Importance of thermal diffusion in high subcritical and supercritical aqueous solutions

We have reviewed our experimental and theoretical studies of irreversible thermodynamics of non-isothermal aqueous systems, with particular emphasis on the investigation of thermal diffusion phenomena, via electrochemical methods. By employing Agar's hydrodynamic theory, and by using the experimentally-derived standard entropies of transport, S₂*⁰, at temperatures from 25 to 125°C, we have calculated high temperature S₂*⁰ values, which exhibit a dramatic rise when the temperature increases above 250°C. Recently reported experimental data for thermal diffusion in NaNO₃ aqueous solutions at supercritical temperatures and pressures by Butenhoff et al. have allowed us to test the validity of our calculations at temperatures up to 450°C. The significance of thermal diffusion in liquid systems with forced convection is discussed, and it is concluded that, as the Lewis number and the Soret coefficient increase, the impact of thermal diffusion on mass transport becomes increasingly important at higher temperatures. We also concluded that the use of an electrochemical cell with forced convection could be a promising experimental technique for determining Soret coefficients for aqueous solutions at elevated temperatures.