Crystallization of calcium carbonate beneath insoluble monolayers: Suitable models of mineral-matrix interactions in biomineralization?

The growth of inorganic materials below negatively charged monolayers is frequently considered to be a suitable model system for biomineralization processes. The fact that some monolayers give rise to oriented overgrowth of calcium carbonate crystals has been interpreted in terms of a geometrical and stereochemical complementarity between the arrangement of headgroups in the monolayer and the position of Ca ions in the crystal plane that attaches to the monolayer. However, comparative investigations of the growth of calcium carbonate beneath monolayers of macrocyclic polyacids have demonstrated that non-directional electrostatic parameters, such as the average charge density or the mean dipole moment of the monolayer, determine the orientation and the polymorph of the overgrowing crystals. The results show that it is possible to control the surface charge densities in monolayers by the appropriate design of amphiphilic molecules. A switch in polymorph occurs above a critical monolayer charge density at which aragonite or vaterite nucleation takes place, presumably via a kinetically controlled precipitation process.