The influence of surface active molecules on the crystallization of biominerals in solution

In the following article studies pertaining to "in situ" interactions of growing biogenic crystals (calcium phosphates, carbonates and oxalates) with, soluble, surface active molecules, including small, highly charged organic molecules, natural and synthetic polymers and synthetic surfactants, are discussed. Such interactions are at the roots of crystallization processes occurring in nature (biological mineralization) and in the controlled production of materials with well defined crystal structure, morphology and phase composition. The main characteristics of the crystals, including crystallographic data, and of the organic molecules, including their molecular structures, are briefly described. Most of the model crystals are crystal hydrates, whose dominant crystal planes are covered with continuous layers of structural water molecules (hydrated layer). The experimental methods reviewed include kinetic experiments determining induction times and/or the rates and rate controlling mechanisms of seeded and unseeded crystallization, techniques for the characterization of the nascent solid phase(s), and techniques, suitable for the assessment of interactions on the molecular level. Numerous examples show that the dominant mechanism underlying host crystal/additive interactions is selective adsorption of the additive at the crystal/solution interface, with the main driving forces ranging from purely electrostatic to highly specific recognition of crystal faces by the additive. Selective electrostatic interactions take place between growing crystals and flexible, highly charged small and macromolecules and/or surfactants because of differing ionic structures and charges of the crystal planes, some of them being shielded by hydrated layers. As in solution, surfactant molecules at high concentrations self-assemble into various superstructures (hemimicelles, bilayers) at the crystal/solution interface. Recognition of crystal planes by rigid small molecules and macromolecules with partial beta-sheet conformation (such as proteins or polyelectrolytes) is highly specific. It requires a dimensional fit between the distances of constituent ions protruding from the affected crystal plane(s) and the distances between functional groups that are part of the additive molecules. The consequences of selective additive/crystal interactions range from changes in crystal growth morphology to changes in the composition of the crystallizing phase. Examples showing the dual role of macromolecules as initiators and retarders of crystallization are discussed.