Nucleation thermodynamic control

It seems also meaningful to recall that, for both PVL and iPP, the metastable chiral modification is not obtained from solution. This fact is hard to rationalize if polymorphic discrimination occurs on the basis of the secondary nucleation site which should exist also in the presence of the solution it rather points to diffusion and to transport problems in the melt, or thermodynamic control in solution. 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

Accounts of nucleation inhibition in the pharmaceutical literature are sometimes confusing because the dependence of the nucleation event (nucleation rate, metastability zone width, or induction time) on supersaturation is not considered. In search of additives that inhibit nucleation, induction times are often measured as a function of additive concentration, while the dependence of the nucleation event on supersaturation is neglected. Results from such studies possibly lead to the erroneous conclusion that the additive inhibited nucleation when indeed the additive decreased the supersaturation and frequently led to an undersaturated state. Hence, the system is under thermodynamic control instead of kinetic control. 

To preserve the droplets as reaction loci, the polymerization has to compete with the bleeding of the emulsion droplets (239). A consequence of these considerations is that the best lyophob for the preparation of emulsion droplets for subsequent polymerization are polymers in form of seed particles. Swelling is a thermodynamically controlled process and consequently, swollen seed particles are not subjected to degradation processes to the same extent as emulsion droplets made by comminution processes. Furthermore, in most cases the chemistry of the seed particles does not interfere with the final properties of the dispersion. For example a seed particle with a diameter of 20 nm represents in final particles of about 100 nm diameter only less than 1% of the total volume. However, seed processes with suppressed nucleation of new particles are not always the method of choice. 

The given approach allows us to differentiate the influence of thermodynamic constraints on hysteresis (related to the Gibbs free energy dependence on size and nucleation barrier) from the influence of kinetic constraints on hysteresis (related to the activation energy for the diffusion across the parent phase-nucleus interface). At high T the hysteresis is related to thermodynamic control of the process, whereas at low T the hysteresis is related to kinetic control of the process. 

Ligands are also involved in the growth of the NPs, and the choice of acid/amine mixtures allows the dissociation of a nucleation reservoir form a growth reservoir and, therefore, with the help of a dihydrogen atmosphere, the growth of large monodisperse nano-objects, iron nanocubes, and cobalt nanorods, the monodispersity of which results from thermodynamic control [67]. 

Flow induced nucleation A Control volume thermodynamics approach... 

Thus far, we have considered mechanisms to explain the relative sizes of 0-D nanostructures, without consideration of their shapes. The observed shapes of nanocrystalline facets may be controlled by either thermodynamics or kinetics. For those reactions under thermodynamic control, growth will occur in such a way to minimize the total interfacial free energy, y - i.e., the energy associated with the nucleation/growth of a surface of a unit area. For a fee structure, y 110 > y 100 > y 111 hence, a single crystal should prefer an octahedral structure to maximize the number of (111) facets. However, as we have seen, a truncated... 

But liquids that behave in this way on cooling are the exception rather than the rule in spite of the second principle of thermodynamics, crystallization usually occurs at lower temperatures (supercooling). This can only mean that a crystal is more easily destroyed than it is formed. Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. The nucleation and growth of a crystal are under kinetic, rather than thermodynamic, control. 

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). 

Thermodynamic inhibitors Antinucleants Growth modifiers Slurry additives Anti-agglomerates Methanol or glycol modify stability range of hydrates. Prevent nucleation of hydrate crystals. Control the growth of hydrate crystals. Limit the droplet size available for hydrate formation. Dispersants that remove hydrates. 

The usual practice for avoiding the plugging of production facilities by hydrates is to add thermodynamic inhibitors, such as methanol or glycol. A newer concept is the injection of low-dosage additives either kinetic inhibitors, which delay nucleation or prevent the growth of hydrate crystals, or hydrate dispersants, which prevent the agglomeration of hydrate particles and allow them to be transported within the flow [880,1387]. Hydrate control is discussed extensively in Chapter 13. Classes of hydrate control agents are shown in Table 11-9, and additives are shown in Table 11-10. 

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