D2O–H2O isotope effect on nuclear magnetic relaxation of alkali halide nuclei and preferential solvation in mixed solvents

Abstract

The effect of substitution of H₂O by D₂O on the nuclear magnetic relaxation of the ionic nuclei ²³Na, ⁸⁷Rb and ⁸¹Br, which all relax by quadrupole interaction, has been investigated in several NaBr and RbBr solutions. The solvents were neat water and binary mixtures of water with methanol, formamide and dimethylsulphoxide. In the neat aqueous solutions it was found experimentally that the isotope effect is similar to that on the viscosity and that even at high salt concentrations, where all water is “hydration water”, this effect is still relatively strong. In the mixed solvents the composition dependence of the isotope effect on the relaxation rates has been studied in the water rich region, showing e.g. in methanol + water mixtures a quite different behaviour for cationic and anionic nuclei. A new method is presented, which, with certain assumptions, allows us to calculate the local mole fraction of water in the inner solvation sphere of the ions from the measured D₂O–H₂O isotope effect as a function of the macroscopic solvent composition. The analysis yielded the result that Na⁺ and Rb⁺ in methanol + water and to a smaller extent also Rb⁺ in formamide + water are preferentially hydrated. In the same way it was found that Br^– is weakly preferentially solvated by methanol and Na⁺ by dimethylsulphoxide, whereas Br^– is not selectively solvated in formamide + water. The results are discussed and compared with those obtained by other methods.