Molecular diffusivity of phenol in sub- and supercritical water: application of the split-flow Taylor dispersion technique.

The binary diffusion coefficient of phenol in aqueous solution was examined from ambient to supercritical water conditions by using the developed split-flow Taylor dispersion technique. The technique significantly simplifies diffusivity measurements in high-temperature and supercritical water, as the sample injection and detection are performed ex situ at ambient conditions. The binary diffusion coefficient of phenol increases from 1.013 × 10(-9) m(2) s(-1) at 298.7 K and 25 MPa to about 34.71 × 10(-9) m(2) s(-1) at 672.9 K and 30 MPa and follows Arrhenius behavior with an activation energy of 15.09 kJ/mol. The diffusion coefficient of phenol in infinitely dilute solution was also calculated by means of molecular dynamics (MD) simulations over a wide temperature and density range (298-773 K and from 0.07 to 1.0 g/cm(3), respectively). A dramatic increase in the diffusivity was observed upon transition into the low density supercritical region. The obtained experimental data agrees well with available literature values and the MD results. At subcritical conditions the experimentally obtained binary diffusion coefficients generally follow the predictions from the Stokes-Einstein equation, with the estimate for the hydrodynamic radius of the solute taken from MD data.