Nucleation, homogeneous

From this section we can summarize the general behavior of confined crystallizable MDs. These generalizations apply to block copolymers that are in the strong segregation regime and that can crystallize within their specific MD without breakout. When a block copolymer component crystallizes within isolated MD structures like spheres, cylinders or lamellae it may nucleate homogeneously. For homogeneous nucleation to take place, several requirements should be met ... 

Kitamura (1989) studied many of these nucleation factors in the competitive crystallization of the a and fi forms of L-glutamic acid. He found that at 25 °C only the a modification nucleates and grows. In this system, at least, the effect of temperature on the relative nucleation rates of the two polymorphs is more remarkable than the effect of the supersaturation ratio as the temperature is increased with a constant supersaturation ratio, the amount of a decreases. He also reported that the p form tends to nucleate in stagnant solutions, while at 25 °C essentially only a nucleated homogeneously. 

Figure 8.49. Microstructure of an orthopyroxene from the Siilwater Complex showing large (100) augite lamellae A, intermediate phase B, and a fine distribution of platelets that nucleated homogeneously. (From Champness 1977.)...

Homogeneous Nucleation Homogeneous nucleation is based on accretion of molecules in the liquid phase. Single species (molecules or ions) come together and form dimers. Dimers become trimers by addition of a molecule, and this accumulation process continues until eventually a stable nucleus forms depending on temperature and supers aturation. 

The observation that nanophase anatase sometimes transforms to nanophase brookite and in other cases nanophase brookite transforms to nanophase anatase implies very closely balanced energetics as a function of particle size. Several competing mechanisms (surface nucleation, interface nucleation, homogeneous nucleation) have been proposed for these reactions as a function of temperature and/or particle size (Zhang and Banfield 1999, 2000a,b). 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

The emphasis of most studies on nucleation and growth of diamond has been placed on the heterogeneous formation of diamond particles and the crystallization and deposition of diamond films on substrate surfaces. Only a limited number of experiments have been conducted to achieve the homogeneous nucleation of diamond in the gas phase at atmospheric and subatmospheric pressures. However, there is evidence that, at least in some cases, diamond can be nucleated homogeneously in the nas phase.I2i H i]... 

Nucleation plays a fundamental role whenever condensation, precipitation, crystallization, sublimation, boiling, or freezing occur. A transformation of a phase a, say, a vapor, to a phase p, say, a liquid, does not occur the instant the free energy of p is lower than that of a. Rather, small nuclei of p must form initially in the a phase. This first step in the phase transformation, the nucleation of clusters of the new phase, can actually be very slow. For example, at a relative humidity of 200% at 20°C (293 K), far above any relative humidity achieved in the ambient atmosphere, the rate at which water droplets nucleate homogeneously is about 10 54 droplets per cm3 per second. Stated differently, it would take about 1054 s (1 year is 3 x 107 s) for one droplet to appear in 1 cm3 of air. Yet, we know that droplets are readily formed in air at relative humidities only slightly over 100%. This is a result of the fact that water nucleates on foreign particles much more readily than it does on its own. Once the initial nucleation step has occurred, the nuclei of the new phase tend to grow rather rapidly. Nucleation theory attempts to describe the rate at which the first step in the phase transformation process occurs—the rate at which the initial very small nuclei appear. Whereas nucleation can occur from a liquid phase to a solid phase (crystallization) or from a liquid phase to a vapor phase (bubble formation), our interest will be in nucleation of trace substances and water from the vapor phase (air) to the liquid (droplet) or solid phase. 

Homogeneous and Heterogeneous Nucleation. Homogeneous and heterogeneous nucleation occur at very high levels of supersaturation, either in the solution, or in the case of heterogeneous nucleation, on other inert particles that are present in the form of crystals or as amorphous solid material. Most industrial crystallizers of the types in commercial use operate at levels of supersaturation far below those at which these types of seeding are expected, except under startup conditions. 

As described in Chapter 2, new crystals may be formed by primary nucleation (homogeneous and heterogeneous), secondary nucle-... 

---