Measurements and group contribution analysis of 0.1 MPa densities for still poorly studied ionic liquids with the [PF6] and [NTf2] anions

• We report 0.1 MPa density data for four still poorly studied ionic liquids.
• We analyze the density data within the group contribution concept.
• Molar volume contributions of three particular atomic groups were determined.
• The molar volume contribution of the alkyl inner CH2 group is (16.99 ± 0.10) cm3 · mol−1.
• Standard reference density values for [CnMIM][NTf2] ionic liquids were developed.

Experimental data over the temperature range from (262 to 365) K are presented on the 0.1 MPa densities for seven imidazolium and pyridinium based ionic liquids with hexafluorophosphate and bis(trifluoromethylsulfonyl)imide anion. Four of them almost have not been studied in this respect yet. The reported density data were obtained using a single-sinker buoyancy method. The estimated expanded combined uncertainty at 0.95 confidence level ranges from 0.1 kg · m−3 for [BMPy][PF6] to 1.0 kg · m−3 for [EMIM][NTf2], that is, from (0.8·10-40.8·10-4 to 6.7·10-4)ϱ6.7·10-4)ϱ (T = 298.15 K). The Krűss K100MK2 tensiometer was used to determine the buoyancy forces. For the contribution to the ionic liquid molar volume at T = 298.15 K and 0.1 MPa follows from the obtained density data for a CH3 group bound to a nitrogen atom in the imidazolium ring a value of (17.65±0.1617.65±0.16) cm3 · mol−1, for a CH3 group bound to a carbon atom in the imidazolium ring a value of (17.71±0.317.71±0.3) cm3 · mol−1, and for an inner CH2 group in the alkyl side chain the value of (16.99±0.0216.99±0.02) cm3 · mol−1, where 0.95 confidence level uncertainties are given arising from the data scatter. Nonlinearity of the dependence of the alkyl side chain contribution to the molar volume on the number of its carbon atoms limits the accuracy attainable using the group contribution method to about 0.1 cm3 · mol−1, that is, 5·10-4ϱ5·10-4ϱ. A model is proposed to generate recommended values of density at 0.1 MPa for 1-Cn-3-methylimidazolium bis(trifluoromethylsulfonyl)imides with n=2n=2 to 14 at temperatures from (260 to 473) K.