Influence of surfactantassembly on the formation of calcium phosphate materials—A model for dental enamel formation

Abstract

Novel calcium phosphate materials were synthesized from solutions containing the surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), water and oil. A range of morphologies was obtained by varying the relative concentrations of the solution components. A material with structural features resembling tooth enamel was produced from a highly viscous reaction solution. This material consisted of bundles of co-aligned filaments, 750 nm–1 µm in length and 250–350 nm wide. Each bundle contained 10–20 filaments, identified as hydroxyapatite crystals. Electron diffraction of the bundles resulted in an arc pattern indicative of elongated aligned crystals, and similar to that known for dental enamel. Systems such as this may be used as models to gain insight into the mechanisms involved in the biomineralization of tooth enamel. Importantly, we have provided new evidence in support of the hypothesis that hierarchical structures in nature result from cooperative interactions between organic assembly and crystal growth. Amorphous calcium phosphate nanoparticles, hollow spheres, spherical octacalcium phosphate aggregates of plates and elongated plates of calcium hydrogen phosphate dihydrate were also obtained under different sets of conditions that altered AOT assembly and solution viscosity. These latter findings illustrate further the significant influence of organic assembly on the formation of calcium phosphate materials.