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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. 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).

New pTxy data are reported for methane + pentane and methane + hexane at pressures up to 14 MPa over the temperature range (173 to 333) K using a custom-built vapor−liquid equilibria apparatus. For methane (1) + pentane (2), a mixture with overall mole fraction z 2 ≈ 0.02 was prepared gravimetrically, and measurements were performed along an isochoric pathway. For the methane (1) + hexane (3) mixture, liquid hexane was pumped into the evacuated cell using an HPLC pump, and then after the addition of methane, isothermal measurements were made at 11 temperatures. Two liquid phases were observed close to the bubble point in the methane + hexane mixture at (183.15 and 233.15) K at pressures of (3.31 and 12.99) MPa, respectively. Our data are compared with previous literature data and with the predictions of the Groupe European de Recherche Gaziere (GERG-2004 XT08) multiparameter equation of state (EOS) and the Peng−Robinson and Advanced Peng−Robinson cubic equations of state implemented in commercial process simulation software. The differences from the GERG-2004 EOS in the liquid phase mole fraction x 1 were up to 0.1 for methane + pentane and up to 0.3 for methane + hexane. The systematic increase in the deviations with pressure, at constant temperature, is clear evidence of the need for tuning of the EOS parameters, especially at high pressure. The differences are smaller for the Peng−Robinson and the Advanced Peng−Robinson EOS; however, all three EOS failed to predict the second liquid phase in methane + hexane. Our data agree with the x 1 values reported by Chen et al. (J. Chem. Eng. Data 1976, 21, 213−219) for the appearance of a second liquid phase.

A commercial differential scanning calorimeter (DSC) was adapted to allow accurate isobaric heat capacity, c p , measurements of cryogenic, high-pressure liquids. At (subcritical) temperatures between (108.15 and 258.15) K and pressures between (1.1 and 6.35) MPa, the standard deviation in the measured c p values was 0.005·c p for methane, 0.01·c p for ethane, and 0.015·c p for propane, which is comparable to both the scatter of c p data for these liquids measured using other techniques and with the scatter of those data sets about the reference equation of state (EOS) values. Three key modifications to the commercial DSC were required to enable these accurate cryogenic, high-pressure liquid c p measurements. First, methods of loading and removing the liquid from the calorimeter without moving the sample cell were developed and tested with high boiling temperature liquids; this modification improved the measurement repeatability. Second, a ballast volume containing a high-pressure vapor phase at constant temperature was connected to the DSC cell so that the liquid sample’s thermal expansion did not cause significant changes in pressure. A third modification was required because the boil-off vapor from the liquid nitrogen used to cool the calorimeter resulted in a temperature inversion, and hence convection, along the tubing connecting the DSC’s sample cell to the ballast volume, that lead to an unstable calorimetric signal at T > 130 K. The modifications to the specialized DSC were tested by measuring c p values for heptane, methylbenzene, and a heptane (1) + methylbenzene (2) mixture with x 1 = 0.38 at atmospheric pressure and temperatures of (228.15, 238.15, 303.15, and 313.15) K. The measured values had relative deviations from those measured adiabatically by Holzhauer and Ziegler (J. Phys. Chem. 1975, 79, 590–604) of less than 0.006·c p , indicating that the specialized DSC could be used for liquids and liquid mixtures at conditions relevant to liquefied natural gas production.

Isobaric heat capacity cp measurements are reported at temperatures between (108 and 168) K for liquid mixtures of CH4 (1) + C3H8 (3) at x1  0.8 and p  5 MPa; CH4 (1) + C4H10 (4) at x1 = 0.96, 0.88, and 0.60 and p  5 MPa; and a five component mixture that is similar in composition to a commercial mixed refrigerant at p = (1.0 and 5.0) MPa. Measurements were made, with an estimated uncertainty of about 0.02•cp, using a specialized calorimeter further modified to prevent preferential condensation of the less volatile component and minimize possible stratification of the mixture during sample loading. Our results for CH4 + C3H8 agreed within 0.02•cp of the values predicted using the GERG-2008 equation of state (EOS) whereas literature cp data over the same temperature range agree within 0.03•cp. In contrast the cp predicted for this mixture with the Peng Robinson EOS (PR-HYSYS) shows an offset of about 0.02•cp from the average of the literature data at 165 K, which increases to 0.045•cp at 120 K. For CH4 + C4H10 our data for x1 = 0.96 and 0.88 are within 0.05•cp of the GERG 2008 predictions; however, at x1 = 0.6, the deviations increase from 0.02•cp at 168 K to nearly 1.1•cp at 118 K. The PR-HYSYS predictions for this binary mixture deviate from the measured cp data by 0.11•cp at 168 K, which increases to 0.21•cp at 118 K. For the mixed refrigerant (mole fraction composition 0.247 CH4 + 0.333 C2H6 + 0.258 C3H8 + 0.076 C4H10 + 0.059 N2) the deviations of the measured cp from GERG 2008 are less than 0.02•cp in the range (168 to 148) K increasing to 0.19•cp at 108 K. The deviations from the predictions of PR-HYSYS range from 0.07•cp at 168 K to 0.15•cp at 108 K. Similar trends are observed for the data of Furtado, who measured the cp of a mixture with a mole fraction composition of 0.359 CH4+ 0.314 C2H6 + 0.327 C3H8 at similar temperatures.