Thermodynamic properties of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid in the condensed state

Heat capacity for 1-butyl-3-methylimidazolium trifluoromethanesulfonate [C4mim]OTf in the temperature range of (5–370) K has been measured in an adiabatic calorimeter. The temperature Tfus = (290.98 ± 0.01) K and the enthalpy of fusion ΔfusHm°=(19.43±0.02) kJ mol−1 were determined. A solid-phase transition was observed near T = 120 K. Thermodynamic functions for the compound in the condensed state were calculated. Trends in the solid–liquid phase change entropies, ∑ΔtrSm, for 1-alkyl-3-methylimidazolium salts [Cnmim]An were analyzed. The increment to ∑ΔtrS was shown to increase with n. It was found that ∑ΔtrSm = 66.4 ± 1.3 J K−1 mol−1 for all the studied [C4mim]An ionic liquids except [C4mim]NTf2. This difference can be partly explained by formation of two distinguishable conformers of the NTf2 anion in the liquid state.