(620g) Measurements of Binary Diffusion Coefficients of Metal Complexes in Supercritical Carbon Dioxide and Liquid Organic Solvents by Transient Response Techniques

Metal complexes are attracting considerable attention in various fields, in particular, innovative syntheses of inorganic, organic and hybrid materials through supercritical fluid and self-assembly technology. Diffusion coefficients as well as other physical properties are essential for estimating mass transfer rates in reactors under supercritical conditions. However, the literature data on diffusion coefficients are limited, and most of the data in supercritical fluids and liquids were measured by electrochemical methods such as cyclic voltammetry and chronoamperometry. In these methods the addition of electrolyte is needed for non-electrolytic solvents. The presence of the electrolyte added may affect the diffusion coefficient values somewhat, and the effect is not clear. On the other hand, the transient response methods such as the Taylor dispersion method and the chromatographic impulse response method are more suitable due to no requirement of electrolyte added. In the present study binary diffusion coefficients, D12, of various metal complexes such as ferrocenes and metal acetylacetonates, Me(acac)n, were measured in supercritical carbon dioxide (308.15 to 333.2 K, and 0.2 to 40 MPa) by the Taylor dispersion with an uncoated capillary column and the chromatographic impulse response method with a polymer coated capillary column. In the Taylor dispersion method measurements were also made in liquid organic solvents such as acetonitrile, methanol, ethanol, hexane, and cyclohexane. The metal complexes studied were ferrocene, 1,1-dimethylferrocene, ethylferrocene, butylferrocene, 1,1'-dibutylferrocene, Pd(acac)2, Pt(acac)2, Co(acac)3, Rh(acac)3, and Ru(acac)3. Over a wide range of solvent viscosity, irrespective of solvent species and of supercritical or liquid state, binary diffusion coefficients for each solute were expressed by a hydrodynamic equation, namely D12/T=aη^b, where T is the temperature, η is the solvent viscosity, a and b are specific constants for each solute. D12 data for all ferrocenes studied were well correlated by the modified hydrodynamic equation, M^0.5D12/T=aη^b, where M is the molecular weight of metal complex. Althoug D12 data for Me(acac)n were roughly represented by the modified hydrodynamic equation, the accuracies were lower than those with ferrocenes because the atomic sizes of centered metals in acac complexes were not proportional to the molecular weights. In plots of the hydrodynamic equation the D12 values obtained in the present study were nearly consistent with the literature data measured by the electrochemical methods such as cyclic voltammetry and chronoamperometry with various organic solvents, when the viscosities of the mixture of a solvent and an electrolyte are used.