In the recent decade, many computational studies as well as physicochemical experiments such as neutron diffraction and NMR spectroscopic measurements have actively been done in order to clarify the structure and dynamics of supercritical water and its solutions. In particular, studies on hydrogen bonding in supercritical water have been attracting much interest from physical chemists. Long-ranged structure and the relevant collective motion have also been investigated in detail. For supercritical solutions, thermodynamic properties such as solubility and the microscopic structure of ion hydration have been studied. Here, recent progress in computational studies on the structure and dynamics of the fluids is reviewed.
The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O-D axis. It is then found that while τ2R decreases rapidly with the temperature on the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied at a temperature above the critical. The comparison of τ2R with the angular momentum correlation time shows that the inertial effect is operative in the rotational dynamics of supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with computer simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.
In this article, we have dealt not only with the dynamic behavior of nitrate anion in aqueous zinc nitrate solution at high temperatures and a fixed pressure but also with photoreduction of benzophenone by N, N, -diethylaniline in supercritical carbon dioxide with time-resolved absorption techniques. In the former cases the perpendicular orientational relaxation time (τ) of the anion significantly decreases with increasing temperature u p to 340°C the values of τ were 1. 86 and 0. 25 ps at 20 and 340°C, respectively. In the latter the rate constant at 40°C decreased greatly with increasing pressure from 9. 8 to 17. 2 MPa, which is considered to be due to the local aggregation of carbon dioxide around benzophenone and/or N, N, -diethylaniline.
To elucidate the effect of ions on the solvent structures in view of the dynamics of solvent molecules, the spinlattice relaxation times (T1) of solvent molecules (D2O and DMSO) in various electrolyte solutions were measured in a relatively wide range of temperature under atmospheric pressure, and those of D2O at moderate temperature under high pressure. From the behaviors of rotational correlation times (τc) and their activation energies (Ea) of solvent molecules (D2O) hydrated to the ions, the ions are classified into structure-making and structure-breaking ions. These characteristics obtained in aqueous solutions were compared with those in non-aqueous solutions (DMSO). Finally the pressure effects on τc in D2O solutions are discussed focusing on the pressure dependence of the hydrophobic hydration.
The purpose of this article is to discuss the effect of pressure on the solvation dynamics and the rotational reorientation dynamics in solution, which have been studied by measuring the picosecond time-dependent fluorescence Stokes shift (TDFSS) and picosecond time-dependent fluorescence rotational depolarization (TDFRD) at high pressures. At the time scale longer than the present time resolution of ca. 20 ps, whose dynamics correspond to the molecular motion in n-alcohol solvent viscosity greater than 0. 2 mPa s, the solvation and rotational reorientation dynamics are well characterized by the simple continuum prediction. In a restricted state as in a micellar environment, a nondiffusive pressure effect is observed.
Analyses of ultrahigh pressure materials by an analytical electron microscope (AEM) are reviewed. AEM provides important advantages for analyzing synthetic and natural ultrahigh pressure materials because it enables us to analyze both structure and composition of these materials down to tens of nm-sized grains. Using AEM, several new high-pressure minerals were discovered in shocked meteorites, and new phases and new phase transformation behaviors were recognized in ultrahigh pressure experiments under the Earth's lower mantle conditions.
Self-propagating high-temperature synthesis (SHS) is currently developed for the production of intermetallic compounds and ceramic powders. SHS has some features, i. e., a very short synthetic time, low energy consumption, high productivity, and furthermore synthesizing novel materials which cannot be prepared by solid state reaction and powder metallurgy. This paper reviews recent research trends and progress related to the structural materials fabricated by the combination of SHS and high-pressure technology. Especially, the characteristics of dense sintered composite materials by spark plasma sintering (SPS) are described, focusing on their microstructures and mechanical properties.