Development of multinuclear, high-temperature, and diffusion NMR probe and dynamics of supercritical aqueous solutions

点击翻译按钮获取中文摘要

In order to elucidate the dynamics of hot water including supercritical, a high-resolution nuclear magnetic resonance (NMR) probe (500 MHz for ^1H) has been developed for multinuclear pulsed field gradient spin-echo (PGSE) diffusion measurements at high temperatures up to 400℃. The convection effect on the self-diffusion measurement is minimized by achieving the homogeneous temperature distributions of±1 and ±2℃, respectively, at 250 and 400℃. The high temperature homogeneity is attained by using the solid-state heating system composed of a ceramic (AlN) with high thermal conductivity comparable with that of metal aluminium. The self-diffusion coefficients D for light (^1H_2O) and heavy (^2H_2O) water are distinguishably measured at subcritical temperatures of 30-350℃ with intervals of 10-25℃ on the liquid-vapor coexisting curve and at a supercritical temperature of 400℃ as a function of water density between 0.071 and 0.251 g/cm^3. The D value obtained for ^1H_2O is 10%-20% smaller than those previously reported because of the absence of the convection effect. At 400℃, the D value for ^1H_2O is increased by a factor of 3.7 as the water density is reduced from 0.251 to 0.071 g/cm^3. Roughly speaking, the observed self-diffusion coefficient of supercritical water appears, although not exactly, inversely proportional to the density or the number of hydrogen bonds in the low-density region covered in this study. The isotope ratio D(^1H_2O)/D(^2H_2O) decreases from 1.23 to 〜1.0 as the temperature increases from 30 to 400℃. Since ^2H_2O is considered to be more structured than ^1H_2O, the isotope effect is interesting to discuss in terms of intermolecular interaction, typically hydrogen bonding, apart from the density and the temperature dependences.