  The mass and momentum balances are theoretically studied in heterogeneous
two-component systems. Following Gibbs the system is presented as two bulk and
a single surface phases. Comparing the equations derived with some typical
rheological models, useful information about the location of the interface is
obtained. It was demonstrated that the surface phase for insoluble surfactants
coincides with the equimolecular interface, while for soluble ones it is placed
on the surface of total mass density zero excess. In both cases the surface
phase is close to the surface of tension and kinematic surface.
