Diffusion of Decamethylferrocene and Decamethylferrocenium Hexafluorophosphate in Supercritical Trifluoromethane

The diffusion coefficients of decamethylferrocene (Fe(Cp*) 2) and decamethylferrocenium hexafluorophosphate (Fe(Cp*) 2PF6) in supercritical trifluoromethane (CHF3) were measured at a temperature of 323.15 K, as a function of density, with tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte, using a voltammetric technique on a platinum microelectrode. The diffusion in subcritical CHF3 was also studied, to analyze the differences in the behavior of both solutes in the low- and high-density regions. Application of the Oldham, Cardwell, Santos, and Bond theory allowed the effect of ion pairing on the limiting current to be considered. The density dependence of the diffusion coefficients of the different species has been discussed on the basis of the continuum hydrodynamic model and the clustering of solvent around solute molecules in the low-density supercritical region. The predictions of the compressible continuum model for the case of the diffusion of the free ions and the ion pair also have been discussed.