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 [N.G. Polikhronidi et al., Phys. Chem. Liq. 52, 657 (2014)] by 5 % of the compressed liquid density and by 75 kelvins. An adiabatic calorimeter was used to measure one-phase (CV1) liquid and two-phase (CV2) liquid + vapor isochoric heat capacities, densities (ρS ), and phase-transition temperatures (TS ) of the ionic liquid (IL) substance. The combined expanded uncertainty of the density ρ and isochoric heat capacity CV measurements at the 95 % confidence level with a coverage factor of 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 (TS, ρS, CV1 and CV2) evaluated at saturation and in terms of key derived thermodynamic properties Cp, CS, [Formula: see text], and [Formula: see text] 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 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 CV2 = f(V,T) from chemical potential [Formula: see text] and from vapor pressure [Formula: see text]. 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.
Keywords: Coexistence curve; Ionic liquid; Isochoric heat capacity; Phase transition; Saturated density.