Protonation of carboxyl latex particles in the presence of a strong cationic polyelectrolyte

The charging behavior of carboxyl latex surface in the presence of poly(diallyl dimethyl ammonium chloride) (DADMAC) was investigated by means of potentiometric titrations and electrophoretic mobility. The titration curves at different ionic strengths show a common crossing point, which coincides with the point of zero mobility, and can be identified with the point of zero charge (PZC). This characteristic behavior is due to the adsorption of the strong cationic polyelectrolyte on the weak acid particle surface, leading to an additional positive charge, and a corresponding shift of the surface potential. These features can be quantified in terms of modified Stern model and the standard electrokinetic model. This model predicts the PZC very well, while the magnitude of the charge and its ionic strength dependence are reproduced only semi-quantitatively.