Rotational Relaxation in Supercritical Carbon Dioxide Revisited: A Study of Solute-Induced Local Density Augmentation

Solute Diffusion into Polymer Swollen by Supercritical CO2 by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography

High-pressure electron paramagnetic resonance (EPR) was used to measure translational diffusion coefficients (D_tr) of a TEMPONE spin probe in poly(D,L-lactide) (PDLLA) and swollen in supercritical CO₂. D_tr was measured on two scales: macroscopic scale (>1 μm), by measuring spin probe uptake by the sample; and microscopic scale (<10 nm), by using concentration-dependent spectrum broadening. Both methods yield similar translational diffusion coefficients (in the range 5-10 × 10^-12 m²/s at 40-60 °C and 8-10 MPa). Swollen PDLLA was found to be homogeneous on the nanometer scale, although the TEMPONE spin probe in the polymer exhibited higher rotational mobility (τ_corr = 6 × 10^-11 s) than expected, based on its D_tr. To measure distribution coefficients of the solute between the swollen polymer and the supercritical medium, supercritical chromatography with sampling directly from the high-pressure vessel was used. A distinct difference between powder and bulk polymer samples was only observed at the start of the impregnation process.

Fluorination effects on rotational correlation times of tris(β-diketonato)aluminum(III) in CO2 by 27Al NMR relaxation measurements

(27)Al NMR longitudinal relaxation times, T(1,obs)((27)Al), of [Al(acac)(3)] and [Al(hfa)(3)] (Hacac = acetylacetone, Hhfa = hexafluoroacetylacetone) in CH(3)CN and CO(2) were measured over a wide range of temperature and pressure. The rotational correlation times, τ(r), of the tris(β-diketonato)aluminum(III) complexes were determined from T(1,obs)((27)Al) using (27)Al quadrupole coupling constants, eQq/h((27)Al), which were also obtained to be 3.11 and 3.22 MHz for [Al(acac)(3)] and [Al(hfa)(3)], respectively, in CD(3)CN by the dual spin probe technique in the present study. At each temperature, τ(r) increased almost linearly with increasing viscosity, η, in both CH(3)CN and CO(2); however, τ(r) in CO(2) at near critical densities deviated appreciably upward, as shown in a similar analogue of bis(acetylacetonato)beryllium(II), [Be(acac)(2)] (Umecky; et al. J. Phys. Chem. B 2002, 106, 11114). The η/T dependence of τ(r) was examined to discuss intermolecular interactions between the complexes and solvent molecules in terms of the fluorination and geometrical effects. The degree of solute-solvent interactions increases in the order [Be(acac)(2)] < [Al(hfa)(3)] < [Al(acac)(3)] in CH(3)CN and [Al(acac)(3)] < [Be(acac)(2)] < [Al(hfa)(3)] in CO(2). The results suggest that dipolar CH(3)CN molecules interact with negatively charged oxygen atoms in the complexes, whereas nonpolar CO(2) prefers fluorinated substituents as well as quasi-aromatic rings in the ligands. Moreover, the relationship between the rotational and translational motions of tris(acetylacetonato)metal(III), [M(III)(acac)(3)], in CO(2) was investigated.

Molecular Dynamics Simulation of the Cybotactic Region in Gas-Expanded Methanol−Carbon Dioxide and Acetone−Carbon Dioxide Mixtures

Local solvation and transport effects in gas-expanded liquids (GXLs) are reported based on molecular simulation. GXLs were found to exhibit local density enhancements similar to those seen in supercritical fluids, although less dramatic. This approach was used as an alternative to a multiphase atomistic model for these mixtures by utilizing experimental results to describe the necessary fixed conditions for a locally (quasi-) stable molecular dynamics model of the (single) GXL phase. The local anisotropic pair correlation function, orientational correlation functions, and diffusion rates are reported for two systems: CO2-expanded methanol and CO2-expanded acetone at 298 K and pressures up to 6 MPa.

Temperature Dependence of Local Density Augmentation for Acetophenone N,N,N‘,N‘-Tetramethylbenzidine Exciplex in Supercritical Water

Local density augmentation around exciplex between acetophenone and N,N,N',N'-tetramethylbenzidine in supercritical water was measured by observing the peak shift of transient absorption spectrum at temperatures from 380 to 410 degrees C and at pressures from 6 to 37 MPa. Large local density augmentation was observed at lower solvent densities. Local density augmentation was evaluated by the excess density, which was defined as the difference between local density and bulk density, and the density enhancement factor, which was defined by the ratio of the local density to the bulk density. The number of solvating molecules was estimated with a Langmuir adsorption model. The excess density was found to exhibit a maximum at approximately 0.15 g cm(-3), which decreased with increasing temperature. The density enhancement factor was found to decrease with increasing temperature; however, its value was much greater than unity at 410 degrees C, which provides evidence that exciplex-water interactions still exist at these conditions. The temperature dependence of local density augmentation around the exciplex in supercritical water was comparable with that in supercritical carbon dioxide, which suggests that the ratios of the solute-solvent and solvent-solvent interactions are comparable between these two systems.

Enhanced cage effects in supercritical fluid solvents. the behavior of diffusive and geminate caged-pairs in supercritical carbon dioxide

The behavior of geminate and diffusive radical caged-pairs arising from the photolysis of dicumyl ketone in conventional and supercritical carbon dioxide (SC--CO(2)) solvents has been examined. The results suggest that locally enhanced solvent density about a solute (solvent/solute clustering) can lead to an enhanced cage effect near the critical pressure in supercritical fluid solvents. This enhanced cage effect is similar in magnitude for both diffusive and geminate caged-pairs.