Kinetic theory of condensation and evaporation. II

The macroscopic temperature and pressure jumps that occur at a condensate surface during a nonequilibrium phase change are calculated using a variational principle on the linearized Boltzmann equation. When evaluated for the BGK collisional model, the macroscopic jumps have values within 1% of a recent numerical calculation. Furthermore, the microscopic density and temperature jumps are estimated using an ansatz for the distribution function of molecules impinging on the surface. The jumps obtained in this way are correct to within 5% of the numerical values when evaluated for the BGK model. Also a simple procedure is described by which the jumps occuring at a surface for which the mass accommodation (condensation) coefficient σ≠1 may be calculated from those for which σ=1. In addition the connection of this kinetic theory with the thermodynamics of irreversible processes is made and the elements of the appropriate Onsager matrix are expressed in terms of the macroscopic jumps for arbitrary values of σ.