A fluorine-19 NMR investigation of specific interactions between fluorocarbons and supercritical carbon dioxide

Supercritical carbon dioxide's density, viscosity, dielectric constant, and solubility parameter can be tuned over a wide range from the gas phase to supercritical phase. Amorphous fluoropolymers exhibits an exceptional solubility in scCO₂. This investigation focuses on using fluorcarbons as a first step in studying the solvation power of scCO₂ and other solvents to determine it a specific interaction exist between scCO₂ and fluorines. The nonlinear 19F chemical shifts originally observed for perfluorohexane in scCO₂ by Dardin et al. are reproducible and do result from the three distinct atomic sites having (conformationally) different accessibility to the solvent. Those observations were extended to include other related solvents, CS₂, scCO₂, sc-Ethane, and sc-Propane. However, the crude attempts by Dardin et al to account for the observed site-specific shifts by merely considering the surface area of these groups was too naïve. The 19F NMR investigation of trans-perfluorodecalin in scCO₂ displayed five dramatic chemical shifts each with a distinct, nonlinear density dependence. The solvent-accessible surface area approach applied to the trans-perfluorodecalin chemical shifts as in the case of perfluorohexane, failed to explain the data. Since the site-specific density dependence of the chemical shifts was so distinct for this solute investigation were extended in other supercritical solvent (ethane and propane). This was indeed the case and prompted for a generic explanation. MD simulations were used in order to get a more fundamental understanding of the solvation of trans-perfluorodecalin in scCO₂. The radial distribution functions at each inequivalent site in the solute was computed and used to explicitly calculate the van der Waals contribution at each site. The data shows that a qualitatively correct agreement between calculations and experiment does result confirming that the origin of the site-specific density dependence of the F-19 shifts is indeed do to the van der Waals interaction. However, the F-19 shifts observed for solutes in carbon dioxide have a rather generic explanation and fail to give us an easy answer to the question about why amorphous fluoropolymers dissolve into scCO₂.