A group contribution method for heat capacity estimation of ionic liquids

Based on experimental liquid heat capacity (C(pL)) data collected from the literature, a second-order group additivity method has been developed for the estimation of the liquid heat capacity of imidazolium-, pyridinium-, and pyrrolidinium-based ionic liquids (ILs) containing hexafluorophosphate (PF(6)), tetrafluoroborate (BF(4)), bis(trifluoromethanesulforyl) amide (Tf(2)N), bromide (Br), ethyl sulfate (EtSO(4)), or trifluoromethane sulfonate (CF(3)SO(3)) as anions, covering wide ranges of temperature (196.36-663.10 K) and liquid heat capacity (264.8-825.0 J mol(-1) K(-1)). It is shown that a good description of the literature data is obtained with the proposed method. For ca. 2400 data points from 19 ILs that have been studied, a mean percent deviation (MPD) of 0.36% with a maximum deviation of <2.5% was observed. From the total data points of estimated heat capacities, 90.2% present deviations of <1% from the experimental values, whereas only 0.9% have deviations of >2%. A correlation of heat capacities with molar volumes of the ILs was also developed for the estimation of heat capacity at 298.15 K. Therefore, the group contribution method developed here can be used to evaluate the liquid heat capacity of new ILs and, as data for new groups of cations and anions become available, can be extended to a larger range of ionic liquids.