Nucleation mechanism heterogeneous

Phyllosilicates are clay-related compounds with a sheet structure such as talc, mica, kaolin, etc. for which the nucleation mechanism of PET is known to be heterogeneous, although still uncertain. 

The kinetics of nucleation of one-component gas hydrates in aqueous solution have been analyzed by Kashchiev and Firoozabadi (2002b). Expressions were derived for the stationary rate of hydrate nucleation,./, for heterogeneous nucleation at the solution-gas interface or on solid substrates, and also for the special case of homogeneous nucleation. Kashchiev and Firoozabadi s work on the kinetics of hydrate nucleation provides a detailed examination of the mechanisms and kinetic expressions for hydrate nucleation, which are based on classical nucleation theory. Kashchiev and Firoozabadi s (2002b) work is only briefly summarized here, and for more details the reader is referred to the original references. 

Cavitation in the rubber particles of PS/high-impact PS (HIPS) was also identified as a heterogeneous nucleation site, using batch-foam processing [15, 16]. The experimentally observed cell densities as a function of the temperature, the rubber (HIPS) concentration, the rubber particle size, and saturation pressure were found to be in good agreement with the proposed nucleation model. Similar nucleation mechanisms of elastomeric particles were claimed for acrylic and di-olefinic latex particles in various thermoplastics [17, 18]. 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un-... 

Wu and Woo [26] compared the isothermal kinetics of sPS/aPS or sPS/PPE melt crystallized blends (T x = 320°C, tmax = 5 min, Tcj = 238-252°C) with those of neat sPS. Crystallization enthalpies, measured by DSC and fitted to the Avrami equation, provided the kinetic rate constant k and the exponent n. The n value found in pure sPS (2.8) points to a homogeneous nucleation and a three-dimensional pattern of the spherulite growth. In sPS/aPS (75 25 wt%) n is similar (2.7), but it decreases with increase in sPS content, whereas in sPS/PPE n is much lower (2.2) and independent of composition. As the shape of spherul-ites does not change with composition, the decrease in n suggests that the addition of aPS or PPE to sPS makes the nucleation mechanism of the latter more heterogeneous. 

The prediction of the evolution of the PSD in Interval II is simpler than that in the ether intervals and it was for this reason that it was discussed first. Even the qualitative features of particle formation in Interval I are in doubt and the relative importance of homogeneous (ije., oligomeric precipitation) versus heterogeneous (i.e., micellar) nucleation mechanisms are not fully understood. For tbis reason, detailed solutions to Eq. (S) in this Interval, when c is nonzero, appear to be premature. Moreover, in many emulsion polymerizations, the precise details of events occurring in Interval I are masked by the subsequent particle growth in Intervals II and III. 

When a fat is emulsified, nucleation is substantially altered compared with the same fat in bulk liquid form. This is primarily because of the distribution of heterogeneous nucleation sites among the emulsion droplets. If there are more droplets than heterogeneous nucleation sites, then some of the droplets will nucleate by a homogeneous nucleation mechanism. That is, as a finely dispersed emulsified system is cooled, one population of droplets nucleates at relatively higher temperatures because of heterogeneous nucleation, whereas another population nucleates at substantially lower temperature because of homogeneous nucleation. 

Heterogeneous nucleation mechanisms can significantly affect the dissolution of metastable solid phases, because this form of nucleation can occur at low driving forces. While the choice of a metastable solid phase with solubility higher than other crystalline modifications is motivated by the expectation of faster dissolution rates, achievement of faster dissolution rates and higher concentrations in solutions is jeopardized by surface-mediated nucleation. We have reported that the surface of the metastable phase of theophylline promoted the nucleation of the stable monohydrate crystals. The observed oriented growth of monohydrate crystals on the anhydrous surface is consistent with a close lattice match between the b and c... 

It seems therefore that little or no stability is to be expected for the point defect aggregates which provide the necessary shear-plane precursors in the homogeneous shear-plane formation mechanisms. These homogeneous nucleation mechanisms are therefore unlikely to operate, and we turn our attention now to a heterogeneous mechanism, in which point defects aggregate at pre-existing planar-defect sites. 

The following section will discuss homogeneous, heterogeneous, and secondary nucleation mechanisms and kinetics. This will be followed by a similar discussion of crystal growth. The reader is directed to the references cited above, and others, for detailed treatment of these phenomena. 

It has therefore been concluded that (i) the most common process of zeolite nucleation relies on a primary nucleation mechanism and (ii) the most probable primary nucleation mode is heterogeneous and centred upon the amorphous phase of the reaction mixture (which for most clear solution syntheses is colloidal in nature). 

When the blend is now further cooled, two possible ways of primary nucleation are possible. In a first case, the matrix phase is nucleated by heterogeneous species present in this phase and instantly, newly created crystals appear. Hence, the crystallization temperature of the matrix will be situated at its bulk T. A second possibility for coincident crystallization occurs in the case one finds again a single crystallization peak for the matrix phase, which however takes place above its bulk T. Some novel mutual nucleating mechanism was suggested in such blends a molten component (minor phase) acts as nucleating substrate for the matrix, which instantaneously crystallizes [Erensch and Jungnickel, 1989]. 

It is desirable to classify the various mechanisms of nucleation as shown in Figure 2.19. Primary nucleation occurs in the absence of crystalline surfaces, whereas secondary nucleation involves the active participation of these surfaces. Homogeneous nucleation rarely occurs in practice, however, it forms the basis of several nucleation theories. Heterogeneous nucleation is usually induced by the presence of dissolved impurities. Secondary nucleation involves the presence of crystals and its interaction with the environment (crystallizer walls, impellers, etc.). 

Figure 6,3 Empirical dependence of induction period of Ni(NH4)2(S04)2 6H2O at 25 °C on relative supersaturation in logarithmic coordinates (after Sohnel and Mullin 1979) Region 1—predominant heterogeneous nucleation mechanism Region III—predominant homogeneous nucleation mechanism, and Region II—possible coexistence of both nucleation mechanisms.

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