Thermodynamics Research Center / ThermoML | International Journal of Thermophysics

ABSTRACT

New measurements are reported for the isochoric heat capacity of the ionic liq. substance 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6mim][NTf2]). These measurements extend the ranges of our earlier study by 5 % of the compressed liq. d. and by 75 K. An adiabatic calorimeter was used to measure one-phase (CV1) liq. and two-phase (CV2) liq. + vapor isochoric heat capacities, densities (s), and phase-transition temps. (Ts) of the ionic liq. (IL) substance. The combined expanded uncertainty of the d. ? and isochoric heat capacity CV measurements at the 95 % confidence level with a coverage factor of k = 2 is estd. to be 0.15 % and 3 %, resp. Measurements are concd. in the immediate vicinity of the liq. + 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 thermodn. properties (Ts, s, CV1 and CV2) evaluated at satn. and in terms of key-derived thermodn. properties CP, CS, W'S, K'TS, (dP/dT)V, and (V/T)'P) on the liq. + vapor phase-transition curve. A thermodn. relation by Yang and Yang is used to confirm the internal consistency of measured two-phase heat capacities CV2, which are obsd. to fall perfectly on a line as a function of sp. vol. at a const. temp. The obsd. linear behavior is exploited to evaluate contributions to the quantity CV2 = f(V,T) from chem. potential CVmicro = - T((d2micro)/(dT2)) and from vapor pressure CVP = VT((d2PS)/(dT2)). The phys. nature and specific details of the temp. and sp. vol. dependence of the two-phase isochoric heat capacity and some features of the other derived thermodn. properties of IL at liq. satn. curve are considered in detail.