Gels, crystal growth

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. 

Controlled crystallization Natural and synthetic single crystals Crystal growth from solutions, gels and by coarsening of solids... 

The quality of crystals can be significantly improved when grown in gel media. Growth in gel reduces both convection of the protein molecules, thus favouring their slow diffusion to the crystal faces. 

Crystallization of TiO occurs during solvothermal treatment at high pressure, and the crystals grow to primary particle size through homo-coagulation. At this point, excess solvent partially suppresses further crystal growth as a result, the particle size becomes smaller than that in the sol-gel method. However, hydrolysis and condensation reactions occur very rapidly in sol-gel synthesis of transition metal oxides, therefore uniform and ultrafme products are difficult to obtain. 

HK Henisch. Crystal Growth in Gels. University Park Pennsylvania State University Press, 1973. 

It has long been observed that very beautiful crystals sometimes develop in gels. Wonderfully fine crystals of sugar frequently grow in ordinary fruit jellies. Many splendid mineral crystals are embedded in silica and appear to have developed while the silica was in the state of a gel. For this geological aspect of the subject, consult the papers of Cornu in the first five volumes of Kolloid-Zeitschrift. The experiments on crystal growth require some time for the proper development of the crystals, but the technique of the method is very simple and easily acquired. 

In zeolite systems chosen for study diffusion in the liquid phase and crystal growth on the crystal-liquid interface were the two major steps in converting gels to mordenite, zeolites A and X, the former being the rate-determining step for mordenite and the latter for zeolite X crystallization. In the mordenite system the effect of seed crystals, with surface areas per unit mass different by an order of magnitude, demonstrated the mechanism of nucleation on the seed crystal surfaces. The data support the hypothesis that crystal growth of the zeolite occurs from the solution phase rather than in the gel phase. 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

The gradual extension with time of the crystallization fields of each phase, starting from some preferential gel composition point, confirms the strong influence of the relative concentration of the components in the system on the crystallization of a specific zeolite (16). Comparison of the same system at another dilution shows generally that a longer time is needed for comparable crystal growth, but also a specific retreat of the... 

McCauley J.W. and Roy R. (1974) Controlled nucleation and crystal growth of various CaCC>3 phases by the silica gel technique. Amer. Mineral. 59, 947-963. 

Whether nucleation is homogeneous, heterogeneous or both there is strong evidence that in subsequent crystal growth the gel, if present, progressively dissolves and that the dissolved material then feeds the growing crystals. This also applies in successive transformations such as ( ) ... 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

Our earlier studies of zeolite-zeolite (10,26) and gel-zeolite (11, 12) transformations have shown that, under the assumption that the crystallization of zeolite is a solution-mediated transformation process (1-9) and that the crystal growth is size-independent (5,6, 12-16), the crystallization (transformation) kinetics can generally be expressed as ... 

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