SURFACE-CHARGE ON FUNCTIONALIZED LATEX SPHERES IN AQUEOUS COLLOIDAL SUSPENSIONS

Many properties of colloidal particles in aqueous suspensions are dete rmined 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 ion izable groups to study the behavior of uniformly charged and zwitterio nic colloids in electrolytes of different ionic strength and pH. The s tructural charge from the ionization of the surface groups is determin ed by acid-base titration. The results can be explained by using the c oncept of a surface pH, but not always by single dissociation constant s of the ionizable groups. Values of pK(a) of 3.75 and 4.75 for carbox ylic groups partly fit the data, but interactions with strong acid or base groups severely modify the titration curves. Model calculations i nclude the chemical behavior as well as the electrostatic effects of t he double layer. For carboxylate latex of 100 nm the results are compa red to effective charges determined by static and electrophoretic ligh t scattering. (C) 1994 Academic Press. Inc.