Viscosity of liquid perfluoroalkanes and perfluoroalkylalkane surfactants

As part of a systematic study of the thermophysical properties of two important classes of fluorinated organic compounds (perfluoroalkanes and perfluoroalkylalkanes), viscosity measurements of four n-perfluoroalkanes and five perfluoroalkylalkanes have been carried out at atmospheric pressure and over a wide range of temperatures (278-353 K). From the experimental results the contribution to the viscosity from the CF₂ and CF₃ groups as a function of temperature have been estimated. Similarly, the contributions for CH₂ and CH₃ groups in n-alkanes have been determined using literature data. For perfluoroalkylalkanes, the viscosity results were interpreted in terms of the contributions of the constituent CF₂, CF₃, CH₂, and CH₃ groups, the deviations from ideality on mixing hydrogenated and fluorinated chains, and the contribution due to the formation of the CF₂-CH₂ bond. A standard empirical group contribution method (Sastri-Rao method) has also been used to estimate the viscosities of the perfluoroalkylalkanes. Finally, to obtain molecular level insight into the behavior of these molecules, all-atom molecular dynamics simulations have been performed and used to calculate the densities and viscosities of the perfluoroalkylalkanes studied. Although both quantities are underestimated compared to the experimental data, with the viscosities showing the largest deviations, the trends observed in the experimental viscosities are captured.