Seeding heterogeneous

Although in some cases heterogeneous seeding can be applied, the seeds in this chapter are understood as crystalline material of the solute to be crystallized. These are usually in milled form, both with respect to chemical purity and with respect to the solid-state form (polymorphism or solvate form). 

Falcaro et al. reported how functional ceramic particles can be used as a heterogeneous seed for the nucleation of MOFs [25]. The authors developed a seeding method using a number of different ceramic materials as heterogeneous seeds to spatially control the position as well as the growth rate of MOFs [25,81,82]. 

The concept of using seeds to induce controlled nucleation of MOFs has been successfully implemented to master MOF formation on a variety of substrates. The nucleation has been obtained using nano- or microparticles of the MOF itself ( homogeneous seeding) however, more recently, MOFs have been grown from nano- or micro-ceramic seeds ( heterogeneous seeding). 

When a process is continuous, nucleation frequently occurs in the presence of a seeded solution by the combined effec ts of mechanical stimulus and nucleation caused by supersaturation (heterogeneous nucleation). If such a system is completely and uniformly mixed (i.e., the product stream represents the typical magma circulated within the system) and if the system is operating at steady state, the particle-size distribution has definite hmits which can be predic ted mathematically with a high degree of accuracy, as will be shown later in this section. 

Figures 4c and 4d demonstrate OCT images of two seeds out of the GMF group after 60 minutes when turgescence has started. One can distinctly detect darker layers of watered zones and the heterogeneous zones of water absorption. The water absorption zones merged in a united system of microcapillary vessels with sizes 50-10 pm. Individual differences in the structure of the water absorption zones in the seeds are also clearly seen.

Figures 4e and 4f show OCT images of two control seeds after 60 minutes when turgescence has started. Similar to the GMF seeds, individual structural differences of the seeds are clearly visible here. However, after the same time period the heterogeneous absorption zones (Fig. 4f) are less expressed than in the GMF seeds (Fig. 4d). The bright area corresponding to highly scattering regions (Fig. 4d) is narrower (about 100 im) in the control than in GMF seeds (about 200 pm). Thus OCT imaging of barley seeds can distinctly visualize water absorption processes within the first hour, as well as, individual variations in different seeds. The variations reflect the phenomenon of biological variability of seeds at the tissue level.

Furthermore no nucleation was observed in the experimental time in any ampoules at any supersaturation levels if there was no stirrer in the ampoule. This implies that the heterogeneous nucleation was Induced by the presence of the stirrer, probably by enhancing the nucleation at the sites where fine scratches exist. This fact suggests that the solution is stable if there are no possible seed crystals in the system. The existence of the D-enantiomer in the seed crystals is therefore very likely to initiate the secondary nucleation after they grow to attain sufficient sizes, which results in the sudden purity decrease of the product crystals. 

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