Hydroxyapatite (HAp) particles, a potential starting material for bone substitutes, with nanopores were synthesized in the presence of cetyltrimethylammonium bromide (CTAB) and P123 as cationic and nonionic surfactants as the structuring units. Effect of nonionic surfactant concentration on surface areas is also investigated. Based on N2 adsorption-desorption isotherms investigation, surface area increased up to 50 m2/g by using P123 and 147 m2/g by using CTAB as porosity agent. Pore structure remained even after the removal of surfactant and calcinations at
Hydroxyapatite, Ca10(PO4)6(OH)2 (denoted as HA), is a form of bioceramics material [
Porosity in a biomaterial implant allows for the possibility of growth of natural bone. In recent years, HA has also been used as an acidic catalyst for Friedel-Crafts alkylation and Knoevenagel reactions [
The synthesis of mesoporous materials can be achieved using a supramolecular templating technique. In 1992 [
Surfactant-templated mesoporous materials have some specific features such as high surface areas and high adsorption capacities including the uniformity and periodicity of tunable mesopores [
However, it is difficult to synthesize nonsilica-based mesoporous oxides, especially calcium phosphate species, are preferentially crystallized in aqueous solutions and then cannot interact with surfactant molecules because of the electrostatic mismatching. Several research groups have reported mesostructured and mesoporous calcium phosphates prepared using surfactants. However the resultant products contained crystalline impurities such as brushite (CaHPO4·2HO), monetite (CaHPO4) and hydroxyapatite (Ca10(PO4)6(OH)2) [
In this study, we attempt to achieve the direct crystallization of mesoporous hydroxyapatite by chemical precipitation method using CTAB and P123 as structure-directing agents.
We succeeded in synthesizing a mesoporous hydroxyapatite with high surface area (140 m2/g). HAp powders were synthesized using the micelle as a template system which was used as the surfactant. The effect of the surfactant on the BET specific average surface area of the particle was studied by varying the P123 surfactant concentration as 0.01, 0.02, and 0.03 M.
Calcium chloride (CaCl2, Merck co.), calcium hydroxide (Ca(OH)2, Merck co.), and diammonium hydrogen phosphate ((NH4)2HPO4, Merck co.), phosphoric acid (H3PO4 85%) were used as calcium and phosphorus sources, respectively.
CTAB and P123 as surfactant were used to synthesize nanoporous hydroxyapatite powders of this study. The chemical structures of P123 and CTAB are shown in Figure
The chemical structure of (a) CTAB (b) P123.
In a successful synthesis run to yield only single-phase (Ca10(PO4)6(OH)2, HA), 5.91 gr diammonium hydrogen phosphate ((NH4)2HPO4) and 5 gr CTAB were dissolved in 70 mL deionized water at room temperature with stirring for 1 hr pH that was adjusted to 10 using sodium hydroxide. 8.21 gr of CaCl2 was weighed into 75 mL of de-ionized water. Subsequently, the CaCl2 solution was added dropwise to the solution mixture, yielding a milky suspension, which was refluxed for 24 h.
The precipitate was filtered and washed several times with distilled water to remove contaminated ions and surfactant. It was dried at 100°C for 24 h. Calcination of powders was carried out at 400°C for 4 h to yield a white powder.
Nanoporous hydroxyapatite powders are also synthesized by using P123 as structure directing agent. Different amount of surfactant P123 was first dissolved in 50 gr of deionized water with overnight stirring at room temperature. Calcium hydroxide Ca(OH)2 and phosphoric acid (H3PO4 85%) were used as calcium and phosphorus sources, respectively.
Aqueous solutions of calcium and phosphorus precursors were prepared by dissolving 2.22 gr of Ca(OH)2 and 1.764 gr of H3PO4 into 10 mL of deionized water. The calcium and phosphorus precursor solutions were added into the surfactant solution which was refluxed for 24 h.
The precipitate was filtered and washed several times with distilled water to remove contaminated ions and surfactant. It was dried at 100°C for 24 h. Calcination of powders was carried out at 400°C for 4 h to yield a white powder.
The X-ray powder diffraction patterns were recorded on a Philips 1830 diffractometer using Cu
Figure
XRD pattern of as-dried nanoporous hydroxyapatite.
The XRD pattern demonstrates that pure and well-crystalline HA is the only phase present.
The peaks within small angle range (2 theta 1–10°) provide information on the arrangement presented by porous structure [
For LAXRD (Figure
LAXRD pattern of as-prepared nanoporous hydroxyapatite using different P123 concentration.
It is believed that
The FTIR spectra of nanoporous hydroxyapatite synthesized using P123 and CTAB as template are shown in Figure
FTIR spectrum of as-prepared nanoporous hydroxyapatite using (a) P123 (b) CTAB.
Adsorption-desorption isotherms and pore size distributions of mesoporous hydroxyapatite calcined at 400°C are shown in Figure
Nitrogen adsorption-desorption isotherms of nanoporous hydroxyapatite after calcination at 400°C.
Figures
Adsorption-desorption isotherms of nanoporous hydroxyapatite after calcination at 400°C (a) 3 wt% P123, (b) 15 wt% P123, and (c) 30 wt% P123.
This resulting HAp precipitate interacts with the micelles to form a gel-like mass by electrostatic interaction. This leads to the formation of an organized structure, where the HAp precipitate is templated by micelles and solidified to form a matrix. During heat treatment, the micelle template decomposes into gases, such as CO2 and HO, and leaves pores in the product. This type of mechanism is known in the silica-based inorganic system.
The surface area of the final powders is controlled by the particle size as well as the pore size and shape, which are determined by the micelle structure during powder preparation. In general, the micelle geometry is controlled by the concentration of the surfactant in solution and by the processing conditions. At different concentrations, the surfactant molecules aggregate into different forms, which influence the pore structure and surface area.
The thermal properties were studied by TGA in the temperature ranging from 25 to 800°C under nitrogen atmosphere, in order to choose precisely the calcinations temperature (Figure
Thermal analysis for nanoporous hydroxyapatite using CTAB as template.
Thermal analysis for nanoporous hydroxyapatite using P123 as template.
FTIR spectrum of hydroxyapatite after calcination at 800°C.
We have demonstrated the synthesis of phase-pure nanoporous hydroxyapatite inorganic using the P123 and CTAB micelle systems, where the nonionic surfactant concentration was found to have an influence on nanoparticle surface area. At the 30 wt% P123 surfactant concentration, HAp particles showed the highest surface area 50 m2/g. By using cationic surfactant CTAB, surface area increased to 147 m2/g.
Research council of Iran University of Science and Technology (IUST, Iran) is appreciated for the financial support.