Spectrophotometric study on solubility of UO2(β-diketonato)2dmso complexes (β-diketonate = acetylacetonate, trifluoroacetylacetonate, hexafluoroacetylacetonate; dmso = dimethyl sulfoxide) in supercritical carbon dioxide

We have investigated the molar extinction coefficients (ɛ) and solubility of UO2(β-diketonato)2dmso (β-diketonate = acetylacetonate (acac), trifluoroacetylacetonate (tfacac), and hexafluoroacetylacetonate (hfacac); dmso = dimethyl sulfoxide) in supercritical CO2 (scCO2) at 40 °C in the pressure range of 10–25 MPa (density; 0.6 < ρ (g cm−3) < 0.9) by using UV–vis spectrophotometer equipped with high-pressure cell. The ɛ values (M−1 cm−1, M = mol dm−3) for UO2(acac)2dmso, UO2(tfacac)2dmso, and UO2(hfacac)2dmso in scCO2 were estimated as 1.06 × 102 at 355 nm, 9.59 × 101 at 440.5 nm, and 7.95 at 486 nm, respectively, and were similar to that in C6H14. We also found out that the solubility of UO2(acac)2dmso, UO2(tfacac)2dmso, and UO2(hfacac)2dmso increases with increasing pressure and in order of UO2(acac)2dmso < UO2(tfacac)2dmso < UO2(hfacac)2dmso, i.e., 2.67 × 10−3 < 1.37 × 10−2 < 3.73 × 10−1 M at 40 °C and 25 MPa. This is consistent with the order of an increase in the number of –CF3 group in the coordinated β-diketonates. Furthermore, the equation proposed by Chrastil was found to hold for the solubility of UO2(β-diketonato)2dmso, that is, the logarithmic plot of solubility versus densities of scCO2 showed the linear relationship. These phenomena suggest that the solubility of UO2(β-diketonato)2dmso is related with CO2 solvation and the strength of interactions of CO2 with the coordinated β-diketonates. In order to examine such interactions, we measured the dependence of 13C chemical shifts of CO2 in scCO2 containing β-diketones on the density of scCO2. As a result, it was proposed that the solubility of UO2(β-diketonato)2dmso is mainly controlled by the van der Waals interactions between scCO2 and particular sites (–CH3 and –CF3 groups) of ligands in UO2(β-diketonato)2dmso.