A multi-particle colloidal probe technique based on the atomic force microscope (AFM) was used to measure the interaction forces between individual charged latex particles with adsorbed cationic poly(amido amine) (PAMAM) dendrimers. The forces near the isoelectric point (IEP) were found to be attractive and stronger than van der Waals interactions. These additional attractions can be rationalized semi-quantitatively with a patch-charge model, which is derived from Debye-Hückel theory. However, the model predicts a larger decay constant for the attractive interaction than observed experimentally. The deviation is probably due to the disordered liquid-like structure of the experimentally investigated system and the finite size of the interacting regions. The amplitude of the attractive interaction is estimated correctly by the patch-charge model including its dependence on the ionic strength and dendrimer generation. The experimental force curves are situated between predictions for constant charge and constant potential boundary conditions. We conclude that the observed additional attractive forces are of electrostatic origin, and that charge regulation effects may play an important role.
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