  Two-component liquid-vapor systems are modeled as two bulk phases divided by
a two-dimensional surface phase and the mass and momentum balances are
theoretically studied. Comparing the derived equations with some typical models
of surface rheology, useful information about the interface location is
obtained. It is demonstrated that the surface phase, set on the surface of
tension, coincides with the equimolecular interface for insoluble surfactants,
whereas it is placed on the surface of zero total mass density excess for
soluble ones. The applicability of the model to surface electrostatics is also
discussed by introduction of a two-dimensional Maxwell equation for the surface
phase.
