Yu. E. Gorbaty

X‐ray scattering in liquid water at pressures of up to 7.7 kbar: Test of a fluctuation model

The pair correlation functions g(r) of liquid water obtained at pressures of up to 7.7 kbar at a constant temperature of 20 °C are discussed. A variety of evidence of the very strong effect of pressure on the structure of liquid water has been found. In particular, an unusual pressure dependence of the shortest intermolecular separation was observed. The results are interpreted in terms of fluctuations of the nearest environment, assuming that some of the structural configurations arising may be distinguished as preferred ones. The SIMPLEX procedure has been used to fit g(r) to the sum of correlation functions corresponding to the heavily destroyed structures of the solid phases of water. The results correlate with the phase diagram for water fairly well. This gives rise to an idea that water may be a suitable object for understanding the very nature of the liquid state.

The pair-correl.Ation functions of water at a pressure of 1 000 bar in the temperature range 25–500°C

Abstract Pair-correlation functions for liquid and supereritical water are reported. The behaviour of these functions shows a lessening of the hydrogen bondings and the tetrahedral ordering in the water structure with increasing temperature. Strong evidence that the distribution of intermolecular distances between nearest-neighbour molecules should be discrete is found. The contributions of structurally distinguishable molecules to the coordination number of water are estimated.

An X-ray study of the effect of pressure on the structure of liquid water

The molecular pair correlation functions of liquid H2O have been obtained in the pressure range 1–2000 bar. A strong structural inhomogeneity of liquid water resulting from fluctuations of the entropy and density has been found. This gives rise to a complicated shape of the distribution of the intermolecular distances in the first coordination sphere. The coordination number is equal to approximately 4·4, but not more than 2·5 of the total number of the nearest molecules can form a tetrahedral short-range order. It has been established that the effects of both pressure and temperature on the water structure are fairly alike; they lead to a decrease of tetrahedral ordering. The rate of decrease of the separation between the nearest molecules is found to be approximately 0·03 A kbar-1; this is much larger than is usually supposed. The results have been interpreted in terms of a percolation model.

An infrared study of water vapour in the temperature range 573–723 K. Dimerization enthalpy and absorption intensities for monomer and dimer

A high-temperature high pressure cell with changeable path length was used to obtain the integrated intensities of the low density vapour in the spectral region λ = 2·7 μm along four isotherms in the range from 573 to 723 K. A linear dependence of the intensity was observed and interpreted in terms of the ‘monomer-dimer’ equilibrium. A dimerization enthalpy of 16·65 ± 3·77 kJ mol-1 has been found using the Arrhenius behaviour of the intensity isotherms. All isotherms intersect at zero pressure giving the total integrated intensity 54·92 ± 0·5 km mol-1. Rough estimation of the total intensity for the water dimer gives 318 ± 120 km mol-1.

Structure and hydrogen bonding of liquid water at high hydrostatic pressures: Monte Carlo NPT-ensemble simulations up to 10 kbar☆

Abstract Monte Carlo computer simulations with the TIP4P intermolecular potential are performed in 9 thermodynamic states of compressed liquid water along the 298 K isotherm. Two distinct pressure ranges are found in the pressure dependence of the OO near-neighbor distances in qualitative agreement with the X-ray diffraction data. Nevertheless, the experimentally observed minimum in this dependence is not reproduced by the present simulations. The evolution of hydrogen-bonded network in liquid water under gradual compression is quantitatively analyzed using the combined geometric and energetic criterion of hydrogen bonding. Despite some bending and weakening of the existing hydrogen bonds, the average topology of the H-bonded network remains intact, and the average number of hydrogen bonds per a water molecule remains constant ( n HB > = 3.2) over the whole pressure range studied, while the increase in density is mainly achieved by an increased packing efficiency of non-bonded nearest and second nearest neighbors.

In situ Raman spectroscopic study of sulfur-saturated water at 1000 bar between 200 and 500°C

Abstract Raman spectra of a hydrothermal fluid, interacting with elementary sulfur, have been measured in the temperature range from 200 up to 500°C at a constant pressure of 1000 bar. The main products of the interaction are H 2 S, SO 2 , [HSO 4 ] − , and S 0 . The temperature dependencies of intensities for characteristic Raman lines of these species have been found. They show that concentrations of H 2 S and SO 2 behave much alike, increasing in the whole temperature range studied, whereas the temperature trends for [HSO 4 ] − and S 0 show maxima at 350–400°C, so that these species practically disappear at higher temperatures. The temperature dependencies for all the main components of the solution reveal features around the critical isotherm at isobaric heating or cooling. At low temperatures, the presence of the [SO 4 ] 2− ion may be suspected. The ion [HS] − exists in the whole temperature range. Also, weak bands of the anion [S 2 O 3 ] 2− seem to occur in the spectra.

Experimental Technique for Quantitative IR Studies of Highly Absorbing Substances at High Temperatures and Pressures

A new high-temperature/high-pressure IR cell with a changeable pathlength is described. The pathlength can be varied immediately during a run at high pressure and temperature with a driving mechanism attached to the body of the cell. This arrangement presents the possibility of avoiding most of the common and specific errors connected with intensity measurements in the difficult case of highly absorbing substances. The cell can be used up to 780 K at a pressure of 100 MPa. The algorithms of data processing and examples of representative spectra of water are also discussed.

The physical state of supercritical fluids

Abstract The temperature dependencies of intensity in the Raman spectra of a sulfur-saturated aqueous solution show a singular behavior near the critical isotherm at isobaric heating or cooling. It has been suggested that the reason for the singularity is a qualitative change in the physical state of a fluid as it turns supercritical. As a probe of the physical state of a fluid, a conditional criterion is suggested based on the analysis of rotational molecular movement in the fluid. In the practically significant range of temperatures and pressures a supercritical water fluid cannot be strictly classified as liquid-like or gas-like. It is a unique transitory state of a substance distinguished by very strong structure fluctuations.

The effect of pressure on hydrogen bonding in water: IR study of νOD HDO at pressures of up to 1500 bar

A seeming contradiction has been revealed between the behaviour of the shortest intermolecular separation roo and that of the frequency of the OH vibration of H2O under compression in liquid water. Decrease of roo with increasing pressure up to 2 kbar seems to be evidence of strengthening of the hydrogen bonding, but the increase in the frequency of νOH H2O seems contradictory. To clear up this question, measurements of the integrated intensity of IR absorption for νOD HDO have been made which definitely show that the energy of the hydrogen bonds decreases under compression at least up to 1.5 kbar. The decrease in the intensity is of about 20%, i.e., essentially more than the estimated accuracy of the measurements. Monte Carlo simulations of the compression of liquid water show that the behaviour of rOO, νOH, and the integrated intensity can be explained by the bending of hydrogen bonds.

High‐pressure high‐temperature Raman cell for corrosive liquids

A high‐pressure high‐temperature cell for Raman studies of fluids at pressures of up to 1000 bar and temperatures up to 800 K is described. An uncommon feature of the cell is the internal vessel assembled of sapphire and gold details so that a liquid under study can come in contact only with these materials. An original technique for filling the internal vessel with a liquid to be studied is also presented.