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The influence of polyaspartate additive on the growth and morphology of calcium carbonate crystals
The addition of low levels of polyaspartate to a supersaturated calcium carbonate (CaCO$\sb3$) solution leads to unusual morphologies in the inorganic phase. Spherulitic vaterite aggregates with helical protrusions, and distorted calcite crystals that contain spiral pits, have been produced. The helical particles are coated with an inorganic membrane that appears to be responsible for the helical twist. The polymer also causes deposition of thin CaCO$\sb3$ tablets and films on the glass substrate. Two distinct types of films are deposited; the first is a mosaic of calcite crystals, and the second is spherulitic vaterite. In situ observations of the crystallization reaction have determined that the thin-film morphology is a result of the phase separation of a hydrated CaCO$\sb3$/polymer liquid-precursor, whereby accumulation of isotropic droplets creates a coating on the substrate, and subsequent dehydration and crystallization yields birefringent CaCO$\sb3$ films. During the amorphous to crystalline transition, incremental growth steps lead to "transition bars" and sectored calcite tablets. This in vitro system was originally modeled after certain aspects of CaCO$\sb3$ biomineralization, in which the soluble proteins extracted from biominerals tend to have high levels of aspartic acid residues. Based on the similarities between features exhibited by the products of this system and those in biominerals, an argument has been presented to suggest that this polymer-induced liquid-precursor (PILP) process is involved in the morphogenesis of CaCO$\sb3$ biominerals. These features include the following: thin CaCO$\sb3$ tablets that grow laterally; tablets that express unstable crystallographic faces; non-faceted single crystals with curved surfaces; spatially-delineated single crystals; sectored calcite tablets; hollow-shell spheres; calcium carbonate cements; and magnesium-bearing calcites. This work has demonstrated that a means of morphological control can be accomplished through non-specific organic/inorganic interactions, whereby the polyelectrolyte transforms the solution crystallization to a solidification process. Not only are such findings of significance to the field of biomineralization, but a better understanding of the interactions between polymers and inorganic materials may be expected to lead to new strategies for crystal and particle engineering.
Gower, Laurie Anne, "The influence of polyaspartate additive on the growth and morphology of calcium carbonate crystals" (1997). Doctoral Dissertations Available from Proquest. AAI9809341.