  Breaking of the inversion symmetry at the interface between different
materials may dramatically enhance spin-orbit interaction in the vicinity of
the interface. We incorporate the effects of this interfacial spin-orbit
coupling (ISOC) into the standard drift-diffusion theory by deriving
generalized boundary conditions for diffusion equations. Our theoretical scheme
is based on symmetry arguments, providing a natural classification and
parametrization of all spin-charge and spin-spin conversion effects that occur
due to ISOC at macroscopically isotropic interfaces between nonmagnetic
materials. We illustrate our approach with specific examples of spin-charge
conversion in hybrid structures. In particular, for a lateral metal-insulator
structure we predict an "ISOC-gating" effect which can be used to detect spin
currents in metallic films with weak bulk SOC.
