Experimental study of the isochoric heat capacity of isobutanol in the critical and supercritical regions

The one- and two-phase isochoric heat capacities (CV) of isobutanol in the critical and supercritical regions have been measured with a high-temperature and high-pressure nearly constant-volume adiabatic calorimeter. The measurements were made in the temperature range from 324 K to 575 K for 23 isochores (16 liquid and 7 vapor) from 73.2 kg m−3 to 772.2 kg m−3. The isochoric heat capacity jump (quasi-static thermograms supplemented by the sensor of adiabatic control) technique have been used to accurately measure of the phase transition parameters (TS, ρS). The total experimental uncertainty of density (ρ), temperature (T), and isochoric heat capacity (CV) were estimated to be 0.06%, 15 mK, and 2–3%, respectively. The critical temperature (TC = 547.65 ± 0.2 K) and the critical density (ρC = 272.95 ± 2 kg m−3) for isobutanol were determined from the measured saturated properties (CVS, TS, ρS) near the critical point. The measured CV and saturated density (TS, ρS) data near the critical point have been analyzed and interpreted in terms of extended scaling type equations for the selected thermodynamic paths (critical isochore and coexistence curve) to accurately calculate the values of the asymptotical critical amplitudes of heat capacity (A0±) and coexistence curve (B0). The experimentally derived value of the critical amplitude ratio A0+/A0−=0.522 is in good agreement with the value predicted by various scaling theories. The measured thermodynamic properties (CV, TS, ρS) of isobutanol near the critical point were also interpreted in the terms of “complete scaling” theory of critical phenomena. In particularly, the contributions of the “complete” and “incomplete” scaling terms on the coexistence-curve singular diameter were estimated. We determined the values of the asymmetry parameters a3 and b2 of the coexistence curve singular diameter. The strength of the Yang–Yang anomaly Rμ for isobutanol was estimated using asymmetry parameters a3 and the contribution of the second temperature derivative of vapor-pressure and chemical potential in the singularity of two-phase CV2. The measured values of saturated one- and two-phase liquid and vapor isochoric heat capacities (C′V1,C″V1,C′V2,C″V2C′V1,C″V1,C′V2,C″V2) and saturated thermal (ρS, TS) properties together with vapor-pressure (PS, TS) data were used to calculate other derived thermodynamic properties such as (KT, ΔHvap, CP, CS, W  , (∂P/∂T)′V(∂P/∂T)′V, (∂V/∂T)′P(∂V/∂T)′P, (d2PS/dT2)(d2PS/dT2), and (d2μ/dT2)(d2μ/dT2) of isobutanol at saturation near the critical point.

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► Isochoric heat capacity of isobutanol in the supercritical region.
► Coexistence curve singular diameter.
► “Complete” and “incomplete” scaling.
► Yang–Yang anomaly strength.