Keiko Nishikawa

Study of inhomogeneity of supercritical water by small-angle x-ray scattering

Small-angle x-ray scattering (SAXS) experiments using synchrotron radiation were carried out for supercritical water along isotherms at the temperatures T=660.0, 661.5, 663.0, 677.0, and 687.5 K, from a gaslike density region to a liquidlike one, including an intermediate density region. The high-temperature and high-pressure sample holder for SAXS measurements suitable for supercritical water was redesigned for more precise measurements. The curves illustrating the density dependence of density fluctuations and correlation lengths show a slight shift of the maximum from critical isochore. The deviations become larger with increasing temperature. The results for the density fluctuations and correlation lengths for supercritical water are compared with those for supercritical CO2 and CF3H at T/Tc=1.02 and 1.06. The comparison allows us to draw the conclusion that the behavior in the long-range inhomogeneity of water in the supercritical state is in discord with the ordinary behaviors for other molecular su...

Inhomogeneity of molecular distribution in supercritical fluids

Abstract Inhomogeneity is the most fundamental concept which characterizes the states of supercritical fluids, and density fluctuation is a measure of the inhomogeneity in the distribution of molecules. To investigate the substance dependence of the inhomogeneity, small-angle X-ray scattering experiments and thermodynamic calculations have been carried out for supercritical CO2 and CF3H and the contour maps and their ridges have been drawn on the phase diagrams. The contour maps and the ridges for both samples are similar and the magnitudes of the density fluctuation are almost the same at the same reduced thermodynamic states. The ridges connect smoothly with the rectilinear diameters. The physical meaning of a ridge is stated in terms of the effective volume occupied by molecules. It is concluded that the ridge is the boundary which separates the supercritical region into two a more liquid-like region and a more gas-like region.

Correlation lengths and density fluctuations in supercritical states of carbon dioxide

Abstract Small-angle X-ray scattering intensities from several thermodynamic states of supercritical CO 2 were measured. The correlation lengths and density fluctuations of the states obtained by Ornstein-Zernike plots are found to form a ridge along the extension of the coexistence curve of gas and liquid to the supercritical region, corresponding to the critical isochore. The states where these parameters take maxima are shown to be closely related to the solubility of the supercritical fluid and the reactivity of the solute in it.

Fluid behavior at supercritical states studied by small-angle X-ray scattering

Abstract The density fluctuations of supercritical CO 2 and CF 3 H were measured for various thermodynamic states by small-angle X-ray scattering and also calculated using the empirical state equations. For each sample, the ridge of the density fluctuations forms along the extension of the coexistence curve of gas and liquid in the P–T phase diagram and seems to be the locus of a higher-order phase transition. In the ρ–T diagram, the contour maps of the density fluctuations of the two samples are similar and the two ridges are indistinguishable from each other. The density fluctuation seems to be one of the very important and universal variables in describing the state of supercritical fluids.

X-ray scattering study of carbon dioxide at supercritical states

Abstract X-ray scattering measurements have been carried out for two thermodynamic states of the supercritical CO 2 fluid in comparison with a high-pressure state of liquid CO 2 . The radial distribution curves of the supercritical fluids show remarkable differences from that of the liquid. The structure of the supercritical fluid is found to be a mixture of high-density regions like clusters and low-density regions like gases. The radius of the clusters is estimated to be a 14 A.

Construction of the Sample Holder and Small-Angle X-ray Scattering Measurement for Supercritical Water.

A high-temperature and high-pressure sample holder suitable for water under supercritical conditions was constructed for small-angle X-ray scattering experiments, which was designed to endure the temperature up to 750 K and the pressure up to 40 MPa. By use of the holder, preliminary experiments for supercritical water were carried out along the isotherm at 663 K with pressures from 22.5 to 29.5 MPa. Isothermal change of the correlation length obtained from the scattering data showed maximum near the critical isochore. The tendency is similar to other supercritical fluids, e.g., carbon dioxide, trifluorometane and ethylene.

An X-ray diffraction study of the structure and molecular motion in liquid carbon disulfide

Abstract X-ray diffraction intensities from liquid carbon disulfide were measured by the energy dispersive method at room temperature. The obtained intensity function was found to be consistent with that reported by Sandler and Narten in 1976 (S.I. Sandler and A.H. Narten, Mol. Phys., 32 (1976) 1543). It was well simulated by the local lattice structure model, where the orthorhombic cell of the crystalline phase was used and the cell parameters were slightly modified. In the optimized local lattice structure, the T-shaped configuration found in the crystalline phase manifests itself, as predicted theoretically by using the reference interaction site model (RISM) of Sandler and Narten. The structural diffusion in the liquid was represented by the mean-square deviations of the interatomic distances given by the Prins relation. The mean-square deviations were also calculated by using the frequency distribution of the librational motion determined by Ruhman et al. (S. Ruhman, B. Kohler, A.G. Joly and K.A. Nelson, Chem. Phys. Lett., 141 (1987) 16) by means of impulsive stimulated light scattering (ISS). They showed overall consistency with those given by the Prins relation. Thus the deviations of the distances within the structure region in liquid carbon disulfide seem to be mainly due to the librational motion of the molecules.

Structure model of liquid water as investigated by the method of reciprocal space expansion

A comparison between experimental and theoretical x‐ray scattering intensities of liquid water has been carried out. In the region of 0–2.5 A−1 of the scattering parameter, the theoretical intensities for a structure model were calculated by means of reciprocal space expansion. It has been shown that the ice I model by Narten and co‐workers cannot account for the experimental intensities in this region where they had difficulty in calculating the theoretical intensities. The structure model has been revised so that the experimental intensities may be accounted for in the whole region of the scattering parameter. In the revised model, the six‐membered ring lying perpendicular to the c axis is flapping, and as a result, the unit cell in the local‐lattice structure is reduced to nearly a half of that for the ice I model. The lattice parameters of the hexagonal unit cell have been determined as a=4.47 A and c=4.49 A with the shortest O...O distance 2.91 A. Water molecules occupy both frame sites and interstit...

Anomalous X-ray scattering from aqueous 2-butoxyethanol at XBE = 0.06 near freezing

Abstract Small angle X-ray scattering was measured from aqueous 2-butoxyethanol of mole fraction X BE = 0.06 at 273, 278 and 298 K. Anomalously strong scattering was observed at 273 and 278 K. The scattering intensity I is proportional to s −3 in the range s −1 , where s is the momentum transfer of scattering, s =4π sin θ/λ. This dependence is in contrast to the Ornstein—Zernicke regime, or more generally Fishers dependence, I ∼ s −2+η , commonly observed near the critical demixing of a binary mixture.