New measurements are reported for the isochoric heat capacity of the ionic liquid substance 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6mim][NTf2]). These measurements extend the ranges of our earlier study (Polikhronidi et al. in Phys Chem Liq 52:657, 2014) by 5 % of the compressed liquid density and by 75 K. An adiabatic calorimeter was used to measure one-phase $$(C_{\mathrm{V1}})$$(CV1) liquid and two-phase $$(C_{\mathrm{V2}})$$(CV2) liquid + vapor isochoric heat capacities, densities $$(\rho _s)$$(ρs), and phase-transition temperatures $$(T_s)$$(Ts) of the ionic liquid (IL) substance. The combined expanded uncertainty of the density $$\rho $$ρ and isochoric heat capacity $$C_\mathrm{V}$$CV measurements at the 95 % confidence level with a coverage factor of $$k = 2$$k=2 is estimated to be 0.15 % and 3 %, respectively. Measurements are concentrated in the immediate vicinity of the liquid + vapor phase-transition curve, in order to closely observe phase transitions. The present measurements and those of our earlier study are analyzed together and are presented in terms of thermodynamic properties $$(T_s$$(Ts, $$\rho _s$$ρs, $$C_{\mathrm{V1}}$$CV1 and $$C_{\mathrm{V2}})$$CV2) evaluated at saturation and in terms of key-derived thermodynamic properties $$C_\mathrm{P}$$CP, $$C_\mathrm{S}$$CS, $$W_\mathrm{S}^{{\prime }}$$WS′, $$K_{\mathrm{TS}}^{{\prime }}$$KTS′, $$\left( {\partial P/\partial T} \right) _{\mathrm{V}}^{\prime }$$∂P/∂TV′, and $$\left( {\partial V/\partial T} \right) _\mathbf{P}^{\prime })$$∂V/∂TP′) on the liquid + vapor phase-transition curve. A thermodynamic relation by Yang and Yang is used to confirm the internal consistency of measured two-phase heat capacities $$C_{\mathrm{V2}} $$CV2, which are observed to fall perfectly on a line as a function of specific volume at a constant temperature. The observed linear behavior is exploited to evaluate contributions to the quantity $$C_{\mathrm{V2}} = f(V, T)$$CV2=f(V,T) from chemical potential $$C_{{\mathrm{V}\upmu }} =-T\frac{\mathrm{d}^{{2}}\mu }{\mathrm{d}T^{2}}$$CVμ=-Td2μdT2 and from vapor pressure $$C_{\mathrm{VP}} =VT\frac{\mathrm{d}^{2}P_{\mathrm{S}} }{\mathrm{d}T^{2}}$$CVP=VTd2PSdT2. The physical nature and specific details of the temperature and specific volume dependence of the two-phase isochoric heat capacity and some features of the other derived thermodynamic properties of IL at liquid saturation curve are considered in detail.

中文翻译：

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从 1-己基-3-甲基咪唑鎓双[（三氟甲基）磺酰基]亚胺的实验两相等容热容量得出的饱和热力学性质

报告了离子液体物质 1-己基-3-甲基咪唑鎓双[（三氟甲基）磺酰基]亚胺（[C6mim][NTf2]）的等容热容的新测量值。这些测量将我们早期研究（Polikhronidi 等人，在 Phys Chem Liq 52:657, 2014 中）的范围扩大了压缩液体密度的 5% 和 75 K。绝热量热计用于测量单相 $$( C_{\mathrm{V1}})$$(CV1) 液体和两相 $$(C_{\mathrm{V2}})$$(CV2) 液体 + 蒸气等容热容，密度 $$(\rho _s )$$(ρs) 和离子液体 (IL) 物质的相变温度 $$(T_s)$$(Ts)。密度 $$\rho $$ρ 和等容热容 $$C_\mathrm{V}$CV 测量值的组合扩展不确定性在 95% 置信水平下，覆盖因子为 $$k = 2$$k= 2 估计分别为 0.15% 和 3%。测量集中在液体 + 蒸气相变曲线的附近，以便密切观察相变。目前的测量结果和我们早期研究的测量结果一起分析，并以热力学性质 $$(T_s$$(Ts, $$\rho _s$$ρs, $$C_{\mathrm{V1}}$$ CV1 和 $$C_{\mathrm{V2}})$$CV2) 在饱和状态和关键推导的热力学属性方面进行评估 $$C_\mathrm{P}$$CP, $$C_\mathrm{S}$ $CS, $$W_\mathrm{S}^{{\prime }}$$WS′, $$K_{\mathrm{TS}}^{{\prime }}$$KTS′, $$\left( {\partial P/\partial T} \right) _{\mathrm{V}}^{\prime }$$∂P/∂TV′，和 $$\left( {\partial V/\partial T} \右) _\mathbf{P}^{\prime })$$∂V/∂TP') 在液相 + 气相转变曲线上。Yang 和 Yang 的热力学关系用于确认测量的两相热容 $$C_{\mathrm{V2}} $$CV2 的内部一致性，观察到它们完美地落在一条线上，作为比体积的函数在恒定温度下。观察到的线性行为被用来评估化学势 $$C_{{\mathrm{V2}} = f(V, T)$$CV2=f(V,T) 对量的贡献V}\upmu }} =-T\frac{\mathrm{d}^{{2}}\mu }{\mathrm{d}T^{2}}$$CVμ=-Td2μdT2 和蒸气压 $$ C_{\mathrm{VP}} =VT\frac{\mathrm{d}^{2}P_{\mathrm{S}} }{\mathrm{d}T^{2}}$$CVP=VTd2PSdT2。