A method for determining the density fluctuations of supercritical fluids absolutely based on small-angle scattering experiments and application to supercritical methanol

Abstract

Density fluctuation is a vital concept for understanding disordered systems. A supercritical fluid is a typical disordered system having extremely large inhomogeneity. To determine the density fluctuations using a scattering method, the key physical quantities are the fluid density and the normalized scattering intensity, as well as the small-angle scattering signals. Here, we propose a methodology to obtain all of these quantities absolutely from a scattering experiment. Normalization of scattering intensity relating to the number of molecules per unit volume was performed using fluid density evaluated directly from in situ measurements of the X-ray absorption coefficients. Conversion of scattering intensity to absolute value concerning scattering volume was achieved utilizing the value of the density fluctuation in the ideal state. An analysis of supercritical carbon dioxide confirmed the validity of the present method. By applying this method, the density fluctuations of supercritical methanol were quantitatively determined for the first time.

Thermal Density Fluctuations and Polymorphic Phase Transitions of Ethane (C2D6) in the Gas/Liquid and Supercritical States

“…Systematic SAXS studies on monomolecular SCFs for temperatures between 2 and 6% above the critical temperature have been carried out and published in recent years by Nishikawa and co-workers. Isothermal experiments on CO 2 and CF 3 H are published in ref ( 23 ), and a more recent one on CO 2 and methanol at T / T C = 1.04 in ref ( 24 ). The isothermal experiments were carried out as a function of pressure, which was later converted into a numerical density using the corresponding equation of state (EOS) from the literature and is plotted as abscissa in the figures.…”