Experimental techniques precipitate sizing

Experimental techniques used in the assessment of kinetics of precipitation and sizing of precipitate must address the specificity of this process. In particular, they need to be concerned with rapid chemical reactions, high initial supersaturation, high-order nucleation kinetics, very short time-scale of concurrently occurring component-phenomena, and small size of the crystals. 

Also discussed are precipitation specific experimental techniques, such as supersaturation measurements, constant composition (CC) method, instantaneous mixing devices, maximum (critical) growth rate experiments, and sizing. Due to the intrinsic difficulties with the direct supersaturation measurements and the microsecond characteristic time scale of precipitation reaction and nucleation, the CC method is used to study the precipitation kinetics. For the same reasons, the critical growth experiments are used to delineate the domain of the reactant feed rate that assures a renucleation-free process and a unimodal CSD. 

A supercritical C02-based technique was applied to produce a biocompatible polymer in form of microspheres. With the SAS process carried out in a batch mode, micronic particles of polymer were obtained with a narrow particle size distribution. Co-precipitation of pharmaceuticals for the production of controlled drug delivery systems has been proposed as a preliminary result, since further analyses are under development. The experimental apparatus is easy-to-use and the purification of the precipitated solid from the solvent can be achieved by suitably increasing the time for solvent removing. 

Experimental methods and techniques for catalyst manufacture are particularly important because chemical composition is not enough by itself to determine activity. The physical properties of surface area, pore size, particle size, and particle structure also have an influence. These properties are determined to a large extent by the preparation procedure. To begin with, a distinction should be drawn between preparations in which the entire material constitutes the catalyst and those in which the active ingredient is dispersed on a support or carrier having a large surface area. The first kind of catalyst is usually made by precipitation, gel formation, or simple mixing of the components. 

Lu and Yeh [174] record an emulsion technique for the synthesis of ZnO. They dissolved zinc acetate in de-ionized water to obtain the aqueous phase, n-heptane was used as the continuous phase, in which a surfactant (Span 80) was added. The two phases in different proportions were mixed continuously for Ih for obtaining homogeneous emulsions. Ammonium hydroxide was added into the emulsions to cause precipitation of zinc. The precipitates were dried and calcined at 700 -1000°C/2 h, which yielded white powders of ZnO. The modal particle size was 0.080 pm, while the mean size was about 0.08-0.09 pm depending on experimental conditions. 

Cerium (IV) oxide nanoparticles were synthesized by Masui et al [236] by use of a two-microemulsion technique. One of the microemulsions contained polyoxyethylene(lO) octylphenyl ether (OP-10) as the surfactant, n-hexyl alcohol as the co-surfactant, cyclohexane as the continuous phase, and an aqueous solution of cerium nitrate as the droplet phase. The second microemulsion was the same except that the droplet phase was an aqueous ammonia solution. The two were mixed to cause precipitation the particles thus obtained were gathered by centrifugation and washing under sonication with methanol, deionized water and acetone. The final treatment involved freeze-drying and vacuum drying. The mean particle size varied with experimental conditions in the range 2.5-4.0 nm. 

Use the maximum entropy technique to identify the most probable distribution of cluster sizes that is consistent with the observed scattering data. ° In this method, the experimental data are fitted by an algorithm which varies the volume fraction of each size class of precipitate. The advantage of this method is that no prior assumptions need be made regarding the form of the size distribution. Furthermore, the size distribution must be positive for all sizes. 

It is even more difficult to estimate not only one but four parameters (nucleation rate, growth rate, agglomeration kernel and disruption kernel) simultaneously from a particle size distribution. The errors are likely to be unacceptably high and it might be impossible to distinguish between the mechanisms involved. Therefore, an alternative sequential technique has been developed to obtain the kinetic parameters nucleation rate, growth rate, and agglomeration and disruption kernels from experimental precipitation data. 

Microparticles can be produced by a simple technique that consists of spraying a polymer, e.g., PLLA, solution in dichloromethane (or dimethylsulfoxide), through a nozzle into a reactor filled with supercritical carbon dioxide (Reverchon et al, 2000). This process is known as supercritical antisolvent precipitation (SAS). The experimental parameters have a limited influence on the particle size (1-4 /im). A modified version of the process, known as the SAS-EM process, allows nanoparticles of a controlled size (30-50 nm) to be produced (Chattopadhay et al., 2002). In order to restrict the use of an organic solvent. Pack and co-workers fed the SAS reactor with a solution of PLLA prepared by homogeneous ring-opening polymerisation in supercritical HCFC-22 (Pack et al, 2003a). 

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