PVTx measurements for dilute water plus 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 wi th a constant-volume method. Measurements were made at five near-critical a nd 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 uncer tainty of density, pressure, concentration, and temperature measurements we re estimated to be less than +/-0.2%, +/- 5 kPa, 0.001 mole fraction, and 1 0 mK, respectively. The derived PVTx data have been differentiated to yield infinite dilution partial molar volumes (V) over bar (infinity)(2). Asympt otic properties for V,l along the critical isotherm-isobar of the pure solv ent (water) are experimentally studied. We show that PVTx measurements for the dilute water + n-hexane mixture confirm the non-classical power-law beh avior of (V) over bar (infinity)(2) along the experimental path of constant T-C-P-C with a critical exponent of 0.79. The values of the Krichevskii pa rameter of 96.4 MPa for the dilute water + n-hexane mixture was estimated f rom PVTx measurements and compared with values calculated from the initial slopes of the T-C-x and P-C-x critical lines and the slope of the vapor-pre ssure P-S-T-S curve at the pure solvent critical point. The excess molar vo lumes of the mixture near the critical point of pure water are calculated u sing measured PVTs properties for mixtures and pure components. (C) 2001 El sevier Science B.V. All rights reserved.