Nuclear magnetic relaxation of alkali halide nuclei and preferential solvation in methanol + water mixtures

Abstract

Nuclear magnetic relaxation rates of the ionic nuclei ³⁵Cl, ⁸¹Br, ¹²⁷I, ²³Na and ⁸⁷Rb in methanol + water mixtures have been measured over the complete mixtures range and extrapolated to zero salt concentration. The relaxation of all these nuclei is controlled by quadrupole interaction; in the theoretical part of this paper a formula is given which describes their relaxation behaviour in the mixtures. In connection with these results a method is presented which uses the quadrupolar relaxation studies as a source of information regarding preferential solvation and which yielded the result that Na⁺ and Rb⁺ are preferentially hydrated whereas Cl^– and Br^– are preferentially solvated by methanol in the mixtures. These results are compared with those derived from chemical shift measurements by other authors and the discrepancy, so revealed, for Cl^–(and Br^–), is discussed.