Structure of water in the liquid and supercritical states by rapid x-ray diffractometry using an imaging plate detector

Abstract

The structure of water in the liquid and supercritical states has been investigated with a newly developed rapid liquid and amorphous x-ray diffractometer using an imaging plate area detector. This new method has enabled us to reduce the measuring time to only one hour for each sample, which is less by a factor of about 100 than the time usually needed with a conventional θ–θ type diffractometer, and thus to measure x-ray scatterings of water at high temperatures and pressures, including supercritical state. In this study the temperature range of 300–649 K with pressures of 0.1–98.1 MPa was covered (Tc=647.3 K, Pc=22.12 MPa, ρc=0.322 g/cm3 for water). Densities of sample water were kept constant at 1.0, 0.95, 0.9, 0.8, and 0.7 g/cm3 by controlling temperature and pressure. The radial distribution functions (RDFs) have shown that the peaks for the second and further neighbors interactions disappear over 416 K and 0.95 g/cm3, showing the breakdown of local tetrahedral icelike structure in water. The analysis of the first peak of the RDFs has revealed that with increasing temperature the coordination number of the first neighbor interaction around 2.9 Å decreases from 3.1 at 300 K and 1.0 g/cm3 to 1.6 at 649 K and 0.7 g/cm3, whereas the interaction around 3.4 Å increases from 1.3 to 2.3 at the corresponding temperatures, resulting in a constant coordination number of around four in the first shell under the nearly constant densities. These findings are discussed with the recent results of computer simulation, neutron scattering, and Raman spectroscopic studies on water at high temperatures and pressures.

Water Structure from Scattering Experiments and Simulation

“…Water structure under ambient conditions has been studied by x-ray 63,69,70,160 and neutron scattering 45,48,72,161,162 over the last 30 years. where ρ is the number density of water and r min is the location of the first minimum in g OO (r).…”

“…Water under pressure has been characterized by both neutron 71,75,116 and x-ray scattering 65,66,160,169,170 . These structural studies indicate that when water is placed under pressure the number of hydrogen-bonds per water molecule does not change by any appreciable amount relative to the ambient state, although they are now bent out of their ideal orientation, and are correspondingly weaker energetically.…”

Large-Angle X-ray Scattering Investigation of the Structure of 2-Propanol–Water Mixtures

“…Then, the two-dimensional intensities of I obsd (x, y) were integrated into one-dimensional data, I obsd ( ), where 2 is the scattering angle [18,19]. The scattering intensities of the solutions alone were obtained by subtracting the intensities of the empty glass capillary from those of the samples.…”

“…Details and performance of the diffractometer have been described in [18,19]. X-rays were generated at a rotary Mo anode operated at 50 kV and 200 mA, and then monochromatized by a flat graphite crystal to obtain MoK radiation ( = 0.7107Å).…”

“…Polarization and absorption corrections were first made for two-dimensional X-ray intensities, I obsd (x,y), where x and y are vertical and horizontal coordinates of the IP, respectively, for a sample solution and an empty glass capillary as described in [18,19]. Then, the two-dimensional intensities of I obsd (x, y) were integrated into one-dimensional data, I obsd ( ), where 2 is the scattering angle [18,19].…”