The paper presents recent experimental results of optical, electrical and thermophysical properties of fluid metals in the vicinity of the critical point. Special attention will be paid to the metal-nonmetal transition which occurs at reduced densities of a fluid metal and the plasma phase transitions which are connected with ionization phenomena and many particle interactions in the dense vapor. The interrelation between the gas-liquid critical point phase transition and these electronic transitions and their effect on the critical phenomena will be discussed.

Solid-fluid equilibrium has been measured in 4He between 7 and 18GPa (220 - 400K). The melting line is fitted up to 280K, for an f.c.c. solid, using an exponential pair potential. Experimental evidence for a b.c.c. phase at high temperature is discussed and compared with two calculations on the relative stability of the two structures.

The melting temperatures of Li, Na, K, Rb, and Cs versus pressure were measured to 32 kbar in a piston cylinder apparatus with hydrostatic pressure medium and in situ pressure measurements. The accuracies in pressure and temperature are ± 0.4 % and ± 0.25° C, respectively. Substantial deviation from measurements using solid, non-hydrostatic pressure media is observed. The adiabat, (δT/δP)s,was measured to 32 kbar and 400°C by adiabatic compression. Ln (δT/δP)s is a linear function of volume with good consistency in the slopes. (δT/δP)s changes insignificantly at the liquid-solid phase transitions. The Grüneisen parameters, calculated from γ = Bs/T (δT/δP)s, decreases with pressure and temperature. Bs is the adiabatic bulk modulus. The γ-volume curves show strong curvature and γ tends to become volume independent at compressions of about 30 %. γ increases by several percent at the liquid-solid transition.

The pressure and temperature parameters which delineate the equilibrium thermodynamic stability fields of the liquid, VI and VII phases, includinq the triple point, were measured for the H2;0 and D2;0 systems over the pressure range, 0.8 to 2.4 GPa, and the temperature interval, 20 to 135 °C. The phenomenon of metastability associated with the liquid-VII phase boundary was observed during this work; and, subsequently, a metastable extension of this coexistence curve was determined from the liquid-VI-VII triple point down to 20 °C for both H2;0 and D2;0. The measurements were made by optical polarizing microscopy in conjunction with a diamond anvil cell equipped with a miniature resistance coil heatinq element. Pressures were measured by the ruby fluorescence method.

The compression of carbon tetrachloride has been measured along twelve isotherms covering a pressure range of 0.1 to 200 MPa and a temperature range of 254 to 298 K. Volume changes were measured with an automated capacitance bridge–one side of the bellows containing the sample serving as one plate of the capacitor. Data were obtained in the liquid, face-centered cubic (fcc), and rhombohedral phases, during melting, during freezing into the fcc phase and during the fcc to rhombohedral phase change. Premelting behavior was observed for both solids. The disappearance of the fcc phase at approximately 273 K and the existence of dual melting curves for the fcc and rhombohedral phases were reaffirmed.

For several decades the behavior of carbon and carbonaceous materials has been observed with unvarying interest. Well known are two polymorphous modifications of carbon, namely, graphite and diamond, each possessing unique physicochemical properties which make them attractive for extensive utilization in numerous industries.

The history of carbon studies at high temperatures and pressures produced quite a few examples of ingenious experimental techniques and experimenter's skill, however, many of those studies only resulted in the emergence of new, hard-to-solve problems. This is precisely the case with the melting and vaporization of graphite.

A piston-cylinder apparatus was used to measure the pressure, volume, temperature, and ultrasonic velocity (P,V,T,u) of fluid NH3 from 195 to 320 K at pressures up to 12 kbar. Over 1200 sets of P-V-T-u and P-V-T data were fitted to a Tait-type equation of state (EOS) by non-linear least-squares minimization. With quadratic terms in T for the two fitted parameters, the rms deviation is ± 0.2% in V and ± 0.9% in u, which is comparable to the experimental error. The simple Tait EOS is useful over the entire fluid range between the vaporization and melting curves up to T exceeding room temperature. Measurements of u and calculations of the constant-pressure heat capacity Cp show regular behavior, differing noticeably from values given by a 44-parameter EOS recently published by NBS. It is concluded that incorporating u in the formulation of an EOS improves its self consistency markedly.

In this work a power lay is used to fit the isothermal pressure dependence of the expansivity α = V−1(∂V/∂T)p of liquid hexane. This functional form is also observed to fit the expansivity and the isothermal compressibility KT = −V−1(∂V/∂P)T data of the three liquids : hexane, argon, carbon dioxide.

Liquid helium was shock-compressed in Hugoniot equation-of-state measurements at pressures of 1.2 and 16 GPa (12 and 160 kbar) using a two-stage light-gas gun [1]. Specimens were initially at a temperature of 4.3 K and a molar volume of 32 cm3/mole. Specimen holders were a refinement of the cryogenic targets used for liquid H2 and D2 experiments [2]. The data are in good agreement with the published theoretical prediction of Young, et al.[3] The high shock temperatures test the repulsive pair potential to an interatomic spacing approaching 1 Å, which is comparable to a solid volume at a temperature of 0 K of about 0.5 cm3/mole.

The calculations of the P-V isotherms of fluid hydrogen isotopes H2 , D2 , and T2 up to their freezing pressures at 200, 300 and 500 K have been performed for 4 kinds of intermolecular potentials by using the modified hard-sphere variational perturbation theory with a quantum correction. By comparing the results with the 300 K experimental equation of state, it has turned out that the agreement is good for the potentials of Silvera-Goldman and Nellis et al. except near the freezing point of H2 . The quantum pressures were estimated, and their dependences on volume, molecular mass, and temperature are discussed quantitatively. We discuss the discrepancy of the P-V isotherms between experiments by Brillouin scattering and theoretical results for H2 near the freezing point, including the recent solid isotherms to higher pressures.

The compressibility of liquids was measured in a gasketed diamond anvil cell using a modified piston displacement method. A pair of metal markers was fixed on the diamonds. The gasket thickness at different pressures was measured by monitoring the distance change between the markers using a micrometer microscope. The area of the gasket hole was measured by means of microphotography. Then the product of the average area and thickness of the gasket hole determined the sample volume. Using this method we obtained the compressibility of a 4:1 v/v methanol-ethanol liquid mixture at hydrostatic pressures up to 8 GPa and found that the liquid volume at 8 GPa decreases to 51.5% of the initial volume. By the least-squares method, we fitted the experimental data to the Tait equation and obtained C=0.4055, B=0.5436 GPa. The relative error is estimated to be about 1.7%. This simple and convenient method could significantly extend the pressure range of liquid compressibility measurements.

The PVT properties and vapor-pressures of water have been measured in the critical region by using a constantvolume apparatus immersed in the constant-temperature bath with the procedure of varying the sample mass along a prescribed isotherm. Ninety-one PVT and five vapor-pressure measurements along 11 isotherms cover the region of temperatures 643 to 673 K and of pressures 19.7 to 38.6 MPa, which corresponds to a density variation from 108 to 618 kg/m3. The experimental uncertainties of temperature, pressure and density are estimated 4 mK, 5 kPa and 0.04%, respectively. Based on these measurements, the critical parameters of water are proposed to be 647.061 K, 22.041 MPa and 322.4 kg/m3 by the scaling law analysis.

A pVT apparatus based on a combination of gas expansion and isochoric method and an ultrasonic apparatus for sound velocity measurements based on phase comparison pulse-echo technique have been developed for precise measurements of isothermal and adiabatic compressibility of gases at pressures up to 10 kbar and temperatures down to the melting line.

The critical line of a nearly ideal binary system generally moves from the critical point of one of the components directly to the critical point of the other component (curve 1 Fig. 1). In a less ideal system, however, the behaviour is quite different. In some cases the curves move from the critical point of the less volatile component (component 2) to lower temperatures and higher pressures (curve 2) and rise again to higher temperatures via a temperature minimum, the critical double point. In other systems, the critical temperature increases continuously from the critical point of the less volatile component when the pressure is increased (curve 3). We assume here that the critical line is not interrupted by the appearance of a solid phase.

An apparatus to measure phase equilibria under pressure is described. The composition of the gas phase was determined using a high pressure, high temperature sampling cell. We have found that compressed natural gas is a very good solvent of hydrocarbons. A possible application of this study is the understanding of processes which lead to migration of oil and the location of oil and gas fields.

Supercritical fluid extraction uses a dense gas as a solvent at a temperature only slightly above its critical temperature and a pressure of a few hundred bars. The process offers the advantages of both conventional distillation and solvent extraction, and permits the facile achievement of difficult separations by highly selective absorption and desorption of various solutes. A valid thermodynamic model is essential for the rational design of such processes. We have made extensive solubility measurements at pressures to about half a kilobar, and we interpret the results in terms of perturbed hard-sphere equations of state. In this, we characterize solution nonidealities in terms of enhancement factors, which are often in the range of 103−108. Even better information about the specific solvent-solute interactions are available from measurements of partial molal volumes in supercritical solutions. These are very difficult measurements, as the effects are most pronounced precisely where the solvent compressibility is greatest, and accurate data are possible only because of the advent of the high-pressure vibrating tube densitometer, which converts density determinations to highly accurate frequency measurements for determinations to a precision of ±0.0001 g/cc. The solute partial molal volumes are many liters negative in the critical region, and these data are interpreted in terms of various equations of state.

Single-pulse backwards stimulated Raman and reflected broadband coherent anti-Stokes Raman spectroscopy (BSRS and RBBCARS) have been used to measure the vibrational frequency shifts of the 992 cm−1 ring-stretching mode of liquid benzene shock-compressed to pressures up to 1.2 GPa. The resulting shifts of ∼7.5 cm−1/GPa in the dynamic experiments are compared to spontaneous Raman scattering measurements of heated samples compressed in a diamond-anvil cell. RBBCARS was used to simultaneously measure the ring-stretching mode vibrational frequencies of liquid benzene / liquid perdeuterobenzene mixtures shock-compressed to pressures up to 1.53 GPa. Additional experiments that demonstrate the difficulty of using polarization sensitive coherent Raman techniques, such as Raman-induced Kerr effect spectroscopy (RIKES), in shock-compressed samples are described.

PTx-phase diagrams and critical curves for binary aqueous systems are briefly discussed. New results of the two-phase boundary surface and the critical curve for the water-nitrogen system to 2500 bar and 400°C are presented as isotherms in the Px-plane. A new extended Carnahan-Starling and squarewell type equation permits calculations of spinodal isopleths of the water-methane system and a prediction of the critical curve. - Excess volumes of supercritical benzene-water mixtures are given as functions of composition and pressure. Excess Gibbs energies and activity coefficients are derived. For the same system the static dielectric constant was measured between 300 and 400 °C in the homogeneous supercritical fluid for all concentrations. At 400 °C and 2000 bar the dielectric constant of pure water is 20 and decreases steeply with the addition of benzene. Addition of water to benzene causes at first only a slow increase above the value of 2 (for pure benzene). A short discussion of the description of the dielectric constant of such polar-nonpolar mixtures for wide ranges of density is given. - The solubility of anthracene in high density water to 250 °C was measured spectroscopically. Data are given. The “enhancement factor” is high at low temperature but decreases to unity at 250 °C and a water density of 1 g . cm−3

Experimental measurements have been carried out to determine the compositions of coexisting gas and liquid phases for binary mixtures of hydrogen with the following substances as the second component: nitrogen, carbon monoxide, argon, methane, ethylene, ethane and carbon dioxide. For most of these mixtures the entire region of gas-liquid equilibrium has been explored for the first time. This region is bounded in pressure-temperature space by the vapor-pressure curve of the heavy component, the gas-liquid critical line (where gas and liquid phases become identical) and the 3-phase region solid-liquid-gas. In all of the systems described here the latter two lines intersect to form a critical end point. The general qualitative features of these phase diagrams are described, and compared to those of helium mixtures studied earlier.

During the past decade, improvements in experimental techniques and computer simulation methods have led to a more comprehensive understanding of the properties of water [1]. Foremost amongst these are X-ray and neutron diffraction which provide information directly of the microscopic structure [2]. Such information is in the form of a structure factor F(k) and its real space Fourier transformation G(r). F(k) and G(r) can be used to test the various models of water used in computer simulation studies. G(r) can also provide a basic picture of the short range structure of water characterised by interparticle separations and coordination numbers.