Supercritical water: Local order and molecular dynamics

Abstract

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, τ0, the residence time and D, the translational diffusion coefficient, have been determined as a function of the density.