NMR probing of structural peculiarities in ionic solutions close to critical point

H-1, Na-23, Cl-35, Br-79, and Br-81 NMR chemical shifts (delta) and signal half widths (Delta(1/2)) have been measured in aqueous electrolyte mixtures [tetrahydrofuran/H2O/NaCl and 3-methylpyridine (3MP)/H2O/NaBr] at different mass fractions of salt (X) in the one-phase region, close to their lower critical solution points (T-CL). Discontinuous changes in slope of delta=f(X) and Delta(1/2)=f(X) have been found in Na-23 and Br-81 NMR spectra of 3MP/water/NaBr solution at X approximate to 0.1 and T=301 K. The dependency of H-1 NMR signals of 3MP is continuous over the whole investigated range of X=0.002-0.2, whereas changes in the slope of H2O chemical shifts are hardly noticeable. In the two-phase region, i.e., at T > T-CL, a doubling of all NMR signals has been observed. The sensitivity of NMR parameters depends more on composition of solution for anions (Cl- and Br-) than for cations (Na+). A very strong relaxation effect for Br-81 nuclei with relaxation rates reaching 14 000 s(-1) was observed. The results are interpreted in terms of ion-molecular clustering and changes in coherency of dipole configurations of water molecules during supramolecular restructuring of solutions.