Concentration dependence of correlation times from proton NMR relaxation in hydrous melts

Abstract

Proton spin–lattice relaxation times (T1) for hydrous melts of Ca(NO3)2 containing 2.8, 4.0, 8.0, and 16.O moles H2O/mole salt (55.5 to 2.32 moles H2O/l) at temperatures between 290 and 390 K are analyzed in terms of intramolecular and intermolecular contributions. Rotational correlation times for water (τrot) are estimated from the relaxation rates in the high temperature (narrow line) region using intermolecular correlation times (τinter) estimated from melt viscosities or from diffusivities of protons or of dilute solute ions (Cd2+). The ratio τinter/τrot increases from about 2.5 in pure water to about 17 in the viscous melt Ca(NO3)2-2.8 H2O, suggesting a much greater medium effect on translational than on rotational motion. An empirical relation was found that quantitatively correlates the dependence of the relaxation rate, varying over three orders of magnitude, with viscosity (or water diffusivity) and the volume concentration of water at all of the compositions and temperatures investigated.