PVTx measurements for dilute water+n-hexane mixtures in the near-critical and supercritical regions

The PVTx relationship of four binary dilute (water+n-hexane) mixtures (0.0021, 0.0050, 0.0850 and 0.0138 mole fraction of n-hexane) were measured with a constant-volume method. Measurements were made at five near-critical and supercritical temperatures of pure water: 643.05, 645.05, 647.05, 649.05 and 651.05 K. The range of pressures was from 8 to 35 MPa. The total uncertainty of density, pressure, concentration, and temperature measurements were estimated to be less than ±0.2%, ±5 kPa, 0.001 mole fraction, and 10 mK, respectively. The derived PVTx data have been differentiated to yield infinite dilution partial molar volumes V̄2∞. Asymptotic properties for V̄2∞ along the critical isotherm–isobar of the pure solvent (water) are experimentally studied. We show that PVTx measurements for the dilute water+n-hexane mixture confirm the non-classical power-law behavior of V̄2∞ along the experimental path of constant TC–PC with a critical exponent of 0.79. The values of the Krichevskii parameter of 96.4 MPa for the dilute water+n-hexane mixture was estimated from PVTx measurements and compared with values calculated from the initial slopes of the TC–x and PC–x critical lines and the slope of the vapor-pressure PS–TS curve at the pure solvent critical point. The excess molar volumes of the mixture near the critical point of pure water are calculated using measured PVTx properties for mixtures and pure components.