Effective Charge of Adsorbed Poly(amido amine) Dendrimers: Transition from Heterogeneous to Homogeneous Charge Distribution

The surface charge density of sulfate latex particles with adsorbed poly(amido amine) (PAMAM) dendrimers of generation 10 was determined from direct force measurements with atomic force microscopy (AFM). The variation of the surface charge density with the adsorbed amount can be related to the effective charge of the dendrimers in the adsorbed state. Two regimes were identified. In the first regime, the adsorbed amount is low and the surface charge distribution strongly heterogeneous. This regime is characterized by an effective charge of about 45 elementary charges. The second regime corresponds to a saturated layer with higher adsorbed amount and a more homogeneous charge distribution. This regime is characterized by a higher effective charge of about 93, which is comparable to the value expected from Poisson-Boltzmann theory. The transition between these two regimes is located in a narrow region just after the charge reversal point.