VOLUMETRIC PROPERTIES OF NEAR-CRITICAL AND SUPERCRITICAL WATER PLUS PENTANE MIXTURES - MOLAR, EXCESS, PARTIAL, AND APPARENT VOLUMES

We present new PVTx measurements for water (A) + pentane (B) mixtures at the critical temperature of water (T-C = 647.05 K) in the pressure range 4 MPa to 41 MPa at x(b) = 0, 0.028, 0.042, 0.061, 0.088, 0.184, 0.391, 0.694, and 1.0. The measurements were performed with a constant -volume piezometer. The sample was confined to a 36.8 cm(3) cylindrica l cell of a corrosion-resistant steel alloy, provided with a steel bal l for stirring. The cell was separated from the rest of the fill and p ressure measurement system by a diaphragm-type null indicator. The tem perature was measured by a 10 Ohm platinum resistance thermometer. The uncertainty in the temperature measurement is less than +/-5 mK. Pres sure was measured by means of a dead-weight gauge with a precision of +/-2 kPa. The composition was determined with an uncertainty of +/-0.0 02 in mole fraction. The volume of the cell was corrected for temperat ure expansion and elastic deformation by means of the known expansion coefficients of the cell. Taking into account the errors of temperatur e, pressure, and concentration measurements, the total experimental un certainty of density, delta rho, was estimated to be less than +/-0.5% . From the PVTx results, the excess, partial, and apparent molar volum es were determined. Analysis of the results for dilute water + pentane mixtures showed that partial molar volume of pentane (solute) and exc ess molar volume of the mixture near the critical point of pure water (solvent) exhibit the behavior predicted by theory. A nonclassical (sc aled) asymptotic relation has been used for the analysis of the partia l and molar volumes behavior along the critical isotherm-isobar.