Calcium phosphate precipitation in catanionic templates Be ´ne ´dicte Pre ´lot * , Thomas Zemb Laboratoire Interdisciplinaire sur l’Organisation Nanome ´trique et Supramole ´culaire, (LIONS), Service de Chimie Mole ´culaire, CEA Saclay, 91191 Gif sur Yvette cedex, France Available online 18 August 2005 Abstract 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 toobtain 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 self- assembly. 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. D 2005 Elsevier B.V. All rights reserved. Keywords: Calcium phosphate; Catanionic; Template; SAXS 1. Introduction Some phosphate rocks and particularly synthetic hydrox- yapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) can accept a series of cationic and anionic substitutions [1] in its structure. Furthermore, HAP may adsorb many various ionic species. In consequence, usage of textured phosphate has been proposed as a promising technology for remediation by the removal and immobilization of heavy metals from con- taminated soil and wastewater [2–4]. Since Mobil first reported the discovery of MCM-41 in 1992 [5,6], there has been a great surge in interest in the study of mesoporous materials. This silicate material and other transition metal oxide analogs consist of high surface area approaching 1400 m 2 g  1 , hexagonally packed array of inorganic tubules ranging from 2 to 10 nm in diameter. They are formed by templating inorganic oxides onto a self- assembled liquid crystal mesostructure [7]. Mesoporous materials are of great interest as catalysts and sorption media because of their large internal surface area, and uniform pore sizes large enough to accommodate hydrated ions. Mixtures of oppositely charged surfactants form a wide range of microstructures at various mixing fractions [8]. Above chain melting, catanionic surfactants can form spherical or rodlike micelles, lamellae, or vesicles with curvature varying with structural charge. Below chain melting temperature, catanionic surfactants form facetted crystals, from discs to polyhedra [9,10]. A great deal of effort is expended to understand the stability of such kind of 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.07.008 * Corresponding author. Present address: LAMMI, CNRS, Univ. Mont- pellier 2, Ba ˆt. 15-CC 015, Place Euge `ne Bataillon, 34095 Montpellier cedex 5, France. Tel.: +33 4 67 14 33 05; fax: +33 4 67 14 33 04. E-mail address: prelot@univ-montp2.fr (B. Pre ´lot). Materials Science and Engineering C 25 (2005) 553 – 559 www.elsevier.com/locate/msec