is a useful tool to measure diffusion coefficients in supercritical fluids. However, the Taylor dispersion method sometimes leads to erroneous results for polar solutes. As compared with diffusion coefficients of vitamin K₃ measured by the Taylor dispersion and CIR methods, the superiority of the CIR method to the Taylor dispersion method is demonstrated. Moreover, the partial molar volume and solubility were estimated from the retention factors of a solute in the CIR method.

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We have studied local structure of supercritical solution using the Raman spectroscopy and theoretical analysis. The obtained results allow us to understand the fluid structure in the vicinity of a solute molecule by decomposing intemorecular interaction into attractive and repulsive components as a function of fluid density. Here, we show several solvation structures, obtained from the attractive-repulsive component analyses. As another topic, nanomaterials have been synthesized by the pulsed laser ablation of solid materials in the supercritical fluid. The generation process of novel nanomaterials and their applications to optical devices are described, i.e. SERS substrate, enhanced-fluorescence substrate, luminescence Si particle, and nano-silicon LED.

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In this article, recent advances in the fundamental and applied researches on supercritical hydrothermal synthesis were reviewed. It was described how the supercritical condition is suitable for nanoparticle synthesis based on the nucleation theory with the unique features of metal oxide solubility and kinetics of the reaction around the critical points. Supercritical water also provides a good atmosphere to synthesize organic modified nanoparticles. The surface modification occurs during the nanoparticle synthesis, which leads to control the morphology of nanocrystals, namely the exposed surface. By this method, the most active surface of cerium oxide, (100) surface, could be exposed outside to increase the catalytic activity. The organic modification increases the affinity between nanoparticles and polymers to fabricate the hybrid materials with high loading of nanoparticles. Extremely high heat-conductive polymers could be fabricated by this method.

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Extraction processes using pressurized fluid of mixture of carbon dioxide and water or organic solvent are reviewed. Gas-expanded liquid has been used for separation and material processing where liquid solvent is expanded by dissolving carbon dioxide at high pressure. Hybrid process using multi-phase fluid of liquid and supercritical fluid is developed and applied to natural material extraction.

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Partial molar volumes and reaction volume of inorganic electrolytes, hydrophobic solutes, and amino acids, estimated from high pressure solubility or other high-pressure method are reviewed.

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