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Agarose, gelatin, and silica gel media have been surveyed as appropriate matrixes for the crystal growth of sodium bromate (NaBrO3). From pure aqueous solution, NaBrO3 crystallizes with a characteristic tetrahedral habit. We have demonstrated that a greater variety of habits (e.g., cubic, tetrahedral, polyhedral, and dendritic) can be reliably obtained from the growth in these gel media by controlling the solute concentration and the gel density. Agarose and silica matrixes suppress the nucleation as the gel density increases, while increasing gelatin concentrations seem to enhance crystal nucleation rates. These observations support the general hypothesis that the gel media influences the relative growth rates by controlling the solute nucleation and diffusion; however, little can be concluded about the possible chemical interaction between the polymeric backbone and the solute.

The crystallization of sodium chlorate (NaClO3) is a classic example of spontaneous chirality, since it is achiral in solution but adopts a chiral form in the solid state. While crystal growth of NaClO3 from pure aqueous solutions yields a 50:50 statistical distribution of d- and l-crystals, large enantiomeric excesses of either d- and l-crystals can be achieved by crystal growth in agarose gel, a naturally occurring chiral polysaccharide. The influence of gel density (0.1-0.75 wt %), temperature, and the diffusion of cosolvents on crystal distribution was discerned from statistical data obtained from 752 gel-mediated crystallization experiments yielding 12,384 individual crystals. These studies demonstrate that the magnitude and direction of the bias can be selectively engineered toward either d- or l-forms by changing the gelation conditions. Aqueous agarose gels infused with 48 wt % NaClO3 at 6 degrees C, favored the growth of d-NaClO3 crystals, with ee's reaching 22% at the highest gel concentrations. Crystal growth under methanol diffusion favored deposition of the opposite enantiomorph, l-NaClO3. The bias in the crystal distribution is enhanced at higher temperatures. Aqueous gels at 24 degrees C infused with methanol cosolvent favored l-NaClO3, with ee's reaching 53%. The changing magnitude and direction of the enantiomorph bias can be ascribed to differences in the agarose conformation and intermolecular interactions between the gel and crystal surfaces that inhibit the formation of the two enantiomers to different extents.

Crystal growth of a model molecular crystal system, asparagine monohydrate (Asn‚H 2 O), was explored in a variety of hydrogel media. Morphologies of crystals grown in agarose, carrageenan, and gelatin gels were compared with those of crystals grown from pure aqueous solutions, as well as those grown from solutions containing various concentrations of related monomers, ions, and cosolvents. The morphological differences were analyzed and interpreted on the basis of complementary interactions at various gel/crystal interfaces. In many cases, the gel polymers were found to have a greater impact on the crystal habit than when solution growth occurred in the presence of comparable concentrations of monomeric or ionic species. These studies clearly indicate that gel methods can provide (at least some systems) a viable alternative means to generate novel crystal morphologies of small molecules that may not be attainable under more conventional solution growth methods.

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