Additive-mediated nucleation

Additive-mediated nucleation offers a convenient approach to the design of desired fiber networks. The molecule of a suitable additive should also satisfy the criteria given in Section 2.4.1.2. The manipulation of nucleation rate and hence the... 

Carter and Ward have identified a surface-mediated nucleation mechanism that involves a geometric shape match between planes of a ledge site on the substrate and planes of prenucleation aggregates. They have applied these concepts to the directed nucleation of polymorphs. ° This work provides us with the attractive possibility that a library of organic seeds can be used to control polymorphism, or to search for unknown polymorphs. Interpretations of molecular assemblies between solute and additive (seeds, substrate surfaces, excipients, and solvents) molecules may prove beneficial in selectively crystallizing or screening polymorphic systems. 

Monocyte-macrophages are the only principal cells of the immune system that can synthesize all the eicosanoids. T and B lymphocytes are interesting exceptions to the general rule that all nucleated cells produce eicosanoids. However, in a B lymphoma cell line, there is non-receptor-mediated uptake of LTB4 and 5-HETE. Interaction between lymphocytes and monocyte-macrophages may cause the lymphocytes to release arachidonic acid from their cell membranes. The arachidonic acid is then used by the monocyte-macrophages for eicosanoid synthesis. In addition to these cells, there is evidence for eicosanoid-mediated cell-cell interaction by platelets, erythrocytes, PMNs, and endothelial cells. 

Nucleation of calcium phosphate precipitation within the matrix vesicles is mediated by phosphatidylserine, which comprises about 8% of the phospholipids of the inner cytosolic membrane surface (Fig. 9.5a). Calbindin in the vesicle (Fig. 9.5b) may also contribute. Rapid mineral growth within the vesicle keeps the concentration of dissolved calcium and inorganic phosphate ions so low that additional Ca2+ and Pi ions spontaneously enter from the extracellular fluid via their respective transporters. Attached type II and type X collagens from cartilage in the growth plate enhance calcium ion transport and calcification during endochondral ossification (Fig. 9.5b). 

The greatest concern with this approach is lack of nucleation of the most stable form. If the starting API does not contain seeds of the stable form, a successful screen will produce nucleation of the stable form within the duration of the slurry experiments in at least one of the solvents. Lack of adequate solubility, or inhibition of nucleation due to solvent-solute interactions, may preclude transformation to the stable polymorph (Gu et al., 2001). Moreover, impurities or additives, even in trace amounts, can dramatically affect the rate of solvent-mediated polymorphic transformation (Gu et al., 2002 Okamata et al., 2004 Mukuta et ah, 2005). Thus, it is important to have starting API with the highest possible purity. Recrystallization prior to starting this or any type of polymorph screen can be used in an attempt to purify the API. While this could reduce an impurity that might stifle solvent-mediated transformation to the most stable polymorph, it should be noted that recrystallization from solution does not... 

Some organisms actively participate in the precipitation of minerals for example, sulfide-oxidizing microbes mediate the crystallization of native sulfur crystals, and microbes are also thought to participate in the precipitation of marcasite and iron oxide minerals. Additionally, the surfaces of organisms provide favorable sites for nucleation and growth of amorphous silica, metal sulfide and metal oxide crystals, and this facilitates mineral precipitation and fossilization of vent fauna (Figure 3). 

The transition was found to be mediated by nucleation and traveling of sharp fronts (Fig. 11a) that indicates a backward bifurcation, although hysteresis has not been identified directly. Rather, a sharp jump in the contrast (pattern amplitude) with increasing voltage has been detected, with some indications that a low contrast pattern already arises at voltages before the jump occurs in Fig. 1 lb. A preliminary, weakly non-linear analysis has exhibited a bifurcation, which is in fact weakly supercritical at low frequencies. If small changes of the parameters and/or additional effects are included (e.g. flexoelectricity and weak-electrolyte effects) the bifurcation could become a more expressed subcritical one [32, 33]. 

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