Surface Charge on Functionalized Latex Spheres in Aqueous Colloidal Suspensions

Many properties of colloidal particles in aqueous suspensions are determined by the presence of chemical groups on their surface and by the particle charge, which results from the ionization of these groups. We use spherical latex particles with carboxyl groups and additional ionizable groups to study the behavior of uniformly charged and zwitterionic colloids in electrolytes of different ionic strength and pH. The structural charge from the ionization of the surface groups is determined by acid-base titration. The results can be explained by using the concept of a surface pH, but not always by single dissociation constants of the ionizable groups. Values of pKa of 3.75 and 4.75 for carboxylic groups partly fit the data, but interactions with strong acid or base groups severely modify the titration curves. Model calculations include the chemical behavior as well as the electrostatic effects of the double layer. For carboxylate latex of 100 nm the results are compared to effective charges determined by static and electrophoretic light scattering.