Controlled crystallization of vaterite from viscous solutions of organic colloids
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Precipitation of calcium carbonate from hydroxyethyl cellulose (HEC) containing aqueous solutions of CaCl2 and Na2CO3 resulted in an uncommon polymorph, vaterite. In contrast to precipitations without HEC, crystallization in the presence of HEC led to a shell-shaped body consisting of organized vaterite platelets. The artificial shell is a composite of stacks of vaterite and about 2% HEC. In the experimental arrangement used, HEC controlled both nucleation and crystal growth of vaterite. Concentration of HEC also affected the platelefs thickness which in turn influenced the shell's morphology as well. These results demonstrate the importance of organic-inorganic interface in controlling crystallization.
I. INTRODUCTION
Biomineralization in living organisms leads to the formation of inorganic-organic composites. Although the content of the organic component in a typical composite is very low (on the order of 1 wt. %), it exerts tremendous control of the mineralization process leading to particles of uniform size, novel crystal morphology, and specific crystallographic orientation.1 The study of biomineralization is expected to bring new ideas and more impetus to the design of novel materials, especially for preparation of low temperature ceramics and cements. It is intriguing to be able to produce materials with novel structural features without resorting to high temperature treatment which is commonly used in ceramic and glass technologies. A good example for this is abalone shells, consisting of 99% of aragonitic CaCO3 and possessing a f lexural strength of about 200 MPa.2 Biomineralization is the result of a complex panoply of interactions at the inorganic-organic interface, involving basically mineral ions and organic functional groups. The latter are usually those of proteins and polysaccharides. It is the protein-polysaccharide complex that is primarily responsible for the formation of a structural frame—a matrix with regions where crystals can grow.3 Such site-directed nucleation involving molecular specificity at the organic/crystal interface also governs the size of the crystals. Thus the organic matrix is seen as fully responsible for number and distribution of nucleation sites. Only those accessible to the ions, i.e., located at the surface, are operative. Organic matrix inhibits nucleation within the bulk. Thus, dea)
Present address: Slovak Technical University, Department of Silicate Chemistry, CS-812, 37 Bratislava, Czechoslovakia. b) Also with The Department of Agronomy. 2928
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 12, Dec 1990
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pending on the tertiary structure of the polymer, the same polymer can either inhibit nucleation or promote it. When a polymer is in solution, its adsorption onto a subcritical nucleus inhibits further growth, while an insoluble polymer will provide a site for nucleation.1'4 The propensity of the biopolymeric matrix to promote nucleation—that is, its degree of preorganization, seems to be the critical condition in the process of
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