Isobaric heat capacity of nanostructured liquids with potential use as lubricants

Isobaric molar heat capacity (Cp) of eight ionic liquids (ILs) was measured from 283.15 K to 338.15 K with a micro DSC III calorimeter with a heating rate of 0.25 K·min−1 using batch cells. Cp values were determined with an expanded uncertainty about 2%. Four of the selected ILs are imidazolium based with the anions hexylsulfate, trifluoromethanesulfonate, tris(pentafluoroethyl)trifluorophosphate and bis(trifluoromethylsulfonyl)imide whereas the other three are pyrrolidinium-based ILs with trifluoromethanesulfonate, tris(pentafluoroethyl)trifluorophosphate or bis(trifluoromethylsulfonyl)imide anions. The last one is tetrabutylphosphonium tris(pentafluoroethyl)trifluorophosphate. Phase transitions in the temperature range are only observed for 1-butyl-2,3-dimethylimidazolium trifluoromethanesulfonate and tetrabutylphosphonium tris(pentafluoroethyl)trifluorophosphate; isobaric molar heat capacities are determined in solid phase for both ILs. The results showed that Cp values increase with temperature and they present the usual strong dependence with the molar mass of the IL: Cp increases with the molecular weight of the IL. Moreover, it was found that isobaric specific heat capacities, cp, for phosphate-based ILs are the lowest, followed by imide-based ones, and ILs with anions containing sulphur atoms show the highest cp values. Finally, a group contribution method was used for predicting molar heat capacity for three of the studied ILs obtaining good agreement with experimental data.