Density dependences of long-range fluctuations and short-range correlation lengths of C H F 3 and C H 2 F 2 in supercritical states

Small-angle x-ray scattering (SAXS) measurements are carried out for supercritical polar fluorocarbons, C H F 3 and C H 2 F 2 , along the isotherm of 1.04 in reduced temperatures with the density range from 0.3 to 1.5 in reduced units. A novel apparatus for determination of absorption factors of the sample fluids is used in the present measurements. The apparatus enables us to detect simultaneously the accurate factors during the observation of the SAXS signals. Long-range fluctuations such as density fluctuations and correlation lengths are evaluated from the obtained SAXS data. The reduced correlation lengths are obtained by normalization by each molecular size, in order to discuss the fluctuations independent of the difference of the individual molecular size. The density fluctuations and the reduced correlation lengths of C H F 3 and C H 2 F 2 are compared with those of C O 2 and H 2 O . The results are as follows: H 2 O > C H 2 F 2 > C H F 3 ∼ C O 2 in the order of magnitude. The fluctuations of C H 2 F 2 are significantly distinguishable from those of C H F 3 and show intermediate aspect between H 2 O and a group of C O 2 and C H F 3 . In addition, the short-range correlation lengths, i.e., the Ornstein-Zernike direct correlation lengths, are firstly discussed from both viewpoints of density and substance dependences. The reduced short-range correlation lengths normalized by individual molecular size are found to trace a universal curve as a function of the reduced density.