Equilibrium thermodynamic state of water vapor and the collisional interaction of molecules

We consider all kinds of double and triple collisional interactions of molecules in a gas. Relation of the coefficients of the virial equation of state to dimer and trimer equilibrium constants is analyzed. The “excluded volume” for water vapor is estimated quantitatively. The volume is interpreted as an effective volume which is characterized by the range of molecular repulsive forces and is related to the collisional interaction of free monomers. Empirical data on water vapor in thermodynamic equilibrium are used for determination of the second and third virial coefficients, which serve for estimating the upper limits of the dimer and trimer equilibrium constants of water in the temperature range 273–1273 K.