Charging and aggregation properties of carboxyl latex particles: Experiments versus DLVO theory

Carboxyl latex particles of two different sizes were used to study the earl y stages of aggregation in dilute colloidal suspensions. The charging behav ior as a function of solution pH was characterized in acid-base titration a nd electrophoresis experiments at fixed ionic strength; absolute aggregatio n rate constants were measured by combined static and dynamic light scatter ing as a function of pH and ionic strength. Up to an ionic strength of 10 m M in a KCl solution, the classical DLVO theory of colloidal stability is se en to work quantitatively. At higher ionic strength, however, well-known di screpancies between theory and experiment are observed. An analysis of the theoretical pair interaction energy suggests that quantitative agreement ca n be achieved when the energy barrier for reaction-limited aggregation lies at surface separations of at least 1-2 nm. This result is consistent with recent measurements of colloidal forces and interaction energies, as well a s with earlier aggregation and deposition studies typically carried out in the unfavorable situation of barriers at subnanometer distances. The theore tical discussion further considers the appropriate choice of a Hamaker cons tant, the effect of nonlinearity in the Poisson-Boltzmann equation on stabi lity predictions, as well as the role of charge regulation and the error in troduced by the Derjaguin approximation.