Supersaturation surface nucleation

Models used to describe the growth of crystals by layers call for a two-step process (/) formation of a two-dimensional nucleus on the surface and (2) spreading of the solute from the two-dimensional nucleus across the surface. The relative rates at which these two steps occur give rise to the mononuclear two-dimensional nucleation theory and the polynuclear two-dimensional nucleation theory. In the mononuclear two-dimensional nucleation theory, the surface nucleation step occurs at a finite rate, whereas the spreading across the surface is assumed to occur at an infinite rate. The reverse is tme for the polynuclear two-dimensional nucleation theory. Erom the mononuclear two-dimensional nucleation theory, growth is related to supersaturation by the equation. 

Keywords Chemical vapour deposition Organometallic chemical vapour deposition Nucleation Supersaturation Surface chemistry... 

When the surface is initially singular, the required ledges can be nucleated by the clustering of supersaturated surface vacancies in monolayer surface cavities or... 

Figure 6.6 can also be used for surface nucleation as a frmction of supersaturation. Because ttie values of are lower than AG for... 

At supersaturations less than the critical supersaturation ratio for surface nucleation, surface —1-5, layer growth has been experimentally... 

Alexander McPherson and Paul J. Shlichta have suggested using insoluble minerals as heterogeneous nuclei for the crystallization of macromolecules. They obtained excellent protein crystals, which could be cleaved from the mineral nucleus and used for X-ray diffraction studies. The mineral is introduced into a supersaturated solution of the material to be crystallized. As supersaturation increases, nucleation occurs on a specific face of the mineral nucleus, and a crystal begins to grow. The orientation and periodicity of the molecules on the nucleus surface promote an oriented overgrowth that has a similar periodicity. 

In general, the initiation of the precipitation process may result from the presence of particulate matter in the bulk water that seeds the crystallization. The process is usually termed heterogeneous nucleation. It is possible for homogeneous nucleation to occur when the nucleation is spontaneous. Once nucleation has occurred, crystals can grow, provided that the solution is supersaturated. Suitable nucleation points on the heat transfer surface facilitate deposit formation on the surface. In turbulent flow, it is possible that crystallites that are formed in the bulk fluid may be carried into regions, where they can redissolve. 

All of the above models assuming two-dimensional surface nucleation predict crystal growth rates at low supersaturation which are much lower than those observed in practice. A possible solution to this problem was put forward by Frank (1949), who proposed a self-generating step creation process involving a screw dislocation. The model was further formalized by Burton et al. (1951) and became known as the Burton-Cabrera-Frank (BCF) model of crystal growth. Detailed discussion of the BCF model can be found in Ohara and Reid (1973) and Nyvlt et al. (1985). 

It should also be noted that Table 4-2 indicates that good results in the final mean particle size can be achieved with small amounts of seed. While this may be true algebraically, it is not necessarily true in an actual crystallization because the limited surface area for growth may result in a build-up of supersaturation and nucleation of an excess of particles. Seed plus nuclei may then lead to a smaller final mean particle size and a wide PSD, including the bimodal distribution. 

At extremely low supersaturations, the deposition rate may be controlled by surface nucleation. 

In summary, the factors that affect crystal growth in inorganic solutions are (i) level of supersaturation, (ii) mechanistic processes (screw-dislocations, surface nucleation, surface reactions, bulk diffusion, and dehydration), (iii) concentration of other (coprecipitating) ions and molecules in the system, (iv) pH and temperature. 

Crystallization of L-SCMC (S-Carboxymethyl-L-cysteine) from DL-SCMC solution, wluch sodium chloride is contained, has been studied for the purpose to develop an optical resolution process by crystallization. In a crystallization process, crystal size and purity and product amount of crystal are important. Grow rates of L-SCMC seeds and surface nucleation of D-SCMC on the surface of L-SCMC seed crystals, which affect crystal size and purity, were reported by Yokota et al. (1). Using an optical resolution process, the product amount of L-SCMC crystal was studied by the decrease of the concentration of L-SCMC in a DL-SCMC solution. In this study, the concentration change of D-SCMC and L-SCMC in a DL-SCMC supersaturated solution, in which L-SCMC seeds were suspended, were observed. The decreasing rate of the concentration of L-SCMC was different depending on whether the solution was classified by dominant component of L-SCMC or D-SCMC. Particular correlative equations were obtained for either case. The operational conditions at which an L-SCMC seed crystal grew without crystallization of D-SCMC were studied by the induction time for surface nucleation of D-SCMC and a method for deciding the volume of crystallizer for optical resolution process was discussed based on the data obtained in this study. 

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