Small-angle X-ray scattering study on the fluctuations of supercritical aqueous solution of n-pentane along the critical isotherm of water

Fluctuation terms of the supercritical aqueous solutions of n-pentane, i.e., those of densities, concentrations, and their correlation terms, were evaluated by small-angle X-ray scattering intensity in combination with isothermal compressibility and partial molar volumes. The fluctuations increased significantly with the fluid density up to the region of middle density, ∼0.2 g cm−3, and then decreased at higher density. In the region where the fluctuations took maxima or minima, the density was lower than those of water critical density. The particle-number fluctuations of individual components obtained from the fluctuation terms show that water molecules are distributed more inhomogeneously than n-pentane, especially in the region of ∼0.3 g cm−3.