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 b etween 4 and 38 MPa and temperatures from 643.15 to 648.15 K over the entire composition range. The measurements were performed at three iso therms: 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%, +/ -10 mK and +/-0.0002. respectively From these results, excess and part ial molar volumes were determined. The uncertainties of the derived re sults are given. Analysis of the results for dilute water + n-alkane m ixtures showed that partial molar volume of n-alkane (solute) and exce ss molar volume of the mixture near the critical point of pure water ( solvent) exhibit remarkable anomalies. The experimental values of mola r volumes are compared with predicted values based upon scaling theory . Analysis of the results confirms the prediction of scaling theory th at along the critical temperature and pressure of water the limiting p artial molar volume of alkane as mole fraction x --> 0 is proportional to x(-gamma/beta delta), where gamma/beta delta approximate to 0.79. Our results contribute to understanding of supercritical solubility in near-critical fluids.