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 degrees 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 degrees 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.