Colloidal reaction-limited aggregation

P. Meakin, The effects of attractive and repulsive interaction on three-dimensional reaction-limited aggregation, J. Colloid Interface Sci. 134 235 (1990). 

The latex concentration was 10 g/L and the aggregating medium was 0.15 mol/L NaCl at pH 3.0 and 20°C, thus leading to a reaction-limited aggregation of the colloids. After transfer of a controlled volume of the suspension in the aqueous polymer solution at pH 3.0, the resulting ionic strength was equal to 0.0225. Figures 14 and 15 show the variation with time of S(t) and... 

Stoll S, Pefferkorn E. Modes of spontaneous and provoked cluster fragmentation. I. During reaction-limited aggregation. J Colloid Interface Sci 1992 152 257-264. 

Reaction-limited aggregation. This occurs if W is larger than a critical value (of the order of 100 ) presumably, the critical value is smaller for larger particles. The regime has not (yet) been seriously considered by colloid scientists. 

Figure 4.19 Postulated structures (A) stable hematite colloids in absence of organics, (B) reaction limited aggregation (RLAJ, (C) diffusion limited aggregation (DLAJ, (D) SPO aggregates with organics, (E) OPS colloids stabilised with organics, (F) OPS colloids stabilised with NOM, (G) OPS colloids stabilised with organics and destabilised with calcium.

In nature, ordered colloidal systems are unusual for two reasons. First, large scale lattices require monodispersity of the individual particle units within the lattice. Second, both diffusion limited aggregation and reaction limited aggregation in solution produce fractal structures rather... 

The basic theoretical approach for the analysis of diffusion controlled reactions is due to Smoluchowski [9] who developed it for the analysis of diffusion limited aggregation of colloidal particles. We discuss the generalization of this approach to the case of rodlike molecules here. The computational method best suited for the simulation of the polymerization of rodlike molecules is Brownian dynamics. We discuss in this review both multiparticle Brownian dynamics and pairwise Brownian dynamics the latter is a hybrid method combining Smoluchowski s [9] theory and Brownian... 

Figure 8. Transmission electron micrographs of typical clusters of gold, silica, and polystyrene colloids, prepared by both diffusion-limited and reaction-limited cluster aggregation and by computer simulation. There is a striking similarity in the structure of the clusters of different colloids in each regime.

Reaction limited (slow) aggregation occurs when repulsion between colloids is strong (thus at low to moderate salt concentrations). The colloidal aggregates have time to interpenetrate and thus compact aggregates of high mechanical strength form (see Figure 4.19B). 

Computer simulations have shown that the value of fractal dimension largely depends on whether the aggregation process is controlled by the diffusion rate of the clusters and single particles or by their chemical reactivity at the time of collision, the latter being mainly controlled by the DLVO forces. This observation, in agreement with experimental work on aerosols and colloids, has led to a new classification of aggregation processes the reaction-limited and diffusion-limited cluster aggregation (RLCA and DLCA respectively) processes. 

Ferretti, R., Zhang, J. and Buffle, J. (1998). Flocculation of hematite with polyacrylic acid fractal structures in the reaction and diffusion-limited aggregation regimes. J. Colloid Interface ScL, 208, 509-517. 

Axford, S.D.T. (1997). Aggregation of colloidal silica Reaction-limited kernel,... 

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