Nucleation and particle size

Feeney P.J., Napper D.H., Gilbert R.G. Coagulative nucleation and particle size distributions in emulsion polymerization. Macromolecules 1984 17 2520-2529 Feeney P.J., Gilbert R.G., Napper D.H.J. Periodic nucleation processes in emulsion polymerization systems. J. Colloid Interface Sci. 1985 107 159-173 Fomasiero D., GrieserF. Analysis of the visible absorption and SERS excitation spectra of silver sols. J. Chem. Phys. 1987 87(5) 3213-3217... 

Electron microscopy of the final latex of the experiments given in Table I showed almost no new nucleation. The particle size distributions were narrow and indicated no noticeable coagulation as well. New nucleation would lead to increased rates whereas coagulation would have the opposite effect. Any decrease in the rate therefore must be due to a decrease in [m], if we assume n to be constant. We therefore determined the tofuene/polymer ratio in the seed latex in the absence and presence of the various additives. Toluene was chosen as the solvent, because it is similar to styrene and allows the measurement of equilibrium solubilities without the risk of polymerization. Table II gives the experimental values of the toluene solubility in the seed as a function of time. The results indicate that the swelling is nearly complete within 5 to 10 min. 

In semibatch emulsion polymerizations the polymer particles are kept monomer-starved to obtain higher rates of polymerization and to permit easier control of the rate and particle size distribution. There are two aspects to the control of PSD. The controlled addition of emulsifier during particle growth stabilizes the particles without further particle nucleation. The second aspect is related to the particle sticky stage which often occurs... 

Initial recrystallization tests were carried out in the Jerguson gauge to determine the parameters, viz., initial NQ concentration, rate of expansion, final pressure level, etc., that combine to yield particles of a certain kind. It is difficult, if not impossible, to predict a priori the particle size, size distribution, shape, and morphology that will result from specific combinations of experimental parameters, and, thus, it is informative to present some simple nucleation and particle growth theory insofar as the theory guided the course of the experimental investigation in its early stages. 

In an anti-solvent recrystallization process, then, particle size and particle size distribution is determined by the interaction between the nucleation rate and the growth rate of crystals, on one hand, and by the rate of creation of supersaturation, on the other hand all three are influenced by the manner of addition of the anti-solvent. Figure 5 is a qualitative picture of simultaneous events that occur when an anti-solvent is added to a solution of a solute that is to be recrystallized. The three zones shown in Figure 5, designated I, II, and III, denote three areas of supersaturation. Zone I is for a supersaturation less than 1, i.e., for actual solute concentrations less than saturation. No growth of particles will occur in this zone (and in fact if there are any particles that are "somehow" present, they will dissolve). In Zone II, the supersaturation is less than the critical value discussed earlier, but "some" nucleation can occur particles that are present in this... 

Alkalinity also has an important influence on the crystallization rate of zeolites. A remarkable example is the crystallization of zeolite A (LTA) from the precursor gel with a batch composition of 5Na20 Al203 2Si02 (100-200) H20.[13] Figure 3.8 shows the effect of different alkalinity (H2O/Na2O=20, 30,40) on the crystallization rate (including induction period and growth rate) and particle size of the product. Clearly, with an increase of alkalinity the crystallization process is speeded up, the particle size is decreased, and the distribution of the particle size is narrowed due to an increased nucleation rate and an increased polymerization rate between polysilicate and aluminate anions. 

As discussed in Chapter 4, the nucleation rate is both species specific and a function of the supersaUiration ratio. The relation between nucleation rate, growth rate, and particle size as a function of the supersaturation ratio is illustrated qualitatively in Fig. 5-1. The acuial rate and supersaturation characteristics, such as metastable zone width, are system specific and can vary over wide ranges. In practice, it has been observed that the nucleation rate may vary from milliseconds to hours, and the metastable zone width may vary from less than 1 mg/ml to tens of mg/ml. 

Necessary seed types and quantities to achieve specific amounts of particle size increase in an all-growth process were calculated (Table 4-2) by a simple relationship relating seed and product size to the amount and particle size of seed to be added. Since the validity of this calculation depends on an all-growth process, simultaneous nucleation—which is virtually always present to some degree—will result in a reduction in the actual particle size of the product as well as an increase in the PSD. This increase often comes in the form of a bimodal... 

The critical mixing factors in a stirred tank at e impeller speed and type, as well as their influence on local turbulence and overall circulation. Since all aspects of these factors cannot be maintained constant on scale-up either locally or globally, the extent to which changes in the crystallizing environment will affect nucleation is extremely difficult to predict. To the mixing issue must be added the uncertainties caused by soluble and insoluble impurities that may be present in sufficiently different concentrations from batch to batch to cause variation in induction time, nucleation rate, and particle size. 

In order to achieve a monodisperse suspension or polydisperse system with particular particle size contribution, it is necessary to control the process of nucleation and particle growth. With most disperse systems, where the particles have some finite solubility, the smaller particles will have higher solubilities than their larger counterparts. With time, molecular diffusion occurs from the smaller to the larger particles, and this results in a shift in the particle size distribution to larger values this process is referred to as Ostwald ripening. 

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