Experimental and simulation study of salt effects and pressure/density effects on oxygen and hydrogen stable isotope liquid-vapor fractionation for 4-5 molal aqueous NaCl and KCl solutions to 400 degrees C

Liquid-vapor equilibrium fractionation factors for D/H and O-18/O-16 exchan ge between concentrated (4.0 to 4.8 molal) aqueous solutions of NaCl and KC l and their respective equilibrium vapor phases have been determined experi mentally up to 413 degrees C. In both cases, strong deviations from the pur e water liquid-vapor fractionation curves are observed. The D/H fractionati on curves of the two salt solutions are almost identical over the entire te mperature range studied and are always located below the pure water curve, thus changing the location of the crossover point to about 200 degrees C. T he strongest fractionation of deuterium into the vapor phase occurs around 330-350 degrees C. The O-18/O-16 fractionation curves for the two solutions are significantly different. Whereas NaCl has hardly any effect below 200 degrees C and then tends to enrich the heavy isotope in the liquid more str ongly than is the case in pure water, KCl causes a depletion of the solutio n relative to pure water below about 100 degrees C and a relative enrichmen t above 200 degrees C. Using the combined results of molecular dynamics simulations of water vapor at various temperatures and densities and ab initio calculations of the vi brational frequencies for various water species, we demonstrate that above 200 degrees C, the measured fractionation factors cannot solely be interpre ted in terms of isotope effects related to ionic hydration in the solution. The simulations indicate significant contributions from isotope effects re sulting from the different vapor pressures/densities of pure water and salt solutions. Furthermore, it is very likely that the density differences bet ween the liquid phases play an important role at high temperatures. The con tributions of these two density effects to D/H fractionation increase along the liquid-vapor curve. In contrast, the contribution of isotope effects r esulting from ionic hydration decreases with increasing temperature. Eviden ce is presented that the D/H isotope effect resulting from ionic hydration in a solution of constant density probably follows a normal linear 1/T-2 de pendence and that the liquid-vapor data below 200 degrees C can be used to constrain the slope of this line. The vapor density effect contribution to the O-18/O-16 liquid-vapor fractionation is apparently small. In contrast t o D/H, the O-18/O-16 isotope effects caused by ionic hydration appear to in crease with increasing temperature and are different for NaCl and KCl. The possible molecular causes for the observed trends are discussed. Copyright (C) 2000 Elsevier Science Ltd.