Liquid-liquid-vapor, liquid-liquid, and liquid-vapor phase transitions in aqueous n-hexane mixtures from isochoric heat capacity measurements

The constant volume and constant composition heat capacity C-VX data for aqueous n-hexane mixtures are reported for seven compositions, (0.6146, 0,7965, 0.9349, 0.9775, 0.9858, 0.9892, and 0.9940) mol fraction of n-hexane, along seven near-critical isochores between 259.34 and 312.50 kg(.)m(-3) in the temperature range from 463 to 522 K at pressures up to 6 MPa. All of these isochores display two features in the heat capacity as a function of temperature: the first peak appears when one of the liquid phases disappears, and the second peak appears when the vapor or liquid phase disappears. These features are interpreted in terms of the liquid-liquid-vapor, liquid-liquid, and liquid-vapor phase diagram, and their consistency is shown with earlier PTx measurements on the three-phase (L-L-G) boundary. Measurements of the isochoric heat capacity (C(V)VTx) of water + n-hexane mixtures were made in a spherical high-temperature, high-pressure nearly constant volume adiabatic calorimeter. The calorimeter was also used as a constant-volume piezometer to measure the PVTx properties. The uncertainties of the measurements are as follows: isochoric heat capacity, 1.0-1.5%; density, 0.1%; temperature, 10 mK, and pressure, 0.05%. The method of quasi-static thermograms was used to precisely measure the liquid-liquid-gas (L-L-G), liquid-liquid (L-L), and liquid-gas (L-G) phase transition temperatures at each fixed density, with an uncertainty of 0.02 K. New saturated densities and pressures are presented on the (LL-G) and (L-G) phase boundaries. The values of temperature, pressure, and density of the upper critical end point (UCEP) in the water + n-hexane mixture were determined from the measurements. Results for the UCEP (T-UCEP = 495.82 K; P-UCEP = 5.25 MPa; rho (UCEP) = 259.9 kg(.)m(-3)) are in good agreement with the values reported by other authors. For the first time the temperature-dependent behavior of C-VX near the UCEP is reported.