Gel crystallization

When dried in an oven, hydrated silica loses its water and becomes a desiccant (a substance that attracts water from the air). You find little packets of silica gel crystals in containers whose contents would be damaged by condensing moisture, such as vitamin bottles, consumer electronics, pepperoni, or leather products. 

It is usual to employ blue self-indicating silica gel crystals which turn a pale pink colour when regeneration by heating in an electric oven is required. 

The TBA gel crystallizes VPI-5 rapidly and reaches a final pH equivalent to that observed with DPA (see Figures 3 and 4). For reasons similar to those outlined in the DPA synthesis, we specify approximately 24 hours of crystallization time when using TBA (in Table I). Notice that the TBA-VPI-5 does not decompose in the mother liquor. We have observed that the TBA-VPI-5 is stable in the mother liquor for many days. It is interesting that the final pH of the TBA and DPA syntheses are approximately the same yet the TBA-VPI-5 is stable while the DPA-VPI-5 is not. 

This series of gels crystallized at 150°C for 3 days yields exclusively mixtures (Figure 5). When no Mg Is present, only the 5 and 18 structures are observed. At the lowest Mg concentration (0.05) the 18 structure coexists with the 34, and as the Mg concentration increases, the relative amount of MAPO-34 increases as the MAP0-5 decreases. At the highest Mg concentration examined the ratio of MAP0-5 to MAPO-34 Is about 2 1. 

Before moving on to our attempt to measure the complete double layer in a swollen propylammonium vermiculite with d = 43.6 A [18], we pause to note that (a) at ionic strengths relevant to cell fluids, namely c 0.12 M [19], the phase-transition temperature in the propylammonium vermiculite system is not so far away from our body temperature and (b) similar temperature-induced gel-crystal transitions are observed in many biochemical systems. An example is the deoxyhemoglobin molecule that causes sickle cell anemia [33], We also note that with both counterions, Tc decreases linearly with the logarithm of the salt concentration. 

FIGURE 9.6 Low-angle neutron scattering intensity as a function of scattering vector Q (A-1) for propylammonium vermiculite immersed in a 0.25 M propylammonium chloride solution at four different temperatures, T = 309 K (continuous line), 311 K (dashed line), 313 K (crosses) and 315 K (solid circles). The low-angle peak at 0.1 A-1 is due to a gel phase with a clay layer spacing of 60 A. The data therefore show the temperature-induced gel-crystal phase transition occurring between 311 and 313 K. Note that body temperature is 311 K and that cell fluids are typically 0.2 M. 

To form a CBC, control over the dissolution of the bases is crucial. The bases that form acid-base cements are sparsely soluble, i.e., they dissolve slowly in a small fraction. On the other hand, acids are inherently soluble species. Typically, a solution of the acid is formed first, in which the bases dissolve slowly. The dissolved species then react to form the gel. When the gel crystallizes, it forms a solid in the form of a ceramic or a cement. Crystallization of these gels is inherently slow. Therefore, bases that dissolve too fast will rapidly saturate the solution with reaction products. Rapid formation of the reaction products will result in precipitates and will not form well ordered or partially ordered coherent structures. If, on the other hand, the bases dissolve too slowly, formation of the reaction products will be too slow and, hence, formation of the gel and its saturation in the solution will take a long time. Such a solution needs to be kept undismrbed for long periods to allow uninterrupted crystal growth. For this reason, the dissolution rate of the base is the controlling factor for formation of a coherent structure and a solid product. Bases should neither be highly soluble nor almost insoluble. Sparsely soluble bases appear to be ideal for forming the acid-base cements. 

PE/i-PP blend films were prepared by gel crystallization from semidilute decalin solution as reported by Balta Calleja et al. (1990b), using ultra-high-molecular-weight PE (M2 = 6 X 10 ) and i-PP (M , = 4.4 x 10 ). In addition to the individual PE and i-PP homopolymer dry gels, Baltd Calleja et al. investigated PE/i-PP compositions of 75/25, 50/50 and 25/75. For all compositions a concentration of about... 

Probably the most widely used methods of reactant preparation are by crystallization, or by precipitation fi-om solution. Sometimes use can be made of a silica-gel crystal growth technique [2], especially when the compound formed is a sparingly-soluble precipitate from aqueous solution. 

The gel chemistry in the synthesis of microporous compounds is very complicated. Important parameters include the pH of the system, the composition, the solubility of components, aging time of the gel, crystallization temperature and time, the concentration... 

Until well after 1945 sorbents and many catalysts were based on amorphous carbons, silica gels and other oxide and mixed oxide gels. Crystals were not considered. However, in the background there existed a remarkable class of micrcporous minerals, the zeolites. In the early 1940 s quantitative single-step separations with zeolites as sorbents were abundantly demonstrated [1,2,3,], separations based not on boiling or freezing point differences between... 

It seems reasonable to suppose that the construction of the nucleus of a zeolite crystal may involve a more complex assembly process than is necessary for simpler substances whose unit cells are smaller and contain far fewer atoms, although there should be no difference in basic principles. The nature of the nucleation process in zeolite systems is also complicated by the nature of zeolite synthesis sols. These contain observable solid phases (amorphous gel, crystals) and components (cations, anions) in true solution. However, there is frequently also a colloidal component, invisible to the naked eye, which itself may be amorphous or crystalline. 

The current and voltage algorithm should account for the above stages of the formation process. Formation of the NAM results directly in the formation of Pb (cf., reaction schemes in Figs. 3.43 and 3.44) and the processes involved are not complicated. This is not the case, however, with the formation of the positive plates. As is evident from the reaction schemes presented in Figs. 3.14 and 3.15, the formation process of the positive plates involves a number of chemical reactions (reactions (19)-(21)), which result in the formation of Pb02 with a gel-crystal form. 

In some zeolite crystallization in heterogenous aluminosilicate systems, temperature is mainly a kinetic factor whereas the nature and composition of crystals are determined by concentrations and correlations of components. Studies of their chemical structure and the peculiarities of crystallization kinetics show that a solubility equilibrium exists between the solid and liquid phases of gels. Gel crystallization on heating is preceded by increased component concentrations in the liquid phase, which determines the composition of the zeolite crystals formed. Decreases in liquid component concentrations are compensated by dissolution of the solid phase of gels. This explains the linear rate of crystal growth during the first part of the crystallization process. Crystal size distribution for zeolite A and the linear rate of crystal growth indicate the autocatalysis of zeolite crystallization. 

These data show that increases in concentrations of all components in the liquid phase of gels take place on heating. This may be connected with the growth of solubility of the aluminosilicate skeleton with increasing temperature. Consequently, increases in concentration of the silicate and aluminate ions in the liquid phase precede the beginning of gel crystallization. 

Treating the amorphous powdered aluminosilicate with NaOH solution under dynamic conditions, Kerr showed (24) that the use of zeolite A crystals as seeds in the liquid phase considerably accelerates the conversion of the amorphous phase into the crystalline one. To understand the mechanism of aluminosilica gel crystallization, it is essential to know... 

Much evidence can be presented in favor of the proposed mechanism of aluminosilica gel crystallization, some of which was reported earlier (37, 39). The most convincing arguments supporting this mechanism are the growth of seed crystals in gels, dependence of the rate of crystal growth upon the concentration of components in the liquid phase, and dependence of the Si/Al ratio in crystals on the composition of the liquid phase. 

Zeolites containing phosphorus in the tetrahedral site in the framework have been synthesized. Phosphorus incorporation in a variety of structural types of zeolite frameworks has been achieved analcime, phillipsite, chabazite, Type A zeolite, Type L zeolite, and Type B ( ) zeolite. The syntheses and properties of some of the new aluminosilicophos-phate zeolites are described. The synthesis technique involves gel crystallization where incorporation of phosphorus is accomplished by controlled copolymerization and coprecipitation of all the framework component oxides, aluminate, silicate, and phosphate, into a relatively homogeneous gel phase. Subsequent crystallization of the gel is carried out at temperatures in the region of 80° to 210°C. Proof and mechanism of framework substitution of phosphorus is based on electron microprobe analysis, infrared spectroscopy, and other characterization. 

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