The outer surfaces of silica gel (50 µm) are coated with a hydrophilic polymer, so that
only the interior of pores are chemically modified by octadecyl radicals (ODS). Since
proteins and other macromolecules cannot enter the pores and are blocked by the
hydrophilic polymer on the outer surfaces, they are quickly eluted without being
retained by the ODS solid phase. In contrast, pharmaceuticals and other induced low
molecular weight compounds penetrate the pores and are retained by the inner
surfaces of the stationary phase.