Spontaneous Homogeneous Nucleation

Spontaneous (Homogeneous) Nucleation This process is quite difficult because of me energy barrier associated with creation of the interfacial area. It can be treated as a kinetic process with the... 

The crystallization of dilferent polymorphs and crystal forms is best understood in terms of nucleation, which is often the rate-determining step (16). In the absence of foreign particles for inducing heterogeneous nucleation, spontaneous homogeneous nucleation can be assumed to occur as a first step in the crystallization process. The rate of homogeneous nucleation J can be expressed as... 

Spontaneous, homogeneous nucleation — it rarely occurs in the supercooled homogeneous melt. 

Many reported cases of spontaneous (homogeneous) nucleation are found on careful examination to have been induced in some way. Indeed, it is generally accepted that true homogeneous nucleation is not a common event. For example, a supercooled system can be seeded unknowingly by the presence of atmospheric dust which may contain active particles (heteronuclei). Aqueous solutions as normally prepared in the laboratory may contain >10 solid particles per cm of sizes <1 pm. It is virtually impossible to achieve a solution completely free of foreign bodies, although careful filtration can reduce the numbers to <10 cm and may render the solution more or less immune to spontaneous nucleation. 

The theoretical superheating limiting conditions at which spontaneous homogeneous nucleation will exist in all the liquid mass can be established from the tangent line to the vapor pressure-temperature curve at the critical point. This represents the limit to which the liquid may be heated before spontaneous nucleation occurs with a vapor explosion (Reid, 1979). This is shown in Eig. 22.2. 

Instead, if during the heating process the liquid temperature reaches, for example, 89°C (point R in Fig. 22.2), during the depressurization the tangent line will be reached (point S in Fig. 22.2). In this case the conditions required (superheating) by the aforementioned spontaneous homogeneous nucleation would exist and a BLEVE explosion would occur. 

The process of crystallization proceeds via two distinct processes crystal nucleation and growth (Garside, 1985). The nucleation kinetics in fine droplets is often different from nucleation in the same liqnid in bulk. In a fine emulsion, the number of droplets exceeds the number of potential nncleation catalysts (impurities) present in the liquid oil. Thus, a proportion of the lipid is effectively catalyst free and must nucleate by other mechanisms. This may either be completely spontaneous homogeneous nucleation or, more probably, some catalytic effect of the droplet surface (Coupland, 2002). In either case, the crystallization temperature is greatly reduced below the melting point and depends both on particle size and on the nature of the emulsifier selected. For example, Higami and co-workers (2003) showed that the crystallization temperature of trilaurin molecules decreased from 18.9 °C, the crystallization temperature of the bulk lipid, to -9.5 °C when emulsified into droplets smaller than 100 nm. 

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