Preferential solvation of Na+ in aqueous mixtures of simple amides. Application of the D2O–H2O isotope effect to 23Na+ nuclear magnetic relaxation

Abstract

The dynamic D₂O—H₂O isotope effect on the magnetic relaxation of ionic nuclei possessing a quadrupole moment has been applied to a quantitative investigation of Na⁺ solvation in aqueous mixtures of the three amides, formamide, N-methylformamide and NN-dimethylformamide, over the complete solvent mixture range. The local mole fraction of water in the inner solvation sphere of the ion has been determined. In water + formamide and water + NMF mixtures only small deviations from the non-preferential solvation behaviour occur. In water + DMF mixtures the data revealed preferential solvation of Na⁺. In the water-rich region Na⁺ is selectively hydrated, but at a mole fraction of water of ca. 0.6 a transition occurred. In the amide-rich region Na⁺ was preferentially solvated by DMF. A similar transition has previously been observed by other authors in DMSO + water mixtures. In a pilot experiment we also studied the salt concentration dependence of preferential solvation in water + DMF mixtures.