We report an anomalous swelling of polymer thin films in carbon dioxide (CO(2)) which is associated (in both locus and form) with the density fluctuation ridge that forms along the extension of the coexistence curve of gas and liquid in the P-T phase diagram. Neutron reflectivity results showed that CO(2) could be sorbed to a large extent ( approximately 60%) in thin polymer films even when the bulk miscibility of the polymer with CO(2) is very poor. The anomalous swelling is found to scale with the polymer radius of gyration (R(g)) and extends to a distance approximately 10 R(g).

Density fluctuation is one of the fundamental parameters which determine the various physicochemical properties of supercritical fluids. When the contour map of density fluctuation is drawn on the phase diagram, there exists a ridge which separates the supercritical region in two. In order to obtain a phenomenological picture with physical clearness, we formulate the density fluctuation and its ridge for the van der Waals fluid. They are expressed by fairly simple equations with reduced temperature (Tr=T/Tc) and number density (nr=n/nc). It is analytically ensured that the law of corresponding states is applicable to the density fluctuation and its ridge and the ridge is different from the critical isochore. The ridge is the locus of the points where the third derivatives of the Gibbs free energy become zero, and that drawn on a density–temperature phase diagram directly connects with the locus of the inflection points of the van der Waals isotherms in the unstable region. From the viewpoint of the valance of volumes occupied by molecules and void, the physical meaning of the ridge is also discussed. The consistent agreements are confirmed in the characteristics of the density fluctuation and the ridge for the van der Waals fluid and several real supercritical fluids.

A titanium high-temperature sample holder for small-angle x-ray scattering (SAXS) was newly constructed. It is applicable to aqueous solutions in the supercritical state up to 750 K and 50 MPa. The use of high-tension titanium for assemblies and high-purity titanium for gaskets enables us to apply the holder to fluids in extreme conditions such as supercritical water, supercritical aqueous solutions, and other corrosive hydrothermal aqueous solutions. Details are presented for the diamond window sealed by a flange set made of titanium. The seal is superior to the conventional unsupported-area-seal by a screw cap and plug for the titanium high-temperature sample holder. As a test of the instrument, the SAXS experiments for supercritical water were made at the isothermal condition of T=662 K with pressures from 22.8 to 29.3 MPa. The first SAXS measurements for supercritical aqueous solution were also carried out.

A sample cell with a small internal volume was constructed to perform small-angle X-ray scattering (SAXS) measurements for poisonous samples such as benzene in the supercritical state. The cell was specially designed to keep temperature and pressure of the samples constant. Use of the cell makes it possible to prepare samples easily and quickly for SAXS measurements.

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.