Nuclear magnetic relaxation and ionic solvation of23Na+ and81Br− in aqueous mixtures of simple amides

Ion-solvent interactions of Na⁺ and Br^– in binary aqueous mixtures of formamide,N-methylformamide (NMF), andN,N-dimethylformamide (DMF) are studied by use of²³Na and⁸¹Br magnetic relaxation times, extrapolated to zero salt concentration. The relaxation times, which are controlled by quadrupolar interaction, have been measured over the complete mixture range and are compared with a simplified theoretical formula. It turned out that the²³Na⁺ relaxation in H₂O-formamide and H₂O-NMF mixtures is in excellent agreement with theoretical predictions, implying nonpreferential solvation of Na⁺ in these systems. Small deviations of experimental from theoretical results in H₂O+DMF possibly indicate weak selective hydration of the cation. In the case of the anionic nuclei⁸¹Br^–, deviations from the theoretical curve occur which are to be expected, especially for systems where hydrophobic effects play a role. On the other hand, it is demonstrated that these deviations can easily be explained within the electrostatic theory by differences in structural details of the anionic solvation sphere in the mixtures compared to the pure solvents.