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

• Properties of ionic liquids with hexafluorophosphate anion were investigated.
• We report density and surface tension data for six still purely studied ionic liquids.
• We have evaluated accuracy of the data by using in a special way the group contribution and parachor concept.
• Odd-even effect in molar volume of members of 1-alkyl-3-methylimidazolium hexafluorophosphate series have been revealed.
• The parachor contribution per alkyl inner CH2 group is 32.67 (mN m−1)1/4cm3mol−1.

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.