Diffusion of water in liquid and supercritical carbon dioxide: An NMR study

Tracer diffusion coefficients have been measured for water in liquid and supercritical carbon dioxide (CO2) from 10 to 35 degreesC 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 angstrom 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.