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

为了研究超临界热水的动力学，研制了一种高分辨率核磁共振（NMR）探针（500 MHz），用于400℃以下高温下的多核脉冲梯度场自旋回波（PGSE）扩散测量。通过在250 ℃和400℃下分别实现±1 ℃和±2℃的均匀温度分布，使对流对自扩散测量的影响最小化。通过使用由具有与金属铝相当的高导热性的陶瓷（AlN）组成的固态加热系统来实现高温均匀性。在30-350℃的亚临界温度和400℃的超临界温度下，分别测量了轻（^1H_2O）和重水（^2H_2O）的自扩散系数D，在汽液共存曲线上，D的间隔为10-25℃，D是水密度在0.071 ~ 0.251 g/cm ^3之间的函数。对于^1H_2O，由于不考虑对流效应，所得D值比以前报道的小10%~ 20%.在400℃时，当水的密度从0.251 g/cm^3降低到0.071 g/cm^3时，^1H_2O的D值增加了3.7倍。粗略地说，所观察到的超临界水的自扩散系数似乎，虽然不完全，在本研究中所涵盖的低密度区域的密度或氢键的数量成反比。在30 ~ 400℃温度范围内，同位素比值D（^1H_2O）/D（^2H_2O）从1.23减小到1.04。由于^2H_2O被认为比^1H_2O更有结构，因此除了密度和温度依赖性之外，从分子间相互作用（通常是氢键）的角度来讨论同位素效应是很有趣的。

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.