Oxygen Isotope Fractionation and the Structure of Aqueous Alkali Halide Solutions

Abstract

Abstract The oxygen isotope effects between H2O and D2O solutions of various alkali-halides and the respective pure solvents have been measured by means of the CO2 equilibration technique. In general, the effects for H2O are smaller than those for D2O. With "hydration numbers" estimated from the angular distributions of the water dipoles around the ions obtained from MD-simulations and with the plausible assumption that in highly concentrated LiCl solutions the effect is purely cationic, the measured effects are separated into effects between the hydration shells of the individual ionic species (Li, Na, K, Cs, CI, Br, I) and bulk water. The cationic effects thus obtained are compared with the corresponding effects between free water-cation-pairs and bulk water calculated on the basis of the energy surfaces published by Kistenmacher et al. It is found that, in general, the trends coincide but the former effects are smaller than the latter ones.