3.3 Oppositely Charged Particles
3.3.1 Simple Colloids
The particles of a mixture of oppositely charged nanoparticles form charged
aggregates (complexes), in which each charged particle is screened by a shell of
oppositely charged particles [107, 108]. The stable core-and-shell aggregates were
confirmed in experiments with the mixtures of oppositely charged metal
nanoparticles [107] When the net charges of positive and negative species are
equal, the aggregates are neutral and condense into a macroscopic drop, i.e., the
system becomes unstable. The theory developed in [109] predicts a possibility of
enhanced aggregation of larger charged particles induced by the small polyions of
the opposite charge. This phenomenon may reflect the partial “condensation” of the
small particles and reduction of the strong electrostatic repulsion between screened
(larger) colloidal particles.
The Brownian dynamic simulation of aggregation between the oppositely
charged particles has shown that heteroaggregation produces more branched
aggregates than the usual diffusive aggregation [110]. Moreover, the cluster discrimination was observed at the late stages of aggregation, when neutral clusters
were disappearing faster than charged ones [111].
The presence of linear chaining with particle charge alternating down a chain in
aggregates of the oppositely charged polystyrene spheres was revealed by different
experimental techniques and was supported by Brownian dynamics simulation
[112]. It was shown that branching of an aggregate composed of oppositely charged
particles may be varied from a linear chain structure (d f ~ 1.2) to a structure of
diffusion-limited aggregates (d f ~ 1.7) by an increase in concentration of the
background electrolyte. Formation of linear chains was explained by the shortrange attraction between the oppositely charged particles and long-range repulsion
between the identical particles. The experiments and the Brownian dynamics
simulations of heteroaggregation between the oppositely charged particles
[113–115] has shown that small silica (diameter ~ 25 nm, negatively charged)
particles covered the surface of large alumina (diameter ~ 400 nm, positively
charged) particles. For small amounts of silica particles, agglomeration of the
silica-covered alumina particles was observed, and it was demonstrated that the
agglomerates were of elongated shape.
3.3.2 Polymers
Behavior of solutions of oppositely charged polymers is rather similar to that of
solutions of oppositely charged colloids. However, higher flexibility of the polymer
chains results in a less ordered structure of the polyion solutions. Pairing of the
oppositely charged polyions, their aggregation, and complexation of anionic and
76
N.I. Lebovka
3.3.1 Simple Colloids
The particles of a mixture of oppositely charged nanoparticles form charged
aggregates (complexes), in which each charged particle is screened by a shell of
oppositely charged particles [107, 108]. The stable core-and-shell aggregates were
confirmed in experiments with the mixtures of oppositely charged metal
nanoparticles [107] When the net charges of positive and negative species are
equal, the aggregates are neutral and condense into a macroscopic drop, i.e., the
system becomes unstable. The theory developed in [109] predicts a possibility of
enhanced aggregation of larger charged particles induced by the small polyions of
the opposite charge. This phenomenon may reflect the partial “condensation” of the
small particles and reduction of the strong electrostatic repulsion between screened
(larger) colloidal particles.
The Brownian dynamic simulation of aggregation between the oppositely
charged particles has shown that heteroaggregation produces more branched
aggregates than the usual diffusive aggregation [110]. Moreover, the cluster discrimination was observed at the late stages of aggregation, when neutral clusters
were disappearing faster than charged ones [111].
The presence of linear chaining with particle charge alternating down a chain in
aggregates of the oppositely charged polystyrene spheres was revealed by different
experimental techniques and was supported by Brownian dynamics simulation
[112]. It was shown that branching of an aggregate composed of oppositely charged
particles may be varied from a linear chain structure (d f ~ 1.2) to a structure of
diffusion-limited aggregates (d f ~ 1.7) by an increase in concentration of the
background electrolyte. Formation of linear chains was explained by the shortrange attraction between the oppositely charged particles and long-range repulsion
between the identical particles. The experiments and the Brownian dynamics
simulations of heteroaggregation between the oppositely charged particles
[113–115] has shown that small silica (diameter ~ 25 nm, negatively charged)
particles covered the surface of large alumina (diameter ~ 400 nm, positively
charged) particles. For small amounts of silica particles, agglomeration of the
silica-covered alumina particles was observed, and it was demonstrated that the
agglomerates were of elongated shape.
3.3.2 Polymers
Behavior of solutions of oppositely charged polymers is rather similar to that of
solutions of oppositely charged colloids. However, higher flexibility of the polymer
chains results in a less ordered structure of the polyion solutions. Pairing of the
oppositely charged polyions, their aggregation, and complexation of anionic and
76
N.I. Lebovka