Mass Diffusion Coefficients and Thermal Diffusivity in Concentrated Hydrothermal NaNO3 Solutions

Abstract

Binary mass diffusion coefficients and thermal diffusivities for hydrothermal sodium nitrate solutions as a function of pressure (270 < P < 1000 bar), temperature (400 < T < 500 °C), and concentration (0.25 < C < 3.0 m) were measured by the laser-induced grating technique. In concentrated hydrothermal NaNO3 systems, the critical slowing down was significant as far as 300 bar from the phase-separation pressure, resulting in binary diffusion coefficients near the critical point that are comparable to values at ambient conditions. Further from the critical point the mass diffusion coefficients plateaued at their ordinary values. Ordinary binary mass diffusion was about 15 times faster than at 25 °C and atmospheric pressure. The ordinary binary mass diffusion coefficients were compared with predictions from hydrodynamic diffusion theory. Experimental results agreed well with predictions from the Stokes−Einstein equation, where the diffusing species was best represented by a hydrated contact ion pair. The Wilk...