Fluids at high pressures and temperatures

With high pressures, supercritical fluids can be studied over a very wide range of density. Fluids with strongly interacting polar molecules for example water are of particular interest. Results for several thermophysical properties are discussed. Density, dielectric constant and self—ionization of water have been determined or predicted to 1000 °C. Beyond 20 GPa very hot water appears to become a dense ionic fluid. Dense, supercritical gases are increasingly applied as solvents. Pressure—temperature—composition diagrams and critical curves to OO 00 and 200 MPa have been determined for binary systems of water with helium, argon, xenon, nitrogen, oxygen, methane, carbon dioxide, benzene and others. An equation of state based on a defined molecular interaction model permits computation of such phase diagrams with polar partners and at high temperature. Extension to ternary systems is possible. Of geochemical interest are the ternary systems water—carbon dioxide—sodium chloride and water-methane-sodium chloride. Within the dense, supercritical aqueous mixtures, oxidation reactions are possible. Steadily burning "hydrothermal flames" have been produced at pressures to 100 MPa by introducing a slow flow of oxygen into supercritical dense watermethane fluids with excess water at 400 °C and above.