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Fluctuation of supercritical fluids

Supercritical fluid is defined as a fluid above the critical temperature (Tc) and the critical pressure (Pc), which is the end point of vapor-liquid equilibrium curve.

Supercritical fluid is single-phase fluid in macroscopic viewpoint. However, large density inhomogeneity of molecular distribution occurs in the system, and the fluids have local structural fluctuation containing both gas-like and liquid-like structures in microscopic and/or meso-scale size. The inhomogeneity is quantitatively derived from the physical parameter of gdensity fluctuationh.

We have studied the fluctuation of supercritical fluids for two decades. For one component systems, we investigate the density fluctuation for various substances from viewpoint of dependence of molecular shape and interaction, and we discuss universality of the density fluctuation. For two component systems, we experimentally obtain the mixing scheme of supercritical solutions by means of concentration fluctuation and Kirkwood-Buff parameter. Furthermore, we recently study dissolution behavior of carbon dioxide into ionic liquids, which dissolve a great amount of carbon dioxide especially in supercritical state.

Figure. White light transmittance through CO2. 1: T >> Tc, P >> Pc. From 2 to 4: T and P to Tc, Pc. 5: Close to the critical point. 6: T < Tc, P < Pc (meniscus is observed).

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