No abstract

No abstract

In this work we study the kinetics of coagulation of monodisperse spherical colloids in aqueous suspension at the early stage of coagulation. We have performed the measurements on a multiangle static and dynamic light scattering instrument using a fiber-optics-based detection system which permits simultaneous time-resolved measurements at different angles. The absolute coagulation rate constants are determined from the change of the scattering light intensity as well as from the increase of the hydrodynamic radius at different angles. The combined evaluation of static and dynamic light scattering results permits the determination of coagulation rate constants without the explicit use of light scattering form factors for the aggregates. For different electrolytes fast coagulation rate constants were estimated. Stability curves were measured as a function of ionic strength using different particle concentrations.

No abstract

Poly(amidoamine) (PAMAM) dendrimers of generations G0, G1, G2, G3, G4, and G6 are investigated by potentiometric (acid−base) titrations. The data are interpreted with a site binding model, which offers the possibility to model the titration curves for all generations of the dendrimers and to describe all dendrimers within a common parameter set. These parameters involve the microscopic ionization constants for each group in the fully deprotonated state and nearest-neighbor pair interaction parameters. From this model we can further obtain all microscopic ionization constants as well as conditional microstate probabilities. The protonation of PAMAM dendrimers first involves protonation of primary amine groups at the outer rim of the dendrimer at high pH, while the tertiary amine groups in the dendrimer core protonate at lower pH. The last group to protonate at low pH is a central tertiary amine.

Carboxyl latex particles of two different sizes were used to study the early stages of aggregation in dilute colloidal suspensions. The charging behavior as a function of solution pH was characterized in acid-base titration and electrophoresis experiments at fixed ionic strength; absolute aggregation rate constants were measured by combined static and dynamic light scattering as a function of pH and ionic strength. Up to an ionic strength of 10 mM in a KCl solution, the classical DLVO theory of colloidal stability is seen to work quantitatively. At higher ionic strength, however, well-known discrepancies between theory and experiment are observed. An analysis of the theoretical pair interaction energy suggests that quantitative agreement can 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 as with earlier aggregation and deposition studies typically carried out in the unfavorable situation of barriers at subnanometer distances. The theoretical discussion further considers the appropriate choice of a Hamaker constant, the effect of nonlinearity in the Poisson-Boltzmann equation on stability predictions, as well as the role of charge regulation and the error introduced by the Derjaguin approximation.

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

We studied systematically aqueous suspensions of amorphous well-characterized silica particles by potentiometric titration, electrophoretic mobility, and time-resolved light scattering. Their charging behavior and aggregation rate constants were measured as a function of pH and ionic strength in KCl electrolytes for three types of particles of approximately 30, 50, and 80 nm in diameter. The charging behavior was consistent with the basic Stern model; the silica particles carry a negative charge, and its magnitude gradually increases with increasing pH and ionic strength. On the other hand, their early-stage aggregation (or coagulation) behavior is complex. The aggregation of the largest particles shows features resembling predictions of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. On one hand, the rate constant decreases sharply with increasing pH at low ionic strengths and attains fast aggregation conditions at high ionic strengths. On the other hand, we observe a characteristic slowing down of the aggregation at low pH and high ionic strengths. This feature becomes very pronounced for the medium and the small particles, leading to a complete stabilization at low pH for the latter. Stabilization is also observed at higher pH for the medium and the small particles. From these aggregation measurements we infer the existence of an additional repulsive force. Its origin is tentatively explained by postulating hairy layers of consisting of poly(silicilic acid) chains on the particle surface.