Nucleation, crystal molecular

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

Liu, X.Y. Generic mechanism of heterogeneous nucleation and molecular interfacial effects. In Advances in Crystal Growth Research-, Sato, K., Nakajama, K., Furukawa, Y., Eds. Elsevier Science Amsterdam, 2001 42-61. 

Another method for the determination of polymer crystallinity was discussed by Duswalt (159). It is based on the ability of the instrument to cool a molten sample rapidly and reproducibJy to a reselected temperature where isothermal crystallization is allowed to occur. A number of crystallization curves for polyethylene obtained isothermalJy at different, preset crystallization temperatures are shown in Figure 7.57. Differences in polymer crystallizability that may be caused by branching, nucleation, or molecular weight effects can be observed. The sensitivity and speed of the method allow pellet-to-pellet variations in a lot of polymer to be examined. 

As predicted by Helfand and Tagami, the interface is the locus of low molecular weight impurities that have been shown to nucleate crystallization, as for example in PS/PP blends [Wening et al, 1990]. Compatibilization by addition of a third component may either reduce or enhance the tendency for crystallization. On the one hand, addition of compatibilizer increases the... 

On September 25-30, 1988 in Los Angeles, California the first ACS Symposium on zeolite synthesis emphasized the importance that gel chemistiy, zeolite nucleation, crystal growth, crystallization kinetics, and structure-directing phenomena have in understanding zeolite (and molecular sieve) synthesis. The objectives of a similar ACS Symposium held in New York on August 25-30, 1990 where expanded to include papers on pillared clay synthesis and on the synthesis of other microporous materials that could be used in catalyst preparation. About 90% of all the chemical processes in the U.S. are based on catalysis and today catalysts have become indispensable to petroleum refining, an industry that in 1990 had sales of 140 billion (U.S. Dept, of Commerce U.S. Industrial Outlook, 1991). 

The process of crystallization may be considered in three stages crystal nucleation, crystal growth, and the equilibrium partially crystalline state. Embryo nuclei may continuously form and grow even in amorphous rubber. However, below a certain critical size they are unstable enough to disappear due to random thermal motions of the molecular chain and may not have measurable macroscopic consequences. 

Following their nucleation, crystals grown from solution typically exhibit regular, planar facets characterised by their Miller indices. Although appearing flat to the naked eye, these crystalline surfaces are rarely so at the molecular level. The various features which make up the nanoscale surface topography of crystal faces are intimately involved in the mechanisms by which crystals grow [48]. 

In the preceding sections, we discussed the formation and transformation of ionic precipitates from bulk electrolyte solutions. We saw that the rates and mechanisms of the processes governing crystallization (nucleation, crystal growth, flocculation, and aging) depend on the experimental conditions such as supersaturation, temperature, and additives. It has been shown that surfactant micelles have a profound influence on crystallization, even to the extent of controlling the nature of the crystallizing phase (Fig. 8). In this section, we concern ourselves with crystallization of molecular crystals and/or inorganic clusters within confined spaces and/or at the oil/water interface such as occurs in emulsions and microemulsions. 

The critical supersaturation for nucleation of molecular crystals from a melt or solution is conveniently achieved by cooling the sample until a crystallization temperature, is reached. Another critical point is the melting temperature, T, ... 

However, commercial chain extended PET showed slower crystallization kinetics as compared to that of bottle grade PET. In particular, the energetic barriers to nucleation and molecular mobility of the chain moving from the melt to the growing crystals were higher for the chain extended PET, suggesting a lower nucleation rate and a lower molecular mobility (see Figure 9.3). 

Where the movement of the interface is similar to crystal growth on molecularly rough surfaces or involving screw dislocations the rate of growth of patches will be determined by the rate of transformation r. Where Volmer two-dimensional nucleation on molecularly smooth interfaces is the growth process the rate of transformation r appears as a factor in the rate expression, Eq. (9). It is possible that the short horizontal limit on the rate at low pressures which was found for ammonium alum (Fig. 12) may eventually be accounted for by considering the behavior of r in Eq. (9) at low pressures. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

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