The combined use of cryo-TEM, dynamic light scattering, and small-angle X-ray and neutron scattering techniques allows a detailed structural model of complex pharmaceutical preparations of soybean lecithin/chitosan nanoparticles used as drug vectors to be worked out. Charge-driven self-organization of the lipid(-)/polysaccharide(+)vesicles occurs during rapid injection, under mechanical stirring, of an ethanol solution of soybean lecithin into a chitosan aqueous solution. We conclude that beyond the charge inversion region of the phase diagram, i.e., entering the redissolution region, the initial stages of particle formation are likely to be affected by a re-entrant condensation effect at the nanoscale. This behavior resembles that at the mesoscale which is well-known for polyion/amphiphile systems. Close to the boundary of the charge inversion region, nanoparticle formation occurs under a maximum condensation condition at the nanoscale and the complexation-aggregation process is driven toward a maximum multilamellarity. Interestingly, the formulation that maximizes vesicle multilamellarity corresponds to that displaying the highest drug loading efficiency.
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