Calcium phosphate precipitation in catanionic templates

Materials Science and Engineering C 25 (2005) 553 – 559 www.elsevier.com/locate/msec

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 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 HAPtemplate 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 hydroxyapatite (HAP, Ca10(PO4)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 contaminated 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 * Corresponding author. Present address: LAMMI, CNRS, Univ. Montpellier 2, Baˆt. 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: [email protected] (B. Pre´lot). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.07.008

study of mesoporous materials. This silicate material and other transition metal oxide analogs consist of high surface area approaching 1400 m2 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 selfassembled 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

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mixtures, their molecular structures as well as solution conditions (e.g., pH, ionic strength, and temperature) [10,11]. Particular behavior of catanionic systems is now used as structure-directing agents in the synthesis of mesoscopically ordered materials [12 –14]. Our objective in this work is to explore a catanionic formulation adapted to the precipitation of mesoporous hydroxyapatite. A first prerequisite is the existence in their phase behavior of a large domain with a hexagonal structure as a network of parallel cylindrical micelles. When HAP synthesis is performed, this hexagonal structure must be conserved during the precipitation. Using cationic surfactants (e.g., CTA+B ) or anionic surfactants (e.g., alkyl phosphate or alkyl sulfate) alone in formulation, hexagonal packing disappears during precipitation. This may be due to a poisoning effect of the surface with the cationic surfactant, or to a too strong interaction between the anionic surfactant and the Ca2+ ions. We describe here a simple and effective mean for controlling electrostatic interactions during mineral precipitation, which is the use of a catanionic system. Mixtures of anionic and cationic surfactants with a phosphate headgroup and a quaternary ammonium, respectively, are used here in order to obtain a template with adjustable surface charge and test the supposed ‘‘charge-matching’’ effect [15]. This effect, if real, should manifest itself by a strong dependence of the template shape on molar fraction (r = [anionic] / ([anionic] + [cationic])) used in the catanionic template, which governs the structural charge per unit area of the surfactant as well as the variability of the growing inorganic network. We first explore the effect of high ionic strength and pH variation on phase diagram. Synthesis of calcium phosphate is then performed using various volume fraction of template and independently various fraction of anionic and cationic.

WAXS (Weak Angle X-ray Scattering) and SAXS (Small Angle X-ray Scattering) apparatus. XRD patterns of powders are recorded, with a Philips X’pert diffractometer equipped with a diffracted-beam monochromator using CuKa radiation. SAXS experiments are performed in flat cells of 0.1 mm thicknesses with Kapton windows on a home-built Huxley-Holmes type, high-flux camera using a pinhole geometry [16]. The X-ray source is a copper rotating anode operating at 15 kW. The Ka1 radiation is selected and separated from high-energy radiations by the combination of a nickel-covered mirror and a bent, asymmetrical cut germanium [111] monochromator. Spectra are recorded with a two-dimensional gas detector of 0.3 m in diameter giving an effective Q-range of 0.2 to 4 nm 1. WAXS analyses are performed on a Guinier-Me´ring small angle X-ray camera. Such apparatus allows a larger Q-range (0.5 – 23 nm 1), and also to observe simultaneously peaks from the mesoscopic structures and from the crystallographic network. A molybdenum anode source, followed by a quartz monochromator, allows a monochromatic beam with high energy (E = 17 keV) to be obtained. An image plate is used as 2D detector [17]. TOC (Total Organic Carbon) analyses are performed on apparatus from Dohrmann Division (DC-180). The CO2 formed by the oxidation of the sample with potassium peroxodisulfate and under UV illumination is quantified with infrared spectrum. TG and DTA (thermogravimetric and differential thermal analysis) are performed in air, by using Setaram TAG balance, with heating rate of 10 -C/min. Nitrogen adsorption isotherms at 77 K are recorded on a step-by-step automatic apparatus (Coulter, SA3100). Prior to adsorption, samples are outgassed overnight at 110 -C. Specific surfaces areas (Ssa) are determined by applying the Brunauer – Emmet– Teller (BET) equation [18] and by using ˚ 2 for a cross section area of nitrogen [19]. 16.3 A 2.3. Synthesis conditions

2. Experimental 2.1. Materials Hydroxyapatite is prepared using NaOH from Fluka Biochemika Microselect, NaH2PO4I2H2O from LabosiFisher Scientific and CaCl2I2H2O from Prolabo. Anionic and cationic surfactants are used to template the HAP precipitation. The anionic is polyoxyethylene (POE) oleyl ether phosphate H+, supplied by Rhodia (HPCII), with phosphate head and tail with 18 C, and 5 EO: Lubrhophos LB400 (CAS 39464-69-2). The cationic surfactant is myristyl trimethyl ammonium bromide (MTA+B ) from Fluka Chemika (CAS 1119-97-7). 2.2. Methods The suspensions or powders obtained from these syntheses are analyzed using XRD (X-ray Diffraction),

2.3.1. Determination of the phase behavior of POE oleyl ether phosphate H+ The phase behavior of MTA+B is already known [20]. In the case of POE oleyl ether phosphate H+, no phase diagram is available in the literature. The study of phase diagram is performed in Na2SO4 at basic pH (> 11.5). Indeed, the HAP synthesis is achieved at high ionic strength and basic pH, by mixing NaH2PO4 0.3 M + NaOH 0.5 M, with CaCl2 0.5 M. Titration of the surfactant is carried out in order to determine the base quantity necessary to obtain pH > 11.5 and for replacing counter ion H+ by Na+. In the case of the POE oleyl ether phosphate H+, this value is 4 mequiv g 1 of surfactant. Without added salt (in water and without NaOH), the dissolution of surfactant is impossible. Therefore, the phase diagram of POE oleyl ether phosphate H+ is performed with three different ionic strengths: water, Na2SO4 0.3 M, Na2SO4 0.9 M and with NaOH 4 mequiv g 1 of surfactant for each ionic strength. The

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various microstructures are characterized with SAXS analysis and/or observed using with crossed polarizers. 2.3.2. Determination of the phase behavior of catanionic template In the cationic mixture, one objective is to keep the same curvature of the micelle as POE oleyl ether phosphate Na+. In that purpose, we couple the anionic surfactant (tail with 18 C, and 5 EO) with a cationic (alkyl trimethyl ammonium bromide) with a tail of approximately the same length. Because of the high viscosity of the mixture of POE oleyl ether phosphate Na+ with octadecyl trimethyl ammonium bromide (18 C) or cetyl trimethyl ammonium bromide (16 C), the myristyl trimethyl ammonium bromide (MTA+B with 14 C) is used. We checked that the hexagonal structure of POE oleyl ether phosphate Na+ is still preserved in the presence of MTA+B . The anionic surfactant POE oleyl ether phosphate Na+ displays hexagonal structure from 30 wt.%. Keeping this 30 wt.% as total concentration of both surfactants, increasing percentages of MTA+B are introduced in the mixture, in Na2SO4 0.9 M, with different molar fraction r = [anionic] / ([anionic] + [cationic], i.e. r = [ether phosphate Na+] / ([ether phosphate Na+] + [MTA+B ]). Because of the high viscosity of POE oleyl ether phosphate Na+, heating of the samples (bain-marie at 65 -C) is necessary to obtain homogeneous mixture. 2.3.3. Synthesis of HAP in catanionic media Precipitation of calcium phosphate without template is successfully performed by mixing NaH2PO4 0.3 M and NaOH 0.5 M with CaCl2 0.5 M. With such conditions, Ca/P ratio is 1.67, and pH sufficient to obtain HAP. Therefore, syntheses with template are performed in the same concentrations, with Ca/P= 1.67 and pH above 11.5 –12. Several initial template concentrations and various molar fraction of anionic and cationic are tested in order to identify the domain in which simultaneously HAP precipitates and surfactants remain hexagonally organized during the whole precipitation process. Five initial surfactant concentrations ([anionic] + [cationic]), from 1 to 5 wt.% (1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% and 5 wt.%) and five molar fractions r (0.33, 0.45, 0.50, 0.55 and 0.67), hence, 25 combinations are tested (fraction and surfactant weight), and the total concentration of surfactant varies from 0.022 M to 0.126 M. The sample contents of surfactants, PO43 , and Ca2+ (number of PO43 and Ca2+ available for 1 TA) are summarized in Table 1. Both concentrated surfactant solutions are first prepared separately, by mixing one surfactant in NaH2PO4 0.3 M, at basic pH. The acidity of each surfactant is basified by NaOH till the required pH necessary for HAP precipitation (11.5 – 12) is obtained. For the cationic MTA+B , we use a solution of NaH2PO4 0.3 M and NaOH 0.5 M; for anionic POE oleyl ether phosphate H+, NaH2PO4 0.3 M and NaOH dosed at 4 mequiv g 1 of surfactant are used. These two solutions are then mixed together, and mixed with NaH2PO4 0.3 M to

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Table 1 Repartition of surfactant, PO34 , and Ca2+ (number of PO34 and Ca2+ available for 1 surfactant) in HAP synthesis in catanionic templates, for 5 surfactant concentrations, and 5 molar fractions wt.% TA Molar fraction

1 2 3 4 5

r = 0.33

r = 0.45

r = 0.50

r = 0.55

r = 0.67

PO4 Ca

PO4 Ca

PO4 Ca

PO4

PO4

Ca

Ca

12 19.9 12.6 21.1 12.9 21.5 13.2 22 13.9 23.1 6 10 6.3 10.5 6.5 10.8 6.6 11 6.9 11.6 4 6.6 4.2 7.0 4.3 7.2 4.4 7.3 4.6 7.7 3 5 3.5 5.3 3.2 5.4 3.3 5.5 3.5 5.8 2.4 4 2.5 4.2 2.6 4.3 2.6 4.4 2.8 4.6

First column indicates weight fraction of catanionic used. The 2 samples for which a mesostructure is observed are underlined in bold and italic characters.

obtain 25 ml of the catanionic system at various weight concentrations and various fractions (r). Finally 25 ml CaCl2 0.5 M are added, and stirring is maintained during 5 min. The suspension is then stabilized during 5 days without stirring.

3. Results and discussion 3.1. Phase behavior 3.1.1. Phase behavior of POE oleyl ether phosphate H+ Below 30 wt.%, the samples are isotropic, and broad peaks are observed in SAXS analysis. The organization is micellar. For 30 and 35 wt.%, SAXS measurements (Fig. 1a) display sharper peaks, whose positions are easily indexable. q 1, q 2 and q 3 are positions of the peaks 1, 2 and 3. We observe the following relations q 2 = q 1 
3, and q 3 = q 1 
4, . . .. This is characteristic for a hexagonal organization of cylindrical micelles (hexagonal structure). These samples are also birefringent. Associated peaks are noted q 10, q 11, q 20, . . .. For POE oleyl ether phosphate H+ at 35 wt.%, q 10 = 0.91 nm 1, d = 6.9 nm, and a = 8 nm (see the cross section in Fig. 2a). As for other surfactant from the esters phosphate family, after basification by NaOH, there is quite no influence of ionic strength on phase behavior. Only one ionic strength (Na2SO4 0.3 M) is shown on this phase diagram (Fig. 2b). For higher surfactant concentrations, the product is highly viscous. This part of the diagram is not explored. 3.1.2. Phase behavior of catanionic system POE oleyl ether phosphate H+ and MTA+B Fig. 1b displays the influence of molar fraction r (r = [ether phosphate Na+ ] / ([ether phosphate Na+] + [MTA+B ])) on SAXS spectrum. One notice a first peak at 0.85 nm 1 (i.e. d = 7.4 nm, and a = 8.5 nm, for a maximum tail length = 2.4 nm). Even if the second peak of hexagonal structure is not clearly identified (1.44 nm 1), these samples are birefringent. This means that surfactant is

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a

b

10

Intensity (a.u.)

1

1

1.44 1.58

0.1

1.77 2.15

0.1

0.85

Intensity (a.u.)

r = 0.22 r = 0.39 r = 0.6 r = 0.86

30% w 35% w

0.91 1.04

0.01 0.01 0

1

2

3

q (nm-1)

0

1

3

2

q (nm-1)

Fig. 1. (a) SAXS produced by POE oleyl ether phosphate, in Na2SO4 0.3 M, pH > 11.5. (b) SAXS analysis of catanionic mixture POE oleyl ether phosphate Na+ and MTA+B , with 30 wt.% of total surfactant, in Na2SO4 0.9 M and at various molar fractions r = [anionic] / ([anionic] + [cationic]).

organized. This birefringency disappears below r = 0.22. As conclusion, we consider that the catanionic template keeps an organization of cylindrical micelles with hexagonal network for r  0.22. 3.2. Calcium phosphate synthesis 3.2.1. Microstructural characterization and identification of the precipitate Before the analysis of the 25 dried powders and their structure, a quick analysis with WAXS apparatus of the suspensions evidences that only few samples display signature characteristic for mesoporous material. Two cases are encountered. Typical shape of spectra is illustrated in Fig. 3a on which only certain conditions are shown. The monotonous decrease appearing on log scale with a slope of å 3.3 is characteristic of a precipitation uncontrolled or reaction limited, which does not produce measurable periodicity. In the second case, in some particular conditions (low surfactant concentration and high fraction r), the first order of the lamellar mesostructure of precipitate may be noticed: first peak at 3.3 nm 1, and second at 6.7 nm 1 (å 2  3.3 nm 1).

a 2R

In order to determine if those peaks are due to the surfactant, or to a mineral organization around the template, TOC analyses are performed. TOC analyses on the supernatants obtained after separation (centrifugation 20 min, 8700 rpm, and then filtration Millipore Millex 0.45 Am, and 0.22 Am) let us identify the samples in which a large amount of surfactant is trapped in the solid phase, and does not stay in solution (Table 2). Only 8 precipitates from the 25 samples display carbon amount below 20% of the initial concentration. These solids are synthesized with 1 or 2 wt.% surfactant, and with fraction r 0.45, 0.50, 0.55, or 0.67. This means that charge matching in this peculiar case occurs in the range + 3 to 10 AC cm 2 structural charge. We considered that the 17 other residues, which contain less than 80% in the solid, are out of interest. The 8 precipitates are washed 5 times with MilliQ water, and 5 times with water – ethanol mixture (1/2 –1/2) (separation by centrifugation). They are then dried in oven (60 – 70 -C). Due to the high signal to noise ratio observed with WAXS analysis (Fig. 3a), solids are characterized with XRD expressed with wave vector q (nm 1). Fig. 3b shows the XRD analysis of solids that exhibit characteristic peaks after drying and washing procedure. Those two precipitates are

b

H

L1

t H/L1 a d

10 20 25 30 35 5 % weigh in POE oleyl ether phosphate Na2SO4 0.3M, pH>11

Fig. 2. (a) Cross section of the hexagonal organization of cylindrical micelles, also describing the porous material. R is the pore radius, t is the wall thickness, d is the inter-reticular plane distance, and a is the cell parameter. a = 2/
3d. (b) Room temperature phase behavior of POE oleyl ether phosphate Na+ in Na2SO4 0.9 M.

B. Pre´lot, T. Zemb / Materials Science and Engineering C 25 (2005) 553 – 559

a 10

b r =0.55 1000 r =0.67

10

3.3

0.1

1

1

6.7

10

100

5.8

5%w, r = 0.33

6.7

0.1

10

1%w, r = 0.55

0

5

10

15

Intensity (a.u.)

Intensity (a.u.)

1

3.3

557

20

25 0

q (nm-1)

1

2

3

4

5

6

7

8

q (nm-1)

Fig. 3. (a) WAXS of 2 precipitates synthesized in catanionic mixture (POE oleyl ether phosphate Na+ and MTA+B ). One has trapped more than 80% of initial surfactant (1 wt.%, r = 0.55), and displays mesostructure, whereas the second one contains less than 80% (5 wt.%, r = 0.33), and exhibits no characteristic peaks. The WAXS experiment is made on the whole suspension, without isolating the powder. (b) XRD of samples synthesized in catanionic mixture (POE oleyl ether phosphate Na+ and MTA+B ), and which have trapped more than 80% of initial surfactant: 1 wt.% of initial template, and mole fraction r = 0.55 and r = 0.67. Experiments are made on washed and dried powder.

synthesized with the following parameters: 1 wt.% surfactant and r = 0.55 or 0.67. The positions of those peaks are: q 10 = 3.3 nm 1, q 11 = 5.8 nm 1, q 20 = 6.7 nm 1. These values are characteristic for a hexagonal organization. The first peak corresponds to a distance d = 1.9 nm, i.e. a cell parameter a = 2.2 nm. This Fa_ value is too small for the pore diameter of an efficient mesoporous material. It might be due to a micropore network, but the BET measurement after calcination shows that samples are not microporous (see below). It cannot be attributed to crystallographic network, because in the case of hydroxyapatite (JCPDS No. 9-432) ˚ , i.e. q = 0.77 nm 1. the first peak is indexable at d = 8.17 A We stress here that according to cross section shown in Fig. 2a, the cell parameter Fa_ in classical hexagonal structures can only be larger or equal to twice the chain length : . However, the microstructure cannot be the one shown here, since we have the length of surfactant : = 2.4 nm and an Fa_ parameter of 2.2 nm, and then we do not have 2: < a. Because : ¨ a, this possible microstructure could be intertwined hexagonal fiber structures with monolayer of surfactant. Such structures consisting of regularly curved monolayers (and not micelles or cylinders) have been

Table 2 TOC analysis of supernatants obtained from HAP synthesized in catanionic surfactant templates, for 5 surfactant concentrations, and 5 molar fractions wt.% TA

r = 0.33

r = 0.45

r = 0.50

r = 0.55

r = 0.67

1 2 3 4 5

79.6 74.6 66.6 60.6 53.5

98.7 83.5 74.2 65.2 57.2

99 86.6 76.6 62.6 56.3

98.7 97.1 75.6 63.4 57.9

98.7 99.1 74 71.9 67.4

The percentage of surfactant trapped in powders is calculated versus the initial concentration. First column indicates weight fraction of catanionic used. The 8 samples, which have trapped more than 80% of initial surfactant, are underlined in bold and italic characters.

predicted [21,22]. These structures have been also found in asymmetric peptides by Aggeli et al. [23]. 3.2.2. Influence of calcination The suspension is then washed and calcined to eliminate the surfactant and to ripen and stabilize the structure. Thermal analyses are performed, in order to choose precisely the calcination temperature. Fig. 4a shows the TGA (Thermogravimetric Analysis) measurements on surfactant, on hydroxyapatite, and on the powder obtained from catanionic syntheses. Following these thermograms on the surfactant or on the mesophase (mixture of calcium phosphate and template), the loss of weight is insignificant after 450 -C. Cracking and release of the surfactant is considered to be achieved at 450 –500 -C, and the possible porosity is then free of the template. Taking into account these thermal analysis, calcinations at various temperature (400 -C, 450 -C, 500 -C, 550 -C, 600 -C) and various heating rates (2, 5 and 10 -C min 1) are performed to check the stability of the final material. Depending on the sample and the temperature, the loss of weight varies from 16% to 23%. In Fig. 4b SAXS patterns evidence that the mesostructure first peak (3.3 nm 1) observed on the suspension or on the washed powder withdraws after calcination at 400 -C. Such behavior is observed for all samples, and is independent of the calcination temperature or the heating rate. In order to check if the material displays high specific surface area and porosity without mesostructure, the specific surface area is determined from nitrogen adsorption isotherms with the BET method. After calcination at 400 -C, the BET surface is about 130 m g 1. This value is approximately the same order of magnitude as the sample synthesized in homogeneous conditions (without surfactants) (110 –120 m2 g 1). Otherwise, the sample is neither microporous nor mesoporous. The release of surfactant by

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a

b

0 HAP

10

6.7

3.3

TG (%)

Mixture

Suspension

5.8 -40

18.1

22.3 1

Washed powder

-60

Surfactant

0.1

Calcined powder

-80 0

Intensity (u.a.)

-20

200

400

600

800 1000

Temperature (°C)

0

5

10

15

20

25

q (nm-1)

Fig. 4. (a) Thermal analysis (TGA/TDA) for the cationic mixture (r = 0.55), for HAP (hydroxyapatite), and for HAP synthesized in catanionic mixture (1 wt.%, r = 0.55). (b) WAXS analysis of suspension, washed and calcined powder synthesized in catanionic mixture (1 wt.%, r = 0.55). Three Bragg peaks show hexagonal structure of precipitate while arrows on the left side identify characteristic peaks of HAP.

soft calcination (400 -C) leads to the removal of the hexagonal mesostructure, but surface area remains constant. On the other hand, for samples calcined at higher temperatures, specific surface areas are even smaller than for the homogeneous synthesis (20 – 30 m2 g 1). The decrease of specific surface area is not due to sintering of hydroxyapatite, because hydroxyapatite is stable at such temperature [24]. Otherwise, the collapse of the hybrid organic – inorganic structure could be responsible for this decrease of specific area. Table 1 shows that for these two samples, more than 13.2 PO43 and 22 Ca2+ are available for one template molecule. Consequently, the content of the two mesostructured powders (1 wt.% and r = 0.55 or 0.67) is mainly mineral. Furthermore, WAXS displays the first peaks of the HAP crystallographic network. The small amount of template is removed, and particles may collapse to form large aggregates of lower specific surface area.

anionic component in the template. For samples with low amount of surfactant (1 wt.%) and an excess of anionic component (r = 0.55 or 0.67), X-ray analysis evidences peaks which are characteristic of a hexagonal network, but with low repetition distance. This means that charge matching in this peculiar case occurs in the range + 3 to 10 AC cm 2 structural charge. Such structure, where the repetition distance is much lower than twice the surfactant chain length, has not yet been described in surfactant selfassemblies. This is not truly mesoporous material; nevertheless, such structure may be made of intertwined hexagonal fiber structures with monolayer of surfactant. The precipitates are not rigid and stable enough to withstand calcination. After calcination, the Fhexagonal_ peaks disappear, and the specific surface area is about 130 m2 g 1.

References 4. Conclusion The objective of this study is to precipitate mesoporous calcium phosphate (hydroxyapatite, HAP) with high specific surface area. Catanionic mixture is used as structuredirecting agents in the synthesis of mesoscopically ordered materials. Catanionic surfactants with phosphate headgroups (polyoxyethylene oleyl ether phosphate) and a quaternary ammonium (myristyl trimethyl ammonium bromide) are used to template the precipitation, and test the supposed ‘‘charge-matching’’ effect using adjustable surface charge. The phase behavior of anionic template shows that ether phosphate is micellar up to 30 wt.%, and then displays a hexagonal structure. Such organization of cylindrical micelles in hexagonal network is still preserved in the presence of large quantity of cationic component (30 wt.%, and down to molar fraction r = 0.22). Synthesis of HAP is then performed using various volume fractions of template and independently various mole fractions of

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