Heat Capacities and Low Temperature Thermal Transitions of 1-Hexyl and 1-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

Several previous measurements of the isobaric heat capacity of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Hmim][Tf2N]) differ relative to the IUPAC recommended value by +/- 8 %. Specifically, the results obtained by differential scanning calorimetry (DSC) showed relative difference from each other and from values determined by adiabatic calorimetry by up to 12 96 and by 6 % on average. The aim of this work was to explore the reason for these discrepancies in DSC measurements. Accordingly, measurements of the isobaric heat capacity and low temperature thermal transitions of [Hmim] [Tf2N] and 1-octyol-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Omim][Tf2N]) made by DSC are reported here. The isobaric heat capacities for both ionic liquids were measured on samples of (5 to 9) g over the temperature ranges (303 to 373) K for [Hmim] [Tf2N] and (288 to 373) K for [Omim] [Tf2N] using steps of 10 K and a scan rate of 0.025 K.min(-1). These heat capacity measurements were consistent, within their estimated relative uncertainty of 3 %, with the values measured by adiabatic calorimetry and with the DSC measurements made at scan rates of less than 1 K.min(-1) on samples of 5 g or greater. In addition, several thermal transitions were observed for these ionic liquids at temperatures down to 140 K. For [Hmim][Tf2N] a melting temperature of (272 +/- 1) K and an enthalpy of fusion of (62 +/- 2) J.g(-1) were measured, which are consistent within the combined uncertainties with those of Shimizu et al. (J. Phys. Chem. B 2006, 110, 13970-13975). After tempering the [Omim] [Tf2N] sample, a melting temperature of (250 +/- 1) K and an enthalpy of fusion of (58 +/- 2) J.g(-1) was obtained, which differ by 1.6 K and 3.7 % respectively from values reported by Paulechka et al. (J. Chem. Thermodyn. 2007, 39, 866-877).