Molecular Interaction of Water Vapor and Air

The factor f w ( T, p ) by which air increases the saturation vapor pressure of water has been measured accurately for temperatures of 20, 50, and 75 °C and pressures to 14 MPa. An account is given of the measurements and of the analysis of the results to yield the interaction second virial coefficient B aw ( T ) and the first experimental values of the interaction third virial coefficient C aaw ( T ) of the vapor−gas mixture. When a Lennard-Jones potential is fitted, the three values of B aw are found to be highly mutually consistent, allowing that coefficient and its derivatives to be calculated for a wide range of temperatures. The three values of C aaw define the magnitude of this coefficient well for temperatures from about 0 to 100 °C, but for want of a reliable theoretical interpretation yield little information concerning its derivatives. The results for B aw ( T ) are compared with published experimental values, and those for both B aw ( T ) and C aaw ( T ) with published theoretical estimates. The work allows the vapor-pressure enhancement factor f w ( T, p ) and the vapor-concentration enhancement factor g w ( T, p ) to be calculated for a wide range of conditions, and substantially improves the accuracy with which the thermodynamic properties of the water−air system can be calculated.