Molecular Nucleation model

Note, however, that a) the size of a stable nucleus depends on the model used (see Sect. 3.5.1), and b) molecular nucleation theory (Sect. 3.8.1) rules out such a mechanism. 

Figure 2.3 Left, reduction models. In the shrinking core or contracting sphere model the rate of reduction is initially fast and decreases progressively due to diffusion limitations. The nucleation model applies when the initial reaction of the oxide with molecular hydrogen is difficult. Once metal nuclei are available for the dissociation of hydrogen, reduction proceeds at a higher rate until the system comes into the shrinking core regime. Right the reduction rate depends on the concentration of unreduced sample (1-a) as f(a) see Expressions (2-5) and (2-6).

The droplet current / calculated by nucleation models represents a limit of initial new phase production. The initiation of condensed phase takes place rapidly once a critical supersaturation is achieved in a vapor. The phase change occurs in seconds or less, normally limited only by vapor diffusion to the surface. In many circumstances, we are concerned with the evolution of the particle size distribution well after the formation of new particles or the addition of new condensate to nuclei. When the growth or evaporation of particles is limited by vapor diffusion or molecular transport, the growth law is expressed in terms of vapor flux equation, given by Maxwell s theory, or... 

Theoretical approaches to nucleation go back almost 80 years to the development of Classical Nucleation Theory (CNT) by Volmer and Weber, Becker and Doring and Zeldovich [9,10,17-20]. CNT is an approximate nucleation model based on continuum thermodynamics, which views nucleation embryos as tiny liquid drops of molecular dimension. In CNT, the steady-state nucleation rate /, can be written in the form / a where jS, is the monomer condensation... 

A surface nucleation model in which attachment sites exist as molecular clusters that, like nuclei, must reach a critical size to be stable and support subsequent crystal growth. Here,... 

In the case of the n-heptyl viologen deposition, nucleation rates of the first molecular layers of this molecule control the deposition rates of subsequent layers. The nucleation reaction follows the instantaneous nucleation model — and is found to be highly sensitive to the chemical and physical nature of the electrode surface prior to deposition. RF-plasma of ion-beam etched surfaces generally show greatly enhanced nucleation and bulk deposition rates. 

Some fundamental aspects of the nucleation process have been investigated by molecular dynamics (MD) methods. In a recent review [44] the advantages and limitations of molecular cluster models in simulating the dynamics of nucleation and phase changes have been discussed. In this approach, molecular dynamic simulations are correlated with experimental nucleation rates extracted from electron diffraction patterns of molecular supersonic jets. The dynamics of freezing of ammonia, CCI4 and water, and the phase transformations of t-butyl chloride have been analysed. A useful feature of the MD computational... 

Although this model does not fit well for macromolecules since it neglects the interconnection between the crystallizable units along the molecule, it has been used to develop nucleation models as shown in Fig. 3.69. As soon as a ledge is created on the surface, growth is assumed to occur by adding new molecular segments (stems)... 

The concept of molecular nucleation was developed to explain some observations not in agreement with the above model of crystallization limited by secondary nucleation [29]. In Fig. 3.73 the molecular length is plotted which is rejected by a crystal growing from the melt or from solution (curves 1 and 2, respectively). These data were obtained by measuring the molar mass of the noncrystaUized molecules of a... 

Abstract. We review how the nucleation mechanism of polymer crystallization could be assigned to intramolecular processes and what are the preliminary benefits for understanding some fundamental crystallization behaviors. The speculative concept of molecular nucleation and the theoretical model of intramolecular nucleation have been elucidated in a broad context of classical nucleation theory. The focus is on explaining the phenomenon of molecular segregation caused by polymer crystal growth. 

From the classical Lauritzen-Hoffman model to the molecular-nucleation concept, and then the intramolecular-nucleation model, polymer crystal nude-... 

Atomistic classical molecular dynamics modeling as well as quantum chemical calculation for small molecular species can give both insight and fundamental data on nucleation and cluster growth processes. 

Both aerosol modeling and more fundamental atomistic and molecular level models have been applied to this problem. Aerosol dynamics modeling has lead to a better understanding of the individual steps that comprise the formation of particles, all the way from nucleation to subsequent growth. Both molecualar orbital and reaction rate theory was used as sources of fundamental data for input to the aerosol dynamics model. A simplistic molecular dynamics computation has been used to explain the particle morphology observed. 

Molecular simulations have reproduced regime-transition phenomena (Hu and Cai 2008). However, the growth front of Regime 1 is rather rough, favoring an alternative interpretation based on the intramolecular secondary nucleation model (Hu and Cai 2008). 

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