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 (CVX) 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 443K to 501K for 8 liquid and vapor isochores from 121.07kgm−3 to 377.5kgm−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 (TS, ρS). 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), 15mK for temperature (T), and 2% to 3% for isochoric heat capacity (CV). The liquid–gas critical temperatures,TC(x), and the critical densities,ρC(x) (lower critical lines) for binary n-hexane+water mixtures were determined from the measured saturated properties (CVS,TS, ρS) near the critical point. The temperature, density, and the pressure at the UCEP (TUCEP=496.13±0.2K, PUCEP=5.220±0.10MPa, ρUCEP=263.92±2kgm−3) were extracted from the measured critical line and three-phase experimental data. For the first time we reported the temperature dependent behavior of CVX near UCEP.