Experimental Study of the Isochoric Heat Capacity of Diethyl Ether (DEE) in the Critical and Supercritical Regions

Abstract

Two-and one-phase liquid and vapor isochoric heat capacities (C V ρT relationship) of diethyl ether (DEE) in the critical and supercritical regions have been measured with a high-temperature and high-pressure nearly constant-volume adiabatic calorimeter. The measurements were carried out in the temperature range from 347 K to 575 K for 12 liquid and 5 vapor densities from 212.6 kg· m −3 to 534.6 kg· m −3 . The expanded uncertainties (coverage factor k = 2, two-standard deviation estimate) for values of the heat capacity were 2 % to 3 % in the near-critical region, 1.0 % to 1.5 % for the liquid isochores, and 3 % to 4 % for the vapor isochores. The uncertainties of density (ρ) and temperature (T ) measurements were 0.02 % and 15 mK, respectively. The values of the internal energy, U (T, V ), and second temperature derivative of pressure, ∂ 2 P/∂ T 2 ρ , were derived using the measured C V data near the critical 123 186 Int J Thermophys (2012) 33:185-219 point. The critical anomaly of the measured C V and derived values of U (T, V ) and ∂ 2 P/∂ T 2 ρ in the critical and supercritical regions were interpreted in terms of the scaling theory of critical phenomena. The asymptotic critical amplitudes ( A + 0 and A − 0 ) of the scaling power laws along the critical isochore for one-and two-phase C V were calculated from the measured values of C V . Experimentally derived values of the critical amplitude ratio for C V A + 0 /A − 0 = 0.521 are in good agreement with the values predicted by scaling theory. The measured C V data for DEE were analyzed to study the behavior of loci of isothermal and isochoric C V maxima and minima in the critical and supercritical regions.

Intermolecular interactions and the thermodynamic properties of supercritical fluids

“…To the best of our knowledge, the existence of C p minima have not been previously observed either in the Lennard-Jones fluid or real fluids. This is in contrast with experimental evidence 48,49 for the existence of such behavior in C V. The magnitude of the minima is much less pronounced than the maxima and it diminishes with increasing temperatures. This means that it could be easily overlooked, particularly if the density increments are not sufficiently small.…”

“…Experimental measurements for the isochoric heat capacity at supercritical temperatures have been reported, 48 which exhibit both maxima and minima curves that converge at a common point. Experimental heat capacity data 49 for diethyl ether provide a recent example of both maxima and minima behavior. The phenomena observed for the Lennard-Jones fluid are qualitatively similar except that the minima curve for diethyl ether first veers to lower densities before connecting to the maxima curve at a density slightly above the critical density.…”

Thermodynamic properties of supercritical n-m Lennard-Jones fluids and isochoric and isobaric heat capacity maxima and minima

“…Recently, 21 a MD investigation of the thermodynamic properties of supercritical 12-6 Lennard-Jones fluids reported maxima and minima values for both C V and C p . The existence of both minima and maxima for C V is well-documented experimentally, [22][23][24] although they are not a universal feature of fluids. Similarly, many real fluids 22 have maxima in C p .…”

Experimental study of the isochoric heat capacity and coexistence-curve singular diameter of sec-butanol near the critical point and Yang-Yang anomaly strength

“…along the critical isochore, C V exhibits an infinite jump ( Figure 2) upon crossing the critical temperature. A detailed view of the derived from isochoric heat capacity measurements values of the internal energy along the critical isochore for DEE [6], DU C , T ð Þ, in the immediate vicinity of the critical temperature, is shown in Figure 3. As one can see from Figure 3, the values of the internal energy derived from C V measurements along the critical isochore as a function of temperature exhibit a scaling type behaviour U( C ,T) / t 1À , i.e.…”

“…Detailed view of the derived from isochoric heat capacity measurements values of internal energy along the critical isochore, DU C , T ð Þ, near the critical point for DEE[6]. Solid curve is the scaling equation.…”