OXYGEN AND HYDROGEN ISOTOPE FRACTIONATION BY HYDRATION COMPLEXES OF LI+, NA+, K+, MG2+, F-, CL-, AND BR-: A THEORETICAL STUDY

Abstract

Hydration complexes of the ions Li+, Na+, Mg2+, F-, Cl-, and Br- that are expected to occur in natural brines, hydrothermal solutions, or volcanic vapors were studied theoretically using quantum mechanical methods, in particular density functional theory. The normal modes of vibration were calculated for minimum energy configurations obtained with the BLYP and B3LYP functionals in combination with the 6-31G** and 6-311G** basis sets. Enthalpies of dissociation for Li+, Na+, and Cl- hydrates calculated from these vibrational spectra are in good agreement with experimental values. From the theoretical normal modes of vibration, the reduced partition function ratios for D/H and 18O/16O substitution in the hydration complexes as well as the isotope fractionation between the complexes and monomeric water vapor were calculated. Following a scheme introduced by Bopp et al. (1974) we used the gas-phase results to model isotope salt effects in aqueous solutions. Good agreement with measured results was obtained for both oxygen and hydrogen isotope effects caused by cations, whereas anion results are less satisfactory, probably due to first shell-second shell interactions, which are not properly accounted for by the calculations. However, the calculations for the gas-phase cation hydration complexes can be utilized to gain insight into the nature of isotope ''salt effects'' in aqueous electrolyte solutions. Hydration-related ''salt effects'' are quantitatively dominated by the changes of O-H stretching frequencies due to the presence of the ions. Lower frequencies such as those related to the ion-water bonds are of minor importance. The distance up to which measurable effects on the O-H frequencies (and hence on the reduced partition function ratios) are induced is shown to be limited to the first hydration shell. These results may also be relevant for a better theoretical understanding of isotope fractionation involving clay minerals, adsorption of water on mineral surfaces, and isotope exchange at mineral-water interfaces.