The objective of this report is to understand the boundary mechanism of a model drug, propranolol hydrochloride, onto poly( isobutylcyanoacrylate)nanoparticles stabilized with a mixture of dextran and dextran sulfate, and to explain the similarity between depletion isotherms and the curves obtained with the dielectric method. The dramatic influence of the sulfate groups has been demonstrated by the dosage of the dextran sulfate in the bulk aqueous medium as well as by the interpretation of the propranolol hydrochloride adsorption isotherm, Due to its sulfate groups, dextran sulfate is able to bind to propranolol hydrochloride by an ion exchange process and the resulting complex moves toward the nanoparticle surface, It is the first time that such a model is proposed. In addition, this model supports and explains the dielectric answer : indeed, we had demonstrated that the dielectric method makes it possible to determine the amount of drug loaded onto the nanoparticles, without any ultracentrifugation or assay of the drug, Finally, the application field of the dielectric method may be enlarged to other areas such as ionic resins.