Thermodynamic Properties at Saturation Derived from Experimental Two-Phase Isochoric Heat Capacity of 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide

Abstract

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 $(C_{\mathrm{V1}})$ liquid and two-phase $(C_{\mathrm{V2}})$ liquid + vapor isochoric heat capacities, densities $(\rho _s)$, and phase-transition temperatures $(T_s)$ of the ionic liquid (IL) substance. The combined expanded uncertainty of the density $\rho$ and isochoric heat capacity $C_\mathrm{V}$ 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 $(T_s$, $\rho _s$, $C_{\mathrm{V1}}$ and $C_{\mathrm{V2}})$ evaluated at saturation and in terms of key-derived thermodynamic properties $C_\mathrm{P}$, $C_\mathrm{S}$, $W_\mathrm{S}^{{\prime }}$, $K_{\mathrm{TS}}^{{\prime }}$, $\left( {\partial P/\partial T} \right) _{\mathrm{V}}^{\prime }$, and $\left( {\partial V/\partial T} \right) _\mathbf{P}^{\prime })$ 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 $C_{\mathrm{V2}}$, 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 $C_{\mathrm{V2}} = f(V, T)$ from chemical potential $C_{{\mathrm{V}\upmu }} =-T\frac{\mathrm{d}^{{2}}\mu }{\mathrm{d}T^{2}}$ and from vapor pressure $C_{\mathrm{VP}} =VT\frac{\mathrm{d}^{2}P_{\mathrm{S}} }{\mathrm{d}T^{2}}$. 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.