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 aq ueous n-hexane mixtures are reported for seven compositions, (0.6146, 0,796 5, 0.9349, 0.9775, 0.9858, 0.9892, and 0.9940) mol fraction of n-hexane, al ong seven near-critical isochores between 259.34 and 312.50 kg(.)m(-3) in t he temperature range from 463 to 522 K at pressures up to 6 MPa. All of the se isochores display two features in the heat capacity as a function of tem perature: the first peak appears when one of the liquid phases disappears, and the second peak appears when the vapor or liquid phase disappears. Thes e features are interpreted in terms of the liquid-liquid-vapor, liquid-liqu id, and liquid-vapor phase diagram, and their consistency is shown with ear lier 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 mad e in a spherical high-temperature, high-pressure nearly constant volume adi abatic calorimeter. The calorimeter was also used as a constant-volume piez ometer to measure the PVTx properties. The uncertainties of the measurement s are as follows: isochoric heat capacity, 1.0-1.5%; density, 0.1%; tempera ture, 10 mK, and pressure, 0.05%. The method of quasi-static thermograms wa s 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 densi ty, with an uncertainty of 0.02 K. New saturated densities and pressures ar e presented on the (LL-G) and (L-G) phase boundaries. The values of tempera ture, pressure, and density of the upper critical end point (UCEP) in the w ater + 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 th e first time the temperature-dependent behavior of C-VX near the UCEP is re ported.