Excess, Partial, and Molar Volumes of n-Alkanes in Near-Critical and Supercritical Water

Densities of solutions of n-pentane, n-hexane, n-heptane, and n-octane in near-critical and supercritical water were measured at pressures between 4 and 38 MPa and temperatures from 643.15 to 648.15 K over the entire composition range. The measurements were performed at three isotherms: 643.15, 647.05, and 648.15 K. A constant-volume piezometer was used to measure the PVTx data. The overall accuracy of the pressure, density, temperature, and mole fraction data are ±0.15%. ±0.5%, ±10mK and ±0.0002, respectively. From these results, excess and partial molar volumes were determined. The uncertainties of the derived results are given. Analysis of the results for dilute water + n-alkane mixtures showed that partial molar volume of n-alkane (solute) and excess molar volume of the mixture near the critical point of pure water (solvent) exhibit remarkable anomalies. The experimental values of molar volumes are compared with predicted values based upon scaling theory. Analysis of the results confirms the prediction of scaling theory that along the critical temperature and pressure of water the limiting partial molar volume of alkane as mole fraction x → 0 is proportional to x−γ/βδ, where γ/βδ ≍ 0.79. Our results contribute to understanding of supercritical solubility in near-critical fluids.