Nucleation heterogenous primary

For mixed homogeneous/heterogeneous primary nucleation, the following empirical expression can be used to describe its rate Kirk-Othmer Encyclopedia of Technology, 1978-1984) ... 

Three different mechanisms for nucleation may occur in any crystallization operation, as discussed by Mersmann (2001, pp. 45ff.) homogeneous primary, heterogeneous primary, and activated secondary. Industrial crystallizers are usually operated under conditions of the last two simultaneously. For simplicity, nucleation as used in this discussion refers to any or all three of the mechanisms that may be important in a particular crystallization. 

Table 3.28. Overview of the phenomena influencing heterogeneous primary nucleation in polypropylene based immiscible blends (after Bartczak et al., 1995).

It is today well accepted that a control of the heterogeneous primary nucleation can favor the formation of different polymorphs. In this context, the ability of membrane-based CTystalUzers to promote the selective formation of stable or metastable polymorphs on polymeric hydrophobic manbranes was confirmed by experimental tests on glycine with selective CTystalUzation of forms a and y (Di Profio et al. 2007a), on paracetamol with the possibility to disamtinate between the monocline stable form of... 

Interfacial energy between two solids can thus be extremely low, if they have different composition but similar crystal lattices in their symmetries and their cell parameters. We can note that this is the origin of the phenomena of heterogenous primary nucleation starting from the liquid phases, whose ultimate demonstration is the epitaxy. This interfacial energy can on the contrary be very high if the networks are very different in molar volumes and from the crystallographic point of view. 

Heterogenous primary nucleation starting from a fluid phase... 

Both homogeneous and heterogeneous mechanisms requite relatively high supersaturation, and they exhibit a high order dependence on supersaturation. These factors often lead to production of excessive fines ia systems where primary aucleatioa mechanisms are important. The classical theoretical treatment of primary nucleation results ia the expressioa (5) ... 

Primary nucleation is the classical form of nucleation. It occurs mainly at high levels of supersaturation and is thus most prevalent during unseeded crystallization or precipitation. This mode of nucleation may be subdivided into either homogeneous viz. spontaneously from clear solution, or heterogeneous viz. in the presence of dust particles in suspension, or solid surfaces. 

The primary nucleation on the surface of the NA is a kind of heterogeneous nucleation [25]. NA is a so called heterogeneity . In this study, all nucleation is limited to the heterogeneous nucleation. The shape of a heterogeneous nucleus is assumed parallelepiped with length of a stem l and the number of stems m and n. Here /, m, and n are counted by the number of atom... 

Note Nucleation may be classified as primary or secondary. Primary nucleation can be homogeneous or heterogeneous if heterogeneous nucleation is initiated by entities having the same composition as the crystallizing polymer, it is called self-nucleation. Secondary nucleation is also known as surface nucleation. 

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