A new high-temperature multinuclear-magnetic-resonance probe and the self-diffusion of light and heavy water in sub- and supercritical conditions

A high-resolution nuclear-magnetic-resonance probe (500 MHz for H-1) has been developed for multinuclear pulsed-field-gradient spin-echo diffusion measurements at high temperatures up to 400 degrees C. The convection effect on the self-diffusion measurement is minimized by achieving the homogeneous temperature distributions of +/- 1 and +/- 2 degrees C, respectively, at 250 and 400 degrees C. 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 ((H2O)-H-1) and heavy ((H2O)-H-2) water are distinguishably measured at subcritical temperatures of 30-350 degrees C with intervals of 10-25 degrees C on the liquid-vapor coexisting curve and at a supercritical temperature of 400 degrees C as a function of water density between 0.071 and 0.251 g/cm(3). The D value obtained for (H2O)-H-1 is 10%-20% smaller than those previously reported because of the absence of the convection effect. At 400 degrees C, the D value for (H2O)-H-1 is increased by a factor of 3.7 as the water density is reduced from 0.251 to 0.071 g/cm(3). The isotope ratio D((H2O)-H-1)/D((H2O)-H-2) decreases from 1.23 to similar to 1.0 as the temperature increases from 30 to 400 degrees C. The linear hydrodynamic relationship between the self-diffusion coefficient divided by the temperature and the inverse viscosity does not hold. The effective hydrodynamic radius of water is not constant but increases with the temperature elevation in subcritical water. (c) 2005 American Institute of Physics.