Recent progress in the structural studies of water in the super- and subcritical conditions is reviewed with an emphasis on the hydrogen bonding. The structural characteristics of water at high temperatures and pressures predicted from computer simulations are briefly presented and compared to the results from neutron scattering experiments. The NMR experiments on water in extreme conditions are then described in detail, and the strength of the hydrogen bonding in supercritical water is discussed in terms of the chemical shift. The NMR chemical shift experiments and the computer simulations support the persistence of the hydrogen bonding in supercritical water.
The structure of high-pressure and supercritical water has been examined up to 510 °C and 40 MPa with in situ laser Raman spectroscopy. At 24 MPa or below, the hydrogen bonding in the tetrahedral configuration becomes weaker and this structure breaks with increasing temperature, especially near the critical temperature, above which small clusters like dimers are dominant species. At higher pressures, a similar but slight structural change occurs; however, the tetrahedral configuration is predominant above the critical temperature. Near the critical point, short-lived tetrahedral configurations are significantly increased and coexist with dimer and monomer structures at equilibrium.
Role of water in the Earth and planets is discussed. Water is one of the most important species controlling the dynamics and evolution of the Earth and planets. Water decreases the melting temperature of the mantle materials, and promotes the magmatism of the Earth. Water has a large effect on the rheology of the crust and mantle minerals such as quartz and olivine, as is called hydrolytic weakening (water weakening). Small amount of water strongly enhances flow and deformation in the crust and mantle. Water is trapped as various hydrous minerals in the deep mantle especially under the low temperature conditions such as in the descending slabs. Water together with other volatile species such as H2, He and CH4, and NH3 is an important constituent in the Jovian planets and satellites. Water bearing supercritical fluid plays very important roles in the deep atmosphere of the Jovian planets.
Recent progress in conventional geothermal drilling technology was briefly reviewed. Then a series of fundamental experiments on water jet drilling a hole in hot dry rocks (HDR) and magma performed by the author was summarized. In the experiments, artificial rocks made of refractory materials and some kinds of rocks at elevated temperatures up to 1100 °C were used and flow pressure of the water jet of room temperature at nozzle exit was changed from 2. 45 to 194 MPa. It was discussed how the temperature and physical properties of the target materials, standoff distance, pressure of the water jet, nozzle diameter, and so on influence the jet penetration patterns, depth and volume in the materials.
In March of 1996 International Association for the Properties of Water and Steam (IAPWS) issued two newly revised releases on surface tensions of ordinary water substance and heavy water substance. This revision includes changes of the recommended values based on the International Temperature Scale 1990 (ITS-90) and new correlation equations giving the surface tension as a function of temperature. This article reports the contents of the new releases.
It has been shown that some polymeric gels undergo continuous or discontinuous volume changes depending on external conditions (e. g., temperature, pH and solvent concentration). Therefore, the gels are expected to be applied as a size-selective extraction solvent and other functional materials. To apply the gel as an extractor or other functional materials, we should study volume changes of the gel in solvent mixtures and concentrations inside and outside the gel. Furthermore, fundamental knowledge of mesh sizes of gel network and interactions between network and solvents have become very important. Therefore, fundamental studies and applications of polymeric gel are reviewed: Thermodynamic models for polymeric gels are reviewed. Finally, pressure-dependent phase transitions in polymeric gels are briefly reviewed and discussed.
Molecular dynamics computer simulation technique has been applied to investigate fluid phase change mechanisms near vapor-liquid interfaces. The simulation enables us to estimate the most relevant parameter, condensation coefficient αc, which is the ratio of condensation flux to vapor collision flux. We found: (i) αc is less than unity even for simple fluids, and αc of associating fluids is much less than that. (ii) There is a strong temperature dependence of αc. (iii) Under the nonequilibrium condition, αc depends also on the density (or pressure) and the temperature of the vapor. A simple evaporation model based on a transition state theory can explain this behavior to some extent. As related topics, molecular dynamics of nucleation processes is also described.