Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

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][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. 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. (J. Chem. Thermodyn.2007, 39, 866-877).