Calcium phosphate precipitation in catanionic templates

A simple and effective mean for controlling electrostatic interactions during mineral precipitation in water is the use of a "catanionic" template. Such strategy is used to precipitate calcium phosphate (hydroxyapatite or HAP) and obtain mesoporous materials. Mixtures of catanionic surfactants with phosphate head-groups (polyoxyethylene oleyl ether phosphate) and a quaternary ammonium (myristyl trimethyl ammonium bromide) are used in order to obtain a template with adjustable surface charge and test the "charge-matching" effect. This effect manifests by a strong dependence of the template shape on molar fraction, which governs the charge per unit area of the surfactant as well as the growth of inorganic network. We first explore the effect of high ionic strength and pH variation on phase diagram of template. A hexagonal structure is observed for anionic surfactant, and such organization is still preserved in the presence of large quantity of cationic component. Synthesis of HAP is then performed using independently various volume fractions of template and various mole fraction of anionic component in the template. For samples with low amount of surfactant and an excess of anionic component, TOC analysis shows more than 80% of the added surfactant is trapped in the precipitate. These samples display in SAXS three peaks that are characteristic of a hexagonal structure. Such structure, where the repetition distance is much lower than twice surfactant chain length, has not yet been described in surfactant selfassembly. This must be a monolayer microstructure, but symmetry group is not known since higher orders cannot be detected. The HAP-template hybrid structure disappears after calcination, and the BET surface of calcined powders is smaller than for HAP particles synthesized in homogeneous conditions. (c) 2005 Elsevier B.V. All rights reserved.