Abstract:
Liquid-vapor equilibrium fractionation factors for D/H and 18O/16O exchange between concentrated (4.0 to 4.8 molal) aqueous solutions of NaCl and KCl and their respective equilibrium vapor phases have been determined experimentally up to 413°C. In both cases, strong deviations from the pure water liquid-vapor fractionation curves are observed. The D/H fractionation curves of the two salt solutions are almost identical over the entire temperature range studied and are always located below the pure water curve, thus changing the location of the crossover point to about 200°C. The strongest fractionation of deuterium into the vapor phase occurs around 330-350°C. The 18O/16O fractionation curves for the two solutions are significantly different. Whereas NaCl has hardly any effect below 200°C and then tends to enrich the heavy isotope in the liquid more strongly than is the case in pure water, KCl causes a depletion of the solution relative to pure water below about 100°C and a relative enrichment above 200°C.Using the combined results of molecular dynamics simulations of water vapor at various temperatures and densities and ab initio calculations of the vibrational frequencies for various water species, we demonstrate that above 200°C, the measured fractionation factors cannot solely be interpreted in terms of isotope effects related to ionic hydration in the solution. The simulations indicate significant contributions from isotope effects resulting from the different vapor pressures/densities of pure water and salt solutions. Furthermore, it is very likely that the density differences between the liquid phases play an important role at high temperatures. The contributions of these two density effects to D/H fractionation increase along the liquid-vapor curve. In contrast, the contribution of isotope effects resulting from ionic hydration decreases with increasing temperature. Evidence is presented that the D/H isotope effect resulting from ionic hydration in a solution of constant density probably follows a normal linear 1/T2 dependence and that the liquid-vapor data below 200°C can be used to constrain the slope of this line. The vapor density effect contribution to the 18O/16O liquid-vapor fractionation is apparently small. In contrast to D/H, the 18O/16O isotope effects caused by ionic hydration appear to increase with increasing temperature and are different for NaCl and KCl. The possible molecular causes for the observed trends are discussed.