Isobaric heat capacity of nanostructured liquids with potential use as lubricants

Abstract Isobaric molar heat capacity ( C p ) 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. C p 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 C p values increase with temperature and they present the usual strong dependence with the molar mass of the IL: C p increases with the molecular weight of the IL. Moreover, it was found that isobaric specific heat capacities, c p , for phosphate-based ILs are the lowest, followed by imide-based ones, and ILs with anions containing sulphur atoms show the highest c p 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.