Polymorph and Orientation Controls of Calcium Carbonate Crystals Achieved by Organic Matrices

Crystallographic properties such as polymorphs and crystal orientations of calcium carbonate formed in biological systems can be controlled by interactions between organic matrices and inorganic substances. We have attempted to reveal such control mechanisms, using a polymerized Langmuir-Blodgett film of 10,12-pentacosadiynoic acid (PDA) as an organic matrix with highly regulated molecular alignment. Only calcite crystals were precipitated on the polymerized PDA film, even though in presence of Mg2+ ions which were reported to enhance aragonite formation in previous works. The calcite precipitates were aligned on the PDA film with their a-axes in parallel to the polymerization direction. Calcite structure does not give a reasonable match over the molecular alignment of the PDA film along its a-axis. A concerted mechanism of geometric matching and stereochemical effect should determine the polymorphs and directions of calcium carbonate crystals on the organic matrices.