Nucleation particle process

Mini emulsion polymerization processes have multiple advantages apparent in reactions conducted in a heterogeneous medium, but they also maintain distinct characteristics observed in classic polymerization processes, such as perceived in emulsion and suspension. The main difference is evident during the nucleation particle process, which does not require the presence of micelles in the medium. This is a result of the nucleation process, which starts directly in the reactive species (monomer droplets) and is dispersed throughout the continuous phase. Hence, after the polymerization reaction, this process yields a final product comprising very stable polymer particles with sizes that range from 50 to 500 nm [25-27],... 

The Penniman-Zoph process involves the preparation of seeds or nucleating particles by the alkaU precipitation of ferrous sulfate. The reaction is carried out at alow temperature using an excess of ferrous ions. The hydroxide is then oxidized to the seeds of hydrated ferric oxide ... 

The reaction is sustained by addition of iron metal which reacts with the sulfuric acid formed, regenerating Fe(n) in solution. To ensure that the desired crystal form precipitates, a seed of a-FeO(OH) is added. However, with appropriate choice of conditions, for example of pH and temperature and by ensuring the presence of appropriate nucleating particles, the precipitation process may be adapted to prepare either the orange-brown y-FeO(OH), the red a-Fe203 or the black Fe304. 

In discussing the mechanisms of the formation of monodispersed colloids by precipitation in homogeneous solutions, it is necessary to consider both the chemical and physical aspects of the processes involved. The former require information on the composition of all species in solution, and especially of those that directly lead to the solid phase formation, while the latter deal with the nucleation, particle growth, and/or aggregation stages of the systems under investigation. In both instances, the kinetics of these processes play an essential role in defining the properties of the final products. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

Lindrud et al. (2001), Johnson and Prud homme (2003)]. With proper design, mixing to the molecular level can be accomplished in less time than the nucleation time, thereby achieving a primarily nucleation-based process for the production of uniform, fine particles. After the nuclei leave the mixing zone, additional crystallization continues in a standard agitated vessel on a well-defined initial number of nuclei with a well-defined size and shape. 

A nucleation-dominated process may be chosen in order to produce fine particles for a product attribute such as bioavailability for a pharmaceutical with low water solubility. The reader is referred to Examples 9-5 and 9-6 for discussion of a process for creating fine particles. Nucleation-dominated processes may be difficult to operate in stirred vessels for the reasons outlined in Section 5.2.1 below and may require intense in-line mixing, as presented in the examples. 

A mechanism for oiling out can be postulated as follows When supersaturation is achieved rapidly such that the concentration is beyond the upper metastable limit—as can often be the case in a nucleation-based process—the substrate is forced to separate into a second phase by the creation of the resulting high solution concentration. However, crystallization is delayed by a slow crystallization rate. This combination may result in the creation of a nonstructured oil or possibly an amoiphous solid. The rates of phase separation and nucleation are relative to each other such that slow nucleation implies only that nucleation was not fast enough to create discrete particles before oil separation. 

Variations of Silica Sol Manufacturing Process. As shown in Figure 4, the four methods, A-l, A-2, B-l, and B-2, use different technical combinations of nucleation, particle growth, and concentration. In any of these methods the raw material aqueous water-glass solution is diluted, and sodium is removed with a cation-exchange resin to obtain an active silicic acid. The characteristics of these four processes are shown in Table I. 

The few observations of nucleation in the free troposphere are consistent with binary sulfuric acid-water nucleation. In the boundary layer a third nucleating component or a totally different nucleation mechanism is clearly needed. Gaydos et al. (2005) showed that ternary sulfuric acid-ammonia-water nucleation can explain the new particle formation events in the northeastern United States through the year. These authors were able to reproduce the presence or lack of nucleation in practically all the days both during summer and winter that they examined (Figure 11.16). Ion-induced nucleation is expected to make a small contribution to the major nucleation events in the boundary layer because it is probably limited by the availability of ions (Laakso et al. 2002). Homogeneous nucleation of iodine oxide is the most likely explanation for the rapid formation of particles in coastal areas (Hoffmann et al. 2001). It appears that different nucleation processes are responsible for new particle formation in different parts of the atmosphere. Sulfuric acid is a major component of the nucleation growth process in most cases. 

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