Experimental study of the one-, two-, and three-phase isochoric heat capacities of n-hexane + water mixtures near the lower critical line. Part I. Experimental results

Abstract The one-, two-, and three-phase isochoric heat capacities ( C VX ) of n -hexane + water mixtures near the lower critical line, including UCEP, have been measured with a high‐temperature and high‐pressure nearly constant‐volume adiabatic calorimeter. The measurements were made in the temperature range from 443 K to 501 K for 8 liquid and vapor isochores from 121.07 kg m − 3 to 377.5 kg m − 3 . The isochoric heat capacity jump (quasi-static thermograms supplemented by the sensor of adiabatic control) technique has been used to accurately measure the liquid–liquid–vapor ( L–L–V ) and liquid–vapor ( L–V ) phase transition parameters ( T S , ρ S ). The expanded uncertainty of the heat-capacity measurements at 95% confidence level with a coverage factor of k  = 2 is estimated to be 0.06% for density ( ρ ), 15 mK for temperature ( T ), and 2% to 3% for isochoric heat capacity ( C V ). The liquid–gas critical temperatures, T C ( x ), and the critical densities, ρ C ( x ) (lower critical lines) for binary n -hexane + water mixtures were determined from the measured saturated properties ( C VS , T S , ρ S ) near the critical point. The temperature, density, and the pressure at the UCEP ( T UCEP  = 496.13 ± 0.2 K, P UCEP  = 5.220 ± 0.10 MPa, ρ UCEP  = 263.92 ± 2 kg m − 3 ) were extracted from the measured critical line and three-phase experimental data. For the first time we reported the temperature dependent behavior of C VX near UCEP.