Diffusion of Water in Liquid and Supercritical Carbon Dioxide: An NMR Study

Abstract

Tracer diffusion coefficients have been measured for water in liquid and supercritical carbon dioxide (CO2) from 10 to 35 °C in the pressure range from 130 to 300 bar. The measurements were performed by means of pulsed field gradient NMR (PFG-NMR) methods incorporating compensation for electrical eddy currents and mass convection. In the NMR active volume, the sample was contained in a 1.4 mm i.d. PEEK tube with provisions for recirculation and external sample loading. The diffusion coefficients are consistent with the Stokes−Einstein equation with “slip” boundary conditions and a hydrodynamic radius of 1.7 A for water in the high temperature and low density region. In the low temperature and high density region, the diffusion coefficients indicate either a trend toward “stick” boundary conditions or the dynamic clustering of water molecules.