Precipitation of Uniform Particles: The Role of Aggregation
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Precipitation of Uniform Particles: The Role of Aggregation C.F. Zukoski, M. K. Chow, G.H. Bogush, and J-L. Look Department of Chemical Engineering, University of Mlinois, Urbana, I1 61801 I. Introduction The conventional mechanism developed by LaMer (1) is often considered as the most relevant model for describing the precipitation of uniform particles. In this model, the concentration of a species is slowly increased above its equilibrium value until a critical concentration is reached and nucleation occurs. The resulting particles consume soluble species and the supersaturation level is reduced until there is a balance between particle growth and the generation of reactive species. At this point nucleation stops. Particle growth then continues by molecular addition of soluble species to the growing particles. Uniformity is achieved through a short nucleation time and a particle growth mechanism where the small particles grow more rapidly than the large particles. In studies of the preparation of uniform particles, the LaMer mechanism is often invoked. Additional proposed mechanisms include precipitation and redissolution through an Ostwald ripening process (2), thermodynamic limits where a single particle size has the lowest free energy (3), and nucleation followed by aggregation (4-11). Distinguishing between these mechanisms has taken on added importance as methods of scaling-up precipitation technologies are sought. While it is unlikely that a single mechanism will be at the heart of all precipitation chemistries, the classes of precipitation mechanisms that result in uniformity must meet common underlying conditions. Among these are that the final solids be colloidally stable. If irreversible aggregation occurs at an appreciable rate, the number density of particles in the suspension will continuously decrease and the average particle size will grow. A second condition that is closely related to the need for colloidal stability is that a constant number density of stable, growing particles be established at some point in the precipitation reaction.
These necessary conditions can be used to compare and contrast various proposed mechanisms for uniform particle formation. In the Ostwald ripening model developed by Lifshitz and Slyozou (12), a constant number density of particles is never reached and standard deviations in particle size distribution reach greater than 50 % of the average diameter. In typical precipitation reactions, once formed, the particle size is found to be stable for extremely long periods of time (months to years) and size distributions considerably narrower than those expected from Ostwald ripening are routinely reported. Consequently, Ostwald ripening alone is unlikely to result in uniformity. In the thermodynamic limitation model, particle size distribution narrows to the particle size with the lowest free energy and a constant number density of particles need not be reached until the end of the reaction. As currently developed, the thermodynamic model of Feenstra and Debruyn (4) sugge
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