Isochoric heat capacity measurements for pure ethanol in the near-critical and supercritical regions

Abstract

Isochoric heat capacities of pure ethanol have been measured with a high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter as a function of temperature and density. Measurements were performed along 22 liquid and vapor near-critical isochores between 193.32 and 365.5 kg m −3. The range of temperature was from 493.10 to 541.34 K. The coverage includes the one- and two-phase regions, the coexistence curve, the near-critical, and the supercritical regions. The total uncertainty of isochoric heat capacity, density, and temperature measurements was estimated to be less than 2–3%, 0.05%, and 15 mK, respectively. Temperatures at saturation, T S( ρ), for each measured densities (isochores) were determined using a quasi-static thermogram technique supplemented by the sensor of adiabatic control. The critical temperature and the critical density ( T C = 514.44 ± 0.2 K and ρ C = 283.7 ± 2 kg m −3) for pure ethanol were extracted from the saturated properties ( C VS, T S, ρ S) near the critical point. The measured C V and saturated density data have been analyzed and interpreted in terms of extended scaling equations for the selected thermodynamic paths (critical isochore and coexistence curve) to accurately calculate the values of the asymptotical critical amplitude ( A 0 ± and B 0). The experimentally derived value of the critical amplitude ratio A 0 + / A 0 − = 0.529 is in good agreement with the values predicted by scaling theory. The divergence of C V2 for pure ethanol is shared between both (d 2 P S/d T 2) and (d 2 μ/d T 2), but the magnitude of the Yang–Yang anomaly strength for ethanol is small ( R μ = 0.12), which means that the contribution of d 2 P S/d T 2 is dominate. It was showed that, the divergence of the coexistence curve diameter, d ρ d/d τ, for ethanol is also shared between the terms B 2 t 1− α and B 4 t 2 β , however, the contribution of the term B 2 t 1− α is dominate.