  Although it has been shown experimentally that a temperature discontinuity
exists at the liquid-vapor interface during evaporation and condensation,
quantitatively modeling this temperature jump has been difficult. The classical
Schrage equation does not give enough information to determine the interfacial
temperature jump. Starting from the Boltzmann transport equation, this paper
establishes three interfacial boundary conditions to connect the temperature,
density, and pressure jumps at the liquid-vapor interface to the interfacial
mass and heat fluxes: one for the mass flux (the Schrage equation), one for the
heat flux, and the third for the density discontinuities. These expressions can
be readily coupled to heat and mass transport equations in the continuum of the
liquid and the vapor phases, enabling one to determine the values of the
interfacial temperature, density, and pressure jumps. Comparison with past
experiments is favorable. A thermomolecular emission model, mimicking
thermionic emission of electrons, is also presented to gain more
molecular-level insights on the thermal evaporation processes.
