Catalyzed nucleation

Figure 3 Fraction of material in the polymerized state, f, as a function of the mass action variable X relative to its value X at the half-way point f =1/2. Indicated are predictions for the isodesmic and the self-catalyzed nucleated polymerization models. Activation constant of the nucleated polymerization Ka = 10-4.

IfX 1, disordered (nonhelical) assemblies do not form in any appreciable quantities. For h< 0, there is a polymerization transition from monomers to helical assemblies that is of the self-catalyzed nucleation type provided flj 3> 1. In the language of the coarse-grained self-catalyzed nucleated assembly model, the transition takes place near Xp exp [f3h] and we are able to assign an activation constant Ka exp [ 3j + 311], The theory of Section 2 approximately applies. 

Scale of Operation The size of the batch being crystallized may influence rate of crystallization. For example, crystallization from an emulsion generally occurs at a lower temperature than for the bulk fat based on the separation of catalyzing nucleation sites. In an emulsion, the catalyzing nucleation sites are more dispersed (spread through the number of droplets) and this leads to nucleation at a lower temperature than the same fat in bulk phase. 

Modern experimental work on the nucleation of liquids was pioneered by Turnbull, who developed droplet dispersion methods during the early 1950s and applied them to a wide variety of metals. His approach was based on the idea that even highly purified liquid samples contain impurities that can catalyze nucleation heterogeneously in the bulk, so that samples of macro-... 

Gamer and Jennings [60] demonstrated that water vapour catalyzed nucleation of aim surfaces following evacuation. Defects generated at the surface diffuse and accmulate at dislocations to develop germ nuclei [32]. The initiation and development of reaction in potassim and chrome alms were contrasted. The nuclei in the potassim aim were bounded by planar faces and the Arrhenius parmeters were "normal", while in chrome aim the nuclei were approximately hemispherical, A and values were large and the nucleus boundaries were not oriented in a simple relationship to the lattice stmcture. 

Crystal morphologies of glycine can be controlled by using surfactant monolayers at the air/water interface as templates. Catalyzed nucleation may, for example, yield 010 pyramids of glycine in supersaturated aqueous phases below surface monolayers or at foam lamellae (Chen et aL, 1998). 

In 1963 Skoda and Van den Tempel observed [77] that the temperature at which crystallization of triglycerides started in emulsified systems was invariably lower than in nonemulsified solutions and depended on the emulsifying agent used. With some emulsifiers T was so low that the authors assumed the existence of homogeneous nucleation. Other molecules effected a rise in T, apparently catalyzing nucleation. It was observed that the catalytic activity was the greater... 

The foregoing studies point to conclusions similar to those of the extensive research on induced crystallization of inorganic and organic crystals under close-packed amphiphilic Langmuir monolayers [88-92]. All these studies show that it is possible to design interfaces that will not only catalyze nucleation but will also orient the nascent crystals in a well-defined and reproducible manner. The... 

Of the geometries he investigated. Wilt [32] showed that the conical pit, with high contact angle, was theoretically the only one able to catalyze nucleation to the point that bubbles would be formed at saturation ratios close to 5. This was important because this is typical of carbonated drinks, and the fact is that such drinks do indeed produce massive numbers of bubbles when the... 

Alumina is frequently used as a carrier for metals, preferably in the y-modification. Under the prolonged effect of temperature, a transition into the a-modification is possible. Likewise, amorphous silica carrier can evolve into the crystalline form by the effect of temperature or of foreign substances such as gases, or the presence of impurities, such as sodium ions which catalyze nucleation. 

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