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) cylindrical cell of a corrosion-resistant steel alloy, provided with a steel ball for stirring. The cell was separated from the rest of the fill and pressure measurement system by a diaphragm-type null indicator. The temperature was measured by a 10 Ohm platinum resistance thermometer. The uncertainty in the temperature measurement is less than +/-5 mK. Pressure was measured by means of a dead-weight gauge with a precision of +/-2 kPa. The composition was determined with an uncertainty of +/-0.002 in mole fraction. The volume of the cell was corrected for temperature expansion and elastic deformation by means of the known expansion coefficients of the cell. Taking into account the errors of temperature, pressure, and concentration measurements, the total experimental uncertainty of density, delta rho, was estimated to be less than +/-0.5%. From the PVTx results, the excess, partial, and apparent molar volumes were determined. Analysis of the results for dilute water + pentane mixtures showed that partial molar volume of pentane (solute) and excess molar volume of the mixture near the critical point of pure water (solvent) exhibit the behavior predicted by theory. A nonclassical (scaled) asymptotic relation has been used for the analysis of the partial and molar volumes behavior along the critical isotherm-isobar.