Crystal vapor

The octahydrate is prepared by dissolving BaO in hot water for several hours, filtering off undissolved impurities, then cooling the solution to effect crystallization. Vapor pressure data for the octahydrate (20) is... 

Chowhan [9] defined different pathways of physical instability of tablet formulations. These physical paths may involve one or more complex physical processes, e.g. change in polymorphism, crystallization, vaporization and adsorption. These pathways and thus the physical tablet parameters, are influenced by different types of variables formulation variables (e.g. solubility and hygroscopicity), in-process variables (e.g. moisture content) and aging variables (e.g. temperature and relative humidity). 

We discussed iu Section 3.12 that there is a mutual relation among spherulitic, dendritic, hopper, and polyhedral crystals, with respect to the driving force. We will see how these mutual relatious appear in real systems, using, as representative examples, low-temperature suow crystals (vapor phase growth) and high-temperature silicate crystals growiug iu silicate solutiou phases. 

An alternative approach is taken in the production of monosodium glutamate (MSG) which, unlike interferon, is secreted into the fermentation broth. The stages of downstream processing for MSG are shown in Figure 14.3. Again, a variety of unit operations, including centrifugation, crystallization, vaporization, and fixed-bed adsorption, are used in this process. 

MOTION OF CRYSTAL/VAPOR AND CRYSTAL/LIQUID INTERFACES... 

Another type of motion of crystal/vapor interfaces occurs when a supersaturation of vacancies anneals out by diffusing to the surface where they are destroyed. In this process, the surface acts as a sink for the incoming vacancy flux and the surface moves inward toward the crystal as the vacancies are destroyed. This may be regarded as a form of crystal dissolution, and the kinetics again depend upon the type of surface that is involved. 

Figure 12.1 Various point and line defects on a vicinal crystal/vapor surface. 1 See Appendix B for further discussion of the structure of surfaces.

Under these conditions, the growth velocity is simply proportional to the undercooling, and the situation resembles crystal growth at a rough crystal/vapor interface where the growth rate is proportional to the excess vapor pressure (Fig. 12.4).5... 

Interfaces may be sharp or diffuse. A sharp interface possesses a relatively narrow core structure with a width close to an atomic nearest-neighbor separation distance. Examples of sharp crystal/vapor and crystal/crystal interfaces are shown in Figs. B.l and B.2. 

Interfaces may also be classified broadly into homophase interfaces and heterophase interfaces. A homophase interface separates two regions of the same phase, whereas a heterophase interface separates two dissimilar phases. Crystal/vapor and crys-tal/liquid interfaces are heterophase interfaces. Crystal/crystal interfaces can be either homophase or heterophase. Examples of crystal/crystal homophase interfaces are illustrated in Figs. B.2, B.4, and B.5. Examples of heterophase crystal/crystal interfaces are shown in Figs. B.6 and B.7. Figure B.6o shows an interface between f.c.c. and h.c.p. crystals where the small mismatch between close-packed lll fcc... 

Figure 20. Plots of DRH relative to bulk crystal (viz. 75%) versus crystal size for various values of crystal/vapor surface tensions. Note the sensitivity of DRH to surface tension. Adapted from Mirabel et al. (2000). Used by permission of the American Chemical Society.

Polydiacetylenes. Dimethanol-diacetylene (H0CH2C=C-C=CCH OH) (DMDA) was used as received from Farehan Chemical Co. Diphenyl urethane-diacetylene -NH-COO-(CH2) -C=C-C=C-(CH2) -OOC-NH- < ) (TCDU) wasused as received from Allied Chemical Co. Both monomers were successfully deposited as epitaxial films on various alkali halides from vapor and solution phases. The following conditions were used for these crystallizations vapor phase DMDA = 40 C at 10 torr, vapor phase TCDU = 150 C at 10 torr, solution (1 wt % in toluene) phase DMDA = 70 C for 30 minutes, solution (.4 wt % in ethyl acetate) phase TCDU = 30 C for 5 minutes. 

Reactions in rigid environments and application to reactions in crystals or at crystal-vapor interfaces... 

Figure 18.55 shows the predicted steady-state temperature and meridional flow streamlines for (a) no melt convection and stationary crystal, (b) no crystal rotation with buoyancy-induced melt flow, (c) modest rotation and mixed melt convection, and (d) high rotation rates with mixed melt convection [213], The emissivity of the crystal-vapor interface of the system was specified to be 0.3, while the melt-crystal interface emissivity was set to 0.9 for the GGG crystal of refractive index 1.8. Additional property values and geometric details are listed elsewhere [215]. The crucible diameter is 200 mm, the crystal diameter is 100 mm, and thermocapillary convection was not included in the analysis. 

Reddish-brown vapor. Two solid modifications exist. One is reddish-brown to black-red, unstable (especially when exposed to visible, ultraviolet or infrared light) and can be stored only by chilling the freshly condensed product in liquid nitrogen with exclusion of light. Rhombic or monoclinic crystals. Vapor pressure (0°C) 24 mm. 

---