Supercritical water: Local order and molecular dynamics
T. Tassaing, Y. Danten, and M. Besnard
Laboratoire de Physico-Chimie Moléculaire,
CNRS (UMR 5803), Université de
Bordeaux I, 351 Cours de la Libération, F-33405 Talence Cedex,
France
Abstract:
This paper is a study of the structure and dynamics of near-critical
and supercritical water for thermodynamic states above the critical
temperature in a wide range of density by infrared absorption and quasi-elastic
neutron scattering. The evolution of the shape of the infrared profiles
associated with the internal vibrational modes of water has been investigated.
In supercritical water, at T = 380 °C and low pressure (density),
in the range 25-50 bar (0.01-0.05 g·cm-3), only monomers
are detected. A progressive increase of the pressure (density) from
50 to 250 bar (from 0.05 to 0.4 g·cm-3) leads to the
appearance of dimers and trimers. In order to obtain information on
the dynamics, we have performed incoherent quasi-elastic neutron-scattering
experiments on light water for several thermodynamic states (200 < T
< 400 °C and 184 < P < 400 bar) corresponding to densities
ranging from 0.2 to 0.9 g·cm-3. The results have been
analyzed using a jump diffusion model and the two parameters of this
model, namely, t0, the residence time
and D, the translational diffusion coefficient, have been determined
as a function of the density.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.
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