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 % 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-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Omim][Tf ₂N]) 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. B2006, 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.