Thermal diffusion of electrolytes in high temperature aqueous solutions

We have reviewed the state of the art in the experimental and theoretical development of irreversible thermodynamics of non-isothermal electrolyte solutions under hydrothermal conditions, with particular emphasis on the study of thermal diffusion phenomena via electrochemical methods. By combining the measured initial and steady state thermoelectric powers of cells incorporating Ag/AgCl electrodes and aqueous LiCl, NaCl, KCl, and RbCl solutions, the standard entropies of transport, S₂^-0, have been derived for temperatures from 25 to 125°C. By employing Agar's hydrodynamic theory and using the experimentally-derived S₂^-0 values for calibrating the theory, we calculated high temperature S₂^-0 values, which demonstrate a dramatic rise of S₂^-0 when the temperature increases from 200 to 350°C. We infer that when the state parameters of an aqueous solution approach the critical point, the entropy of transport of an electrolyte solution assumes a very large value and we conclude that thermal diffusion becomes an important mass transfer process.