Influence of low-molecular-weight and macromolecular organic additives on the morphology of calcium carbonate

Abstract

The influence of a range of soluble biological and related molecules on the crystallization of CaCO₃ from aqueous supersaturated solution has been studied by optical and scanning electron microscopy and X-ray diffraction. The efficacy of monofunctional additives to induce morphological changes increased with overall anionic charge. For anions of the same charge, the effect was reduced with decreasing partial charge density on the oxygen atoms of the ligand. Additional factors, such as the distance between ligands and conformation were important for multifunctional molecules. Orthophosphate, sulfate, various phosphonates, a polysaccharide associated with coccoliths of Emiliania huxleyi and alginate interacted specifically or pseudo-specifically with crystal faces approximately parallel to the c axis indicative of a bidentate binding motif. Phenyl phosphonate stabilized {0112} faces, inferring a tridentate interaction due to steric constraints. The bone protein, osteocalcin was found to be a non-specific inhibitor whereas a bone proteoglycan monomer and polygalacturonate had minimal morphological effect. A carboxylated hyperbranched polymer gave oriented nucleation owing to partial segregation of the macromolecule at the air/water interface. Fish anti-freeze glycopeptides and polyvinyl alcohol induced the precipitation of vaterite possibly by affecting the kinetics of cation dehydration. Multifunctional additives such as diphosphonates, the coccolith polysaccharide and alginate were also effective at promoting crystal aggregation.