Group contribution and parachor analysis of experimental data on densities and surface tension for six ionic liquids with the [PF6] anion

Experimental data over the temperature range from 268 to 365 K are presented on the 0.1 MPa density and surface tension for three 1-Cn-3-methylimidazolium hexafluorophosphates with n = 5, 7, and 9. In addition, experimental data on surface tension dependence on temperature are presented for hexafluorophosphates with [C3MIM], [C4MMIM] and [C4MPy] cation. For all of these ionic liquids there exists only sporadic data sets or they have not yet been studied at all in these two respects. The reported density data were obtained using a single-sinker buoyancy method. The expanded combined uncertainty at 0.95 confidence level in the density and surface tension measurements is estimated to be, respectively, from 0.3 to 0.7 kg m−3 and from 0.05 to 0.07 mN m−1. To determine the buoyancy and surface forces, the Krűss K100MK2 tensiometer was used. A group contribution model is proposed to generate recommended values of density at 0.1 MPa for 1-Cn-3-methylimidazolium hexafluorophosphates with n = 2–9 at temperatures from 273 to 365 K. The worth-case relative error of the recommended density values is estimated to 0.1%. The homologous series of the imidazolium-based hexafluorophosphates shows statistically discernible odd-even effects in the molar volume, and in enthalpy and entropy of surface formation at the level of the order of magnitude of 10−1 cm3mol−1, 100 mJ m−2 and 10−3 mJ m−2K−1, respectively.