Isochoric Heat-Capacity Measurements for Pure Methanol in the Near-Critical and Supercritical Regions

Abstract

Isochoric heat-capacity measurements for pure methanol are presented as a function of temperature at fixed densities between 136 and 750 kg·m −3 . The measurements cover a range of temperatures from 300 to 556 K. The coverage includes the one-and two-phase regions, the coexistence curve, the near-critical, and the supercritical regions. A high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter was used for the measurements. Uncertainties of the heat-capacity measurements are estimated to be 2-3% depending on the experimental density and temperature. Temperatures at saturation, T S (ρ), for each measured density (isochore) were measured using a quasi-static thermogram technique. The uncertainty of the phase-transition temperature measurements is 0.02 K. The critical temperature and the critical density for pure methanol were extracted from the saturated data (T S , ρ S ) near the critical point. For one near-critical isochore (398.92 kg·m −3 ), the measurements were performed in both cooling and heating regimes to estimate the effect of thermal decomposition (chemical reaction) on the heat capacity and phase-transition properties of methanol. The measured values of C V and saturated densities (T S , ρ S ) for methanol were compared with values calculated from various multiparametric equations of state (EOS) (IUPAC, Bender-type, polynomial-type, and nonanalytical-type), scaling-type (crossover) EOS, and various correlations. The measured C V data have been analyzed and interpreted in terms of extended scaling equations for 163 0195-928X/07/0200-0163/0 © 2007 Springer Science+Business Media, LLC 164 Polikhronidi et al.the selected thermodynamic paths (critical isochore and coexistence curve) to accurately calculate the values of the asymptotical critical amplitudes (A ± 0 and B 0 ).

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

“…This behavior was expected, since it is similar to observations for other heavy organic molecules at compressed-liquid state conditions. Our earlier studies [27,28] made us aware that linear behavior should be observed for internally consistent two-phase C V as a function of specific volume V . Thus, we took advantage of the linearity with respect to temperature to interpolate isochoric two-phase C V to integral values of temperature.…”

“…Since the method (experimental details, the physical basis and theory of the method, procedures, uncertainty assessment) and apparatus have been described in our earlier publications (see, for example [27,28]), only essential information will be briefly given here. Isochoric heat-capacity C V ( V,T ) measurements were performed with a high-temperature, high-pressure, and nearly constant-volume adiabatic calorimeter.…”

“…To precisely determine the phase-boundary parameters, T S and ${\rho }_S^{\text{'}}$, we used a combination of the quasi-static thermogram technique (readings of PRT, T-τ plot at isochoric heating of the sample) and barogram technique (readings of the pressure transducer, P-τ plot). Details of both techniques have been described in [27,28]. The thermo- and barograms provided records of the changes in temperature ΔT , changes in pressure ΔP , and the finite-difference derivative ${false(\partial P/\partial Tfalse)}_{\mathrm{V}}=\underset{\mathrm{normal\Delta }T\to 0}{\text{lim}}{false(\mathrm{normal\Delta }P/\mathrm{normal\Delta }Tfalse)}_{\mathrm{V}}$, as a function of elapsed time τ .…”

“…Furthermore, we propose a method of separation and extraction of the distinct C V2 contributions, chemical potential ( C Vμ ) and vapor-pressure ( C VP ) terms from the measured total two-phase ( C V2 ) isochoric heat capacity of IL by using the Yang-Yang relation. The measurements of the one- ( C V1 ) and two-phase ( C V2 ) isochoric heat capacity of [C6mim][NTf2] were made in the immediate vicinity of liquid-vapor phase transition temperatures for two liquid filling densities of (1323.92 and 1344.14) kg·m −3 using a precise adiabatic technique previously described in our publications [27,28]. …”

“…For example, C V measurements can be used to accurately determine the locus of the phase transition temperatures T S ( ρ ). A sharp discontinuity of the measured C V occurs at the liquid-vapor phase transition temperature T S ( ρ ) for each filling density ( ρ ) (see, for example, [27,28]). In the present work we routinely use the existence of a sharp discontinuity as the measurement criterion to obtain the liquid-vapor saturation curve T S ( ρ ) of a substance during continues heating the system at constant density.…”