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Journal of Nanoscience and Nanotechnology Vol. 12, 1–8, 2012
Surfactant-Assisted Hydrothermal Synthesis of Hydroxyapatite Nanopowders Meriame Bricha1 2 , Younes Belmamouni1 2 , El Mokhtar Essassi1 2 , José M. F. Ferreira3 , and Khalil El Mabrouk1 ∗ 1
Nanotech, Institute of Nanomaterials and Nanotechnology. MAScIR, Moroccan Foundation for Advanced Science, Innovation and Research Avenure, De l’Armée Royale, Madinat El Irfane 10.100, Rabat, Morocco 2 Laboratoire de Chimie Organique Hétérocyclique, Pôle de Compétences Pharmacochimie, Universite Mohammed V-Agdal, BP 1014 Avenue Ibn Battouta, Rabat, Morocco 3 Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Rod-like hydroxyapatite nanoparticles (n-HAp) with a highly ordered nanostructure were prepared by hydrothermal synthesis from calcium chloride, and phosphoric acid, as calcium and phosphorus sources, respectively. Various surfactant families such as cationic (CTAB), anionic (SDS) and nonionic (Triton X-100) were used as regulators of the nucleation and crystal growth. The synthesized nanopowders were characterized using X-ray diffraction (XRD), Fourier transform infrared spectrograph (FTIR) and transmission electron microscopy (TEM). The rod-like morphology was obtained regardless of the surfactant used during the hydrothermal treatment, but the aspect ratio of the crystals was found to be surfactant dependent. The mechanism of crystal growth as well-oriented nanostructure is discussed.
Keywords: Hydrothermal, Hydroxyapatite, Nanopowder, Surfactant, Morphology, Electron
1. INTRODUCTION Hydroxyapatite (denoted as HAp), with the chemical formula Ca10 (PO4 6 (OH)2 , was introduced by Albee in 1920 for the first time and applied in a variety of forms. Currently, it is commonly used as raw material for the preparation of artificial bone grafts.1 2 It is also a promising material as reinforcing filler for composites,3 4 sensor agents5 6 and chromatomedium for simple and rapid fractionation of proteins and nucleic acids.7 8 The features of nano-hydroxyapatite powders (n-HAp), including the small size of particles/crystallites, chemical and mineral compositions analogue to those of natural bone, make n-HAp an important material for applications in bone regeneration9 and tissue engineering.10 The properties of n-HAp, including bioactivity,11 biocompatibility,12–14 solubility,15 16 osteoconduction,17 18 osteoinduction19 20 and absorption usually depend on the control of the particles’ composition, particle size distribution, morphology and surface area, etc., and therefore, on the powder preparation method. For these reasons, it is of great importance developing synthesis ∗
Author to whom correspondence should be addressed.
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methods focused on the precise control of particle size, morphology and chemical composition of n-HAp. A number of techniques have been developed to prepare hydroxyapatite nanopowders with various morphologies. They include solid state reaction,21 22 sol–gel,23–25 hydrothermal,26–28 and microemulsion or emulsion methods.29 30 Recently, surfactant-mediated hydrothermal processes have shown great promises for the elaboration of nanoparticles with controlled morphology.31 32 While conventional methods tend to originate irregular morphologies and particle aggregates larger than 100 nm, the hydrothermal method proved to have some advantages in preparing crystalline materials (including salts, metal oxides, etc.) at relatively low temperatures, but the control on morphology is still unsatisfactory.33 Surfactant template systems offer the opportunity of exploring the complementarity geometric, electrostatic and stereochemical molecular/ionic interactions in an attempt to control the nucleation and growth of inorganic materials.34 Shanthi and coworkers35 reported the preparation HAp nanorods with ≈ 20 nm diameter and 100–120 nm length by co-precipitation at ambient temperature and pressure in the presence of a cationic surfactant.
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doi:10.1166/jnn.2012.6664
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Microscopy.
Surfactant-Assisted Hydrothermal Synthesis of Hydroxyapatite Nanopowders
The same research group has successfully prepared shell-like nano-HAp spheres of varied morphology, with uniform size (≈ 200 nm) using Cetrimide as surfactant.36 Saha coworkers37 synthesized HA nanopowder by reverse microemulsion technique, using calcium nitrate and phosphoric acid as precursor salts dissolved in water. It was found that CTAB played a significant role in regulating the morphology of the nanoparticles. This work reports on the synthesis of n-HAp nanopowders in the presence of different kinds of surfactants to explore the effect of the type of surfactant (cationic, anionic, and non-ionic) on the formation of calcium phosphates. Cetyl trimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and Triton X-100 [C14 H22 O(C2 H4 O)n ] were used as surfactants. In our experiment, we investigate the behaviour of different surfactants in preparation and discussed the difference between them. Their influence on the nucleation, crystal growth and morphology of n-HAp is discussed.
2. MATERIALS AND METHODS
Calcium chloride (CaCl2 > 97%) and cetyltrimethylammonium bromide (CTAB: CH3 (CH2 15 N(CH3 3 Br) were purchased from Fluka. Sodium dodecyl sulfate > 99% (SDS: CH3 (CH2 11 OSO3 Na) was purchased from Merck. Phosphoric acid H3 PO4 (85%) and ammonia NH4 OH (25%) were bought from RH. Ammonia was used to regulate the pH value. Triton X-100 was purchased from SA. All the products were used as received without further purification.
solution (Ca2+ ) and then the mixture was drop wise to PO3− 4 solution. However, Triton X-100 was added directly to the anionic solution. 2.3. Characterization of n-HAp Samples The crystalline phases of all synthesized samples were identified by X-ray diffraction (XRD) using a D8 Discover X-ray diffractometer. Scans were run from 10 to 80 (2) with a step of 0.02 using CuK (wavelength of 1.5418 Å) incident radiation. Cell parameters of the HAp phase were calculated through TOPAS 4.2 (Bruker AXS). The average crystal size, D, was calculated using the Debye-Scherrer equation:38 Dhkl = K/1/2 cos
(1) where Dhkl is the length of the crystallite in a direction perpendicular to the reflection planes (hkl), K is the Sherrer constant approximately equal to unity and related to crystallite shape and chosen as 0.9 for the elongated apatite crystallites, is the X-ray wavelength equal to 1.5418 Å for CuK radiation, the Bragg’s diffraction angle (in degree), 1/2 the full width at half maximum (FWHM) expressed in radians for the peak of the (300) plane, representative of the crystallite size along the a-axis, and of the peak of (002) plane, which represents the crystallite size along the c-axis. Fourier transform IR spectra were collected on a FTLA 2000 within the range of 4000–400 cm−1 to confirm the presence of OH and PO4 functional groups. The morphology of the particles was assessed by TEM (Tecnai G2 Series from FEI Company) observations.
3. RESULTS 2.2. Preparation of n-HAp Nanopowders Aqueous solution of 29.35 mM of CaCl2 and 17.58 mM of H3 PO4 (which corresponds to a Ca/P ratio of 1.67) were used as precursors to prepare n-HAp. Then, 0.7 mM of CTAB was added to the PO3− containing solution. 4 The resulting solution was kept under a slow stirring for about 30 min to ensure the cooperative interaction and the completion of the self-assembly process. The solution of PO3− 4 /surfactant was added drop wise (about 60 drops/min) to the Ca2+ solution with a volume ratio equal to 1:1, under moderate stirring at ambient temperature and normal pressure yielding a milky suspension. The NH4 OH solution was used to adjust the pH value at 10–10.5. The suspension was stirred for further half an hour. It was then transferred into Teflon lined autoclave and heated at 90 C in an oven for 24 h. The precipitate was then centrifuged at 10000 rpm for 10 min, washed with de-ionized water and oven dried at about 100 C for 24 h. The powder was then calcined in a furnace at 550 C for 6 h, yielding a white powder. The above procedure was repeated but in the presence of SDS or Triton X-100. In the case of anionic surfactant, the SDS was mixed with cationic 2
The wide angle (2 > 10 X-ray diffraction patterns of the obtained samples are shown in Figure 1. The patterns have been normalized to enable a better comparison. It can
211 300
002 Intensity (a.u.)
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102 210
301
213 004 321 312
222 310 311 113
322 313
(b)
(a) 20
25
30
35
40
45
50
55
60
2θ (°) Fig. 1. XRD patterns of calcined (550 C, 6 h) n-HAp nanopowders prepared at 90 C for 24 h in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100. The three patterns have been normalized.
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(c)
(b)
(a) OH OH PO 4000
3500
3000
2500
2000
1500
PO
1000
500
Wavenumber (cm–1) Fig. 2. FTIR spectra of n-HAp nanopowders prepared in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100.
Table I. Calculated lattice parameters and the crystallite sizes of n-HAp powders calcined at 550 C for 6 h. Refined lattice data for HAp phase (Å) Sample code ICDD no. 09-432 n-HAp-CTAB n-HAp-SDS n-HAp-TX-100
Crystallite size (nm)
a axis (Å)
c axis (Å)
Cell volume (A
X002
X300
9.4180 9.4856 9.4374 9.4740
6.8840 6.8573 6.8843 6.8947
528.8000 534.3413 530.8150 535.8020
– 46.2358 29.6758 25.2822
– 41.0844 13.3424 17.5938
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be seen that all the diffraction peaks could be indexed to hexagonal HAp crystals. There is a good consistency between the data from our samples prepared by hydrothermal surfactant assistance and those reported in the standard data base, with lattice dimensions of a = b = 9 418 Å, c = 6 884 Å, with space group P63 /m (176), (Joint Committee for powder Diffraction Standards JCPDS No. 09-0432). No crystalline phases other than pure HAp were detected in the XRD patterns, confirming the formation of single phase n-HAp crystals in all samples. Thus, standard HAp with hexagonal structure was formed during synthesis, which remained stable after calcination. The FTIR spectra presented in Figures 2(a)–(c) provide further confirmation about the apatite nature of the hydrothermally synthesized samples in the presence of CTAB, SDS, and Triton X-100, respectively. All n-HAp powders exhibit characteristic bands due to PO3− 4 ions ( 1 -966 cm−1 , 3 -1020 and 1120 cm−1 , 4 -574 and 609 cm−1 39 which tend to coincide with the results of the previous study, with the strong PO stretching bands at about 1100 and 600 cm−1 being particularly well noticed. The presence of hydroxyl group, characteristic of the standard HAp, could be confirmed in the case of n-HAp powder synthesised in the presence of the non-ionic surfactant from the stretching vibration at 3570 cm−1 and libration mode at 630 cm−1 . The peak at 3570 cm−1 is attributed to stretching vibration of OH− ions in the HAp lattice. However, it is difficult to explain why the presence of hydroxyl group is not apparent in the n-HAp powders synthesised in
the presence of the other surfactants. The highly sensitive FTIR results indicated that no surfactant molecules eventually adsorbed in the as synthesized powders remained in the samples after the heat treatment. The crystallites size data derived from the XRD peaks 002 (conscripted as X002 ) and 300 (conscripted as X300 ) are collected in Table I.41 The crystallite size measured from X002 matches to c axis, while X300 value corresponds to a axis. The results of these calculations show clearly that all the values are comprised between ∼13 and ∼46 nm size for all the powders. These results show that although pure HAp has been obtained irrespective of the kind of added surfactant, the crystallite size and the intensity of diffraction peaks are dependent on type of surfactant used. The crystallite size scale reasonable well with the molecular size of the surfactants/micelles. But the XRD peaks are sharpener in the case n-HAp-Triton X-100 powder (Fig. 1(c)), followed by that prepared in the presence of cationic surfactant (CTAB, Fig. 1(a)). Accordingly, the XRD peaks are broadest for the HAp-SDS sample (Fig. 1(b)). The morphological features of calcined hydroxyapatite nanopowders are revealed in the Transmission Electron Microscopy (TEM) micrographs shown in Figure 3. It can be seen that the average dimensions of calcined n-HAp-CTAB particles (Fig. 3(a)) are about 15 nm diameter and 75 nm length, corresponding to an aspect ratio (i.e., length/diameter) of about 4.8. The n-HAp-SDS particles are much thinner and longer (Fig. 3(b)) with average diameter and length of about 11 nm and 137 nm, respectively, and an aspect ratio of about 12.4. The morphology of the particles formed in the presence of Triton X-100 (Fig. 3(c)) vary between the sphere-like shape with dimensions in the range of 19–30 nm and the needle shape with the lengths and widths of 79 nm and 21 nm, respectively. The aspect ratio is about 4.4 for the needle shaped particles. This less distinctive morphology might be attributed to the fact that the non-ionic surfactant is less prone to form an ordered structure in the solution to template the synthesized inorganic particles. These results are roughly consistent with those obtained from the XRD data. The corresponding crystallite size distributions are also shown in Figure 4. In the TEM measurements (using “Image J” software) more than 20 particles were randomly chosen in different areas in each powder sample. Two main features emerge from this figure: (i) the XRD size distributions determined along the directions 002 and 300 are
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Fig. 3. TEM micrographs and their respective EDAX patterns of n-HAp nanopowders prepared in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100.
very different, accounting for the anisotropy nature of the particles as observed by TEM; (ii) there is an excellent qualitative agreement between the distributions determined by XRD and TEM. The quantitative differences between the X-ray and the TEM data are probably due to the fact that the particles might not be monocrystalline but instead result from the coalescence of some growing nuclei. While TEM images reflect the size of the polycrystalline entities, XRD is able to discriminate the features of the composing individual crystallites. 4
4. DISCUSSION The surface active agents, also called surfactants, are generally referred to as being amphiphilic. They usually consist of a hydrophilic polar head with high affinity for water and a hydrophobic tail with a completely opposite behaviour towards water, as schematized in Figure 5(a). Their first property is the ability bridge two immiscible phases. For example, in an aqueous solution, surfactants have the tendency to migrate to the interfaces, particularly J. Nanosci. Nanotechnol. 12, 1–8, 2012
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160 n-HAp CTAB n-HAp SDS n-HAp Triton X-100
140
Fraction
120 100 80 60 40 20 0 Lm
Dm
L/D TEM
TEM
D002
D300
XRD
Fig. 4. Bar diagram of the size distributions obtained from XRD and TEM micrograph. For each sample more than 20 particles were counted. Lm is the average length. Dm is the average diameter.
Hydrophobic tail Hydrophilic head group
(a)
In aqueous
Phase2: liquid or vapor
surfacatnt solution
Phase1: aqueous
This is the case of synthetic products like betaines or sulfobetaines and natural substances such as aminoacids and phospholipids. For the present study we selected the cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS), and Triton X-100 as cationic, anionic and non-ionic surfactants, respectively, in an attempt to shed further light on the mechanisms involved in the formation of HAp nanoparticles. Their structures are schematised in Figure 6. Surfactants have been used as additives to template the crystallization processes, but their mechanisms of action are still poorly understood.44 45 The presence of surfactants tends to exert an overall inhibitory action on the overall crystallization process, with specific effects depending on the type and concentration of surfactant. Within the pH values of interest in the present work, CTAB molecules dissociate completely in the aqueous solution, forming cationic CTAB+ species. Above CMC, these species tend to self-assemble in direct cylindrical micelles exposing the positive shells to water molecules, as schematized in Figure 7(a). Therefore, when the PO3− solution is 4 mixed with the CTAB solution, the PO3− ions are read4 ily attracted towards ionized heads forming CTAB+ –PO3− 4 complexes (which might be mono-, bi-, multi-dentate) onto the surface of the cylindrical micelles and decreasing the total charge and the freedom of the PO3− 4 ions (Fig. 7(b)). Upon the subsequent addition of the Ca2+ containing solution, Ca9 (PO4 6 clusters are likely formed, driven by (a) N+ Br–
(b)
O Micellization
(c) (c)
Fig. 5. Schematic representation of a surfactant molecule (a), its behaviour in aqueous solution (b) to minimize the interfacial tension, and the self-assembly after reaching the critical micelle concentration (CMC) (c).
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O
O
(b)
O
S
O– Na+
H O n
Fig. 6. Surfactant structures: (a) Cationic (CTAB), (b) anionic (SDS), (c) non-ionic (Triton X-100).
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the air-liquid interface, Figure 5(b).42 As a consequence, the interfacial tension decreases and interfacial area might be easily increased through the formation of air bubbles, justifying their use as detergents, foaming agents and froth flotation additives in mineral separation. At low surfactant concentrations, the molecules are present in solution as separate species. Beyond a critical concentration, the individualised molecules start to selforganize to form molecular aggregates called micelles, measuring between 1 nm and 1 m, to minimize the contacts between hydrophobic parts of surfactant molecules and water molecules, Figure 4(c). The concentration at which the molecules begin to form these aggregates is defined as the critical micelle concentration (CMC). “Direct” micelles are formed in water with the surfactant polar heads facing outwards and the hydrophobic chains remain on the inside of the micelle. Conversely, assemblies of micelles called “reverse,” are formed in an organic medium with the hydrophilic polar head inside the micelle and the hydrocarbon chains forming a hydrophobic outer layer in contact with the organic medium.43
Surfactants are broadly classified into the following four groups based on the ionic charges they carry on the hydrophilic head group: (i) Anionic, with the hydrophilic group carrying a negative charge, such as carboxyl (RCOO− M+ ) or sulphate + (ROSO− 3 M ; (ii) Cationic, with the hydrophilic group bearing a positive charge, as for example, the quaternary ammonium salts (R4 N+ X− ) or amine salts; (iii) Amphoteric and zwitterionic, in which the surface— active position bears both positive and negative charges; (iv) Non-ionic, where the hydrophilic group has no net electrical charge.
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Fig. 7. Scheme of the formation process of crystalline HAp nanorods in the presence of CTAB.
the remaining electrostatic interactions, thus leading to the nucleating and growth of rod-like n-HAp crystals (Fig. 7(c)). The formation of ionic-surfactant complexes means that the effectiveness of the collisions between the hydroxyapatite forming ions decreases, explaining their inhibitory action on the overall crystallization process.44 45 The cylindrical micelles configuration formation is thus expected to exacerbate the well-known tendency for the nanocrystalline HAp particles to grow up along c-axis44 giving raise to the formation of HAp nanorods.45 The formation of ionic-surfactant complexes and the consequent decreased efficiency of the relevant collisions towards the formation of hydroxyapatite mean that ions are added to the surfaces of n-HAp particles more slowly but in a more ordered manner, resulting in crystalline powders (Fig. 1). The addition of ions to the crystal surfaces is helped by the high temperature/pressure used in the hydrothermal treatment. The n-HAp-CTAB complexes might coalesce and contribute to control the size and morphology of the obtained nanoparticles and account for their polycrystalline nature.34 45 This coalescence would explain the quantitative differences observed in Figure 4 between the size distributions data determined from XRD and TEM. A similar reasoning could be argued to interpret the anisometric growth of n-HAp particles in the presence of the other surfactants. The only difference expected when using SDS is the early formation of SDS− –Ca2+ complexes that precedes the formation of Ca9 (PO4 6 clusters upon mixing the two HAp precursor solutions. It is likely that the specific interactions between the surfactant molecules and the different crystal surfaces, which might differ especially among the basal plans (perpendicular to c-axis) and the lateral plans (parallel to c-axis), also contribute in different extents to the anisometric growth of n-HAp particles.46 These effects related to the adsorption of surfactant molecules onto crystal faces are thermodynamically 6
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driven and may greatly govern the crystallization and crystal growth.4950 The molecular structure of Triton X-100 (Fig. 6(c)) consists of a hydrophilic polyethylene oxide group (head) and a hydrophobic or lipophilic hydrocarbon (tail). Being a non-ionic surfactant, the possibility of electrostatic interactions may be eliminated. Above the CMC, it forms an isotropic colloidal mixture of spherical micelles and hexagonally ordered cylinders, with the polyethylene oxide group located at the outer shell and the hydrophobic tail at the micelle core.47 Lee coworkers48 suggested that the polyethylene oxide group could coordinate with Ca2+ ions through hydrogen bonds, forming calcium complexes that mediate the reaction with the PO3− solution. These 4 intermolecular interactions would enable nucleation and particle growth to proceed, upon mixing the reagent solutions, according to the template micelles. These theoretical arguments are in good agreement with our experimental TEM observations that reveal a mixed morphology for the n-HAp particles from spherical to rod-like.47 Concerning the size of the produced nanoparticles, we perceive that this factor is strongly dependant on the surfactant used in the experiment. Each surfactant has its sharp and characteristic critical micelle concentration above which the dissolved species combine to form micelles.42 51 The structure of these micelles was assumed to be an aggregate of from 50–100 molecules with a radius approximately equal to the length of the hydrocarbon chain of the surfactant. Thus, for ionic surfactants, the average size of CTAB micelles is expected to be bigger than that of SDS micelles. Further, the CMC effectively decreases with increasing alkyl chain length. These different features explain why nanoparticles with smaller diameters were obtained in the presence of SDS. In the case of the nonionic surfactant (Triton X-100), its lower solubility in comparison to the ionic ones makes the CMC to decrease (Table II). The shape and the structure of the resulting micelles are not so quite defined and their average size increases. The space between micelles also increases, thus inducing a steric hindrance that further contributes to increase the size of the produced particles (Fig. 8).51 The expected stronger interactions between the ionic surfactants and the HAp-forming ions, in comparison to those involving the non-ionic surfactants, mean that the Ca2+ and PO3− ions are likely to form from mono- to 4 multi-dentate complexes. This stronger sequestering effect Table II. Typical CMC values of the surfactants used 42 and the respective crystallite sizes of the n-HAp powders derived thereof. Surfactant CMC (mM) Length of synthesized n-HAp (nm) Diameter of synthesized n-HAp (nm) Surfactant length (Å)a
CTAB
SDS
Triton X-100
0.92–1.0 75 15 22.2
8.5 137 11 18.4
0.55 79 21 59.9
Note: a Calculation was done by Chem 3D Pro software.
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the different crystal surfaces tend to hinder atomic arrangement and lead to decreased crystallinity. The results from the different characterization techniques used (FTIR, XRD and TEM) show good consistency. The size and shape of the obtained self-assembled rod-like nano-crystalline HAp powders are similar to those of the hydroxyapatite mineral existing in human bones, making them good candidate materials for tissue engineering applications and drug delivery systems.
(a)
(b)
(c)
Acknowledgments: This work was funded through by MASCIR—Moroccan Foundation for Advanced Science, Innovation and Research. The support from CICECO is also greatly appreciated.
References and Notes
Fig. 8. Shematic presentation of the cross-sectional view of cylindrical micelles: (a) CTAB, (b) SDS, (c) Triton X-100.
5. CONCLUSIONS Rod-like nano-crystalline HAp particles have been successfully prepared by hydrothermal synthesis in the presence of different surface active agents. The obtained results show that the ionic surfactants are better suited to promote and control the anisometric growth of n-HAp particles. The strong interactions between the dissolved CTAB and SDS species and water molecules lead to the formation of well-defined cylindrical micelles that act as templates for the nucleation and growth of rod-like n-HAp particles. The low solubility of the smaller and non-ionic Triton X-100 molecules stimulates them to self-organize but not in a so quite defined manner leading to the formation of smaller sized micelles and resulting particles. Stronger specific interactions between the surfactant molecules and J. Nanosci. Nanotechnol. 12, 1–8, 2012
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exerted by the ionic surfactants might hinder the immediate release of the surfactants species after an effective collision between two HAp-forming ions. Shall this happens, the adsorbed surfactants species will disturb the atomic arrangement upon building the HAp crystal lattice. Therefore, a less ordered structure is expected to form in the presence of ionic surfactants. Although the as adsorbed surfactant species will be thermally degraded upon calcination, explaining why no surfactant molecules were detected in the samples heat treated at 550 C for 6, Figure 2, it is likely that this disturbing role in the atomic arrangement will have reflexes in the degree of crystallinity obtained after being treated at this moderate temperature. This explains why the sharpness of XRD peaks of the powders n-HAp-CTAB (Fig. 1(a)) and n-HAp-SDS (Fig. 1(b)) is less in comparison to that n-HAp-Triton X-100 (Fig. 1(c)).
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Surfactant-Assisted Hydrothermal Synthesis of Hydroxyapatite Nanopowders 27. H. Ivankovic, G. G. Ferrer, E. Tkalcec, S. Orlic, and M. Ivankovic, J. Mater. Sci. Mater. Med. 20, 1039 (2009). 28. K. Lin, J. Chang, R. Cheng, and M. Ruan, Mater. Let. 61, 1683 (2007). 29. N. I. Ponomareva, T. D. Poprygina, S. I. Karpov, M. V. Lesovoi, and B. L. Agapov, Russ. J. Gen. Chem. 80, 905 (2010). 30. M. Salarian, M. S. Hashjin, S. S. Shafiei, R. Salarian, and Z. A. Nemati, Ceram. Int. 35, 2563 (2009). 31. Q. Zhao, T. Wang, J. Wang, L. Zheng, T. Jiang, G. Cheng, and S. Wang, App. Surf. Sci. 257, 10126 (2011). 32. M. Yoshimura, H. Suda, K. Okamoto, and K. Ioku, J. Mater. Sci. 29, 3399 (1994). 33. L. Yan, Y. Li, Z. X. Deng, J. Zhuang, and X. Sun, Inter. J. Inorg. Mater. 3, 633 (2001). 34. P. M. S. L. Shanthi, M. Ashok, T. Balasubramanian, A. Riyasdeen, and M. A. Akbarsha, Mater. Let. 63, 2123 (2009). 35. P. M. S. L. Shanthi, R. V. Mangalaraja, A. P. Uthirakumar, S. Velmathi, T. Balasubramanian, and M. Ashok, J. Coll. Interf. Sci. 350, 39 (2010). 36. S. K. Saha, A. Banerjee, S. Banerjee, and S. Bose, Mater. Sci. Eng. C 29, 2294 (2009). 37. H. P. Klug and L. E. Alexander, Wiley, New York (1959). 38. S. Kannan, A. F. Lemos and J. M. F. Ferreira, Chem. Mater. 18, 2181 (2006).
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39. X. L. Tang, X. F. Xiao, and R. F. Liu, Mater. Let. 59, 3841 (2005). 40. A. Siddharthan, S. K. Seshadri, and T. S. S. Kumar, J. Mater. Sci. Mater. Med. 15, 1279 (2004). 41. D. Myers, Surfaces, Interfaces, and Colloids, Principles and Applications, Second Edition, John Wiley & Sons (1999), p. 21. 42. F. M. Menger and C. A. Littau, J. Am. Chem. Soc. 113, 1451 (1991). 43. E. Bouyer, F. Gitzhofer, and M. I. Boulos, J. Mater. Sci. Mater. Med. 11, 523 (2000). 44. Y. Wang, S. Zhang, K. Wei, N. Zhao, J. Chen, and X. Wang, Mater. Let. 60, 1484 (2006). 45. S. Kannan, F. G. Neunhoeffer, J. Neubauer, A. H. S. Rebelo, and J. M. F. Ferreira, J. Am. Cer. Soc. 91, 1 (2008). 46. K. Byrappa and T. Ohachi, Crystal Growth Technology, William Andrew Inc., Norwich, New York (2003), p. 533. 47. G. Muller, J. J. Métis, and P. Rudolph, Crystal Growth—From Fundamentals to Technology, First Edition (2004), p. 21. 48. S. V. Ahir, P. G. Petrov, and E. M. Terentjev, Langmuir 18, 9140 (2002). 49. M. H. Lee, S. G. Oh, K. D. Suh, D. G. Kim, and D. Sohn, Colloids and Surfaces. A. Physicochemical and Engineering Aspects 210, 49 (2002). 50. K. Holmberg, B. Jôonsson, B. Kronberg, and B. Lindman, John Wiley & Sons, Ltd. (2002).
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Received: 26 June 2012. Accepted: 18 July 2012.
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J. Nanosci. Nanotechnol. 12, 1–8, 2012
Journal of Nanoscience and Nanotechnology Vol. 12, 1–8, 2012
Surfactant-Assisted Hydrothermal Synthesis of Hydroxyapatite Nanopowders Meriame Bricha1 2 , Younes Belmamouni1 2 , El Mokhtar Essassi1 2 , José M. F. Ferreira3 , and Khalil El Mabrouk1 ∗ 1
Nanotech, Institute of Nanomaterials and Nanotechnology. MAScIR, Moroccan Foundation for Advanced Science, Innovation and Research Avenure, De l’Armée Royale, Madinat El Irfane 10.100, Rabat, Morocco 2 Laboratoire de Chimie Organique Hétérocyclique, Pôle de Compétences Pharmacochimie, Universite Mohammed V-Agdal, BP 1014 Avenue Ibn Battouta, Rabat, Morocco 3 Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal Rod-like hydroxyapatite nanoparticles (n-HAp) with a highly ordered nanostructure were prepared by hydrothermal synthesis from calcium chloride, and phosphoric acid, as calcium and phosphorus sources, respectively. Various surfactant families such as cationic (CTAB), anionic (SDS) and nonionic (Triton X-100) were used as regulators of the nucleation and crystal growth. The synthesized nanopowders were characterized using X-ray diffraction (XRD), Fourier transform infrared spectrograph (FTIR) and transmission electron microscopy (TEM). The rod-like morphology was obtained regardless of the surfactant used during the hydrothermal treatment, but the aspect ratio of the crystals was found to be surfactant dependent. The mechanism of crystal growth as well-oriented nanostructure is discussed.
Keywords: Hydrothermal, Hydroxyapatite, Nanopowder, Surfactant, Morphology, Electron
1. INTRODUCTION Hydroxyapatite (denoted as HAp), with the chemical formula Ca10 (PO4 6 (OH)2 , was introduced by Albee in 1920 for the first time and applied in a variety of forms. Currently, it is commonly used as raw material for the preparation of artificial bone grafts.1 2 It is also a promising material as reinforcing filler for composites,3 4 sensor agents5 6 and chromatomedium for simple and rapid fractionation of proteins and nucleic acids.7 8 The features of nano-hydroxyapatite powders (n-HAp), including the small size of particles/crystallites, chemical and mineral compositions analogue to those of natural bone, make n-HAp an important material for applications in bone regeneration9 and tissue engineering.10 The properties of n-HAp, including bioactivity,11 biocompatibility,12–14 solubility,15 16 osteoconduction,17 18 osteoinduction19 20 and absorption usually depend on the control of the particles’ composition, particle size distribution, morphology and surface area, etc., and therefore, on the powder preparation method. For these reasons, it is of great importance developing synthesis ∗
Author to whom correspondence should be addressed.
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methods focused on the precise control of particle size, morphology and chemical composition of n-HAp. A number of techniques have been developed to prepare hydroxyapatite nanopowders with various morphologies. They include solid state reaction,21 22 sol–gel,23–25 hydrothermal,26–28 and microemulsion or emulsion methods.29 30 Recently, surfactant-mediated hydrothermal processes have shown great promises for the elaboration of nanoparticles with controlled morphology.31 32 While conventional methods tend to originate irregular morphologies and particle aggregates larger than 100 nm, the hydrothermal method proved to have some advantages in preparing crystalline materials (including salts, metal oxides, etc.) at relatively low temperatures, but the control on morphology is still unsatisfactory.33 Surfactant template systems offer the opportunity of exploring the complementarity geometric, electrostatic and stereochemical molecular/ionic interactions in an attempt to control the nucleation and growth of inorganic materials.34 Shanthi and coworkers35 reported the preparation HAp nanorods with ≈ 20 nm diameter and 100–120 nm length by co-precipitation at ambient temperature and pressure in the presence of a cationic surfactant.
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Microscopy.
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The same research group has successfully prepared shell-like nano-HAp spheres of varied morphology, with uniform size (≈ 200 nm) using Cetrimide as surfactant.36 Saha coworkers37 synthesized HA nanopowder by reverse microemulsion technique, using calcium nitrate and phosphoric acid as precursor salts dissolved in water. It was found that CTAB played a significant role in regulating the morphology of the nanoparticles. This work reports on the synthesis of n-HAp nanopowders in the presence of different kinds of surfactants to explore the effect of the type of surfactant (cationic, anionic, and non-ionic) on the formation of calcium phosphates. Cetyl trimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and Triton X-100 [C14 H22 O(C2 H4 O)n ] were used as surfactants. In our experiment, we investigate the behaviour of different surfactants in preparation and discussed the difference between them. Their influence on the nucleation, crystal growth and morphology of n-HAp is discussed.
2. MATERIALS AND METHODS
Calcium chloride (CaCl2 > 97%) and cetyltrimethylammonium bromide (CTAB: CH3 (CH2 15 N(CH3 3 Br) were purchased from Fluka. Sodium dodecyl sulfate > 99% (SDS: CH3 (CH2 11 OSO3 Na) was purchased from Merck. Phosphoric acid H3 PO4 (85%) and ammonia NH4 OH (25%) were bought from RH. Ammonia was used to regulate the pH value. Triton X-100 was purchased from SA. All the products were used as received without further purification.
solution (Ca2+ ) and then the mixture was drop wise to PO3− 4 solution. However, Triton X-100 was added directly to the anionic solution. 2.3. Characterization of n-HAp Samples The crystalline phases of all synthesized samples were identified by X-ray diffraction (XRD) using a D8 Discover X-ray diffractometer. Scans were run from 10 to 80 (2) with a step of 0.02 using CuK (wavelength of 1.5418 Å) incident radiation. Cell parameters of the HAp phase were calculated through TOPAS 4.2 (Bruker AXS). The average crystal size, D, was calculated using the Debye-Scherrer equation:38 Dhkl = K/1/2 cos
(1) where Dhkl is the length of the crystallite in a direction perpendicular to the reflection planes (hkl), K is the Sherrer constant approximately equal to unity and related to crystallite shape and chosen as 0.9 for the elongated apatite crystallites, is the X-ray wavelength equal to 1.5418 Å for CuK radiation, the Bragg’s diffraction angle (in degree), 1/2 the full width at half maximum (FWHM) expressed in radians for the peak of the (300) plane, representative of the crystallite size along the a-axis, and of the peak of (002) plane, which represents the crystallite size along the c-axis. Fourier transform IR spectra were collected on a FTLA 2000 within the range of 4000–400 cm−1 to confirm the presence of OH and PO4 functional groups. The morphology of the particles was assessed by TEM (Tecnai G2 Series from FEI Company) observations.
3. RESULTS 2.2. Preparation of n-HAp Nanopowders Aqueous solution of 29.35 mM of CaCl2 and 17.58 mM of H3 PO4 (which corresponds to a Ca/P ratio of 1.67) were used as precursors to prepare n-HAp. Then, 0.7 mM of CTAB was added to the PO3− containing solution. 4 The resulting solution was kept under a slow stirring for about 30 min to ensure the cooperative interaction and the completion of the self-assembly process. The solution of PO3− 4 /surfactant was added drop wise (about 60 drops/min) to the Ca2+ solution with a volume ratio equal to 1:1, under moderate stirring at ambient temperature and normal pressure yielding a milky suspension. The NH4 OH solution was used to adjust the pH value at 10–10.5. The suspension was stirred for further half an hour. It was then transferred into Teflon lined autoclave and heated at 90 C in an oven for 24 h. The precipitate was then centrifuged at 10000 rpm for 10 min, washed with de-ionized water and oven dried at about 100 C for 24 h. The powder was then calcined in a furnace at 550 C for 6 h, yielding a white powder. The above procedure was repeated but in the presence of SDS or Triton X-100. In the case of anionic surfactant, the SDS was mixed with cationic 2
The wide angle (2 > 10 X-ray diffraction patterns of the obtained samples are shown in Figure 1. The patterns have been normalized to enable a better comparison. It can
211 300
002 Intensity (a.u.)
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102 210
301
213 004 321 312
222 310 311 113
322 313
(b)
(a) 20
25
30
35
40
45
50
55
60
2θ (°) Fig. 1. XRD patterns of calcined (550 C, 6 h) n-HAp nanopowders prepared at 90 C for 24 h in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100. The three patterns have been normalized.
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(c)
(b)
(a) OH OH PO 4000
3500
3000
2500
2000
1500
PO
1000
500
Wavenumber (cm–1) Fig. 2. FTIR spectra of n-HAp nanopowders prepared in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100.
Table I. Calculated lattice parameters and the crystallite sizes of n-HAp powders calcined at 550 C for 6 h. Refined lattice data for HAp phase (Å) Sample code ICDD no. 09-432 n-HAp-CTAB n-HAp-SDS n-HAp-TX-100
Crystallite size (nm)
a axis (Å)
c axis (Å)
Cell volume (A
X002
X300
9.4180 9.4856 9.4374 9.4740
6.8840 6.8573 6.8843 6.8947
528.8000 534.3413 530.8150 535.8020
– 46.2358 29.6758 25.2822
– 41.0844 13.3424 17.5938
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be seen that all the diffraction peaks could be indexed to hexagonal HAp crystals. There is a good consistency between the data from our samples prepared by hydrothermal surfactant assistance and those reported in the standard data base, with lattice dimensions of a = b = 9 418 Å, c = 6 884 Å, with space group P63 /m (176), (Joint Committee for powder Diffraction Standards JCPDS No. 09-0432). No crystalline phases other than pure HAp were detected in the XRD patterns, confirming the formation of single phase n-HAp crystals in all samples. Thus, standard HAp with hexagonal structure was formed during synthesis, which remained stable after calcination. The FTIR spectra presented in Figures 2(a)–(c) provide further confirmation about the apatite nature of the hydrothermally synthesized samples in the presence of CTAB, SDS, and Triton X-100, respectively. All n-HAp powders exhibit characteristic bands due to PO3− 4 ions ( 1 -966 cm−1 , 3 -1020 and 1120 cm−1 , 4 -574 and 609 cm−1 39 which tend to coincide with the results of the previous study, with the strong PO stretching bands at about 1100 and 600 cm−1 being particularly well noticed. The presence of hydroxyl group, characteristic of the standard HAp, could be confirmed in the case of n-HAp powder synthesised in the presence of the non-ionic surfactant from the stretching vibration at 3570 cm−1 and libration mode at 630 cm−1 . The peak at 3570 cm−1 is attributed to stretching vibration of OH− ions in the HAp lattice. However, it is difficult to explain why the presence of hydroxyl group is not apparent in the n-HAp powders synthesised in
the presence of the other surfactants. The highly sensitive FTIR results indicated that no surfactant molecules eventually adsorbed in the as synthesized powders remained in the samples after the heat treatment. The crystallites size data derived from the XRD peaks 002 (conscripted as X002 ) and 300 (conscripted as X300 ) are collected in Table I.41 The crystallite size measured from X002 matches to c axis, while X300 value corresponds to a axis. The results of these calculations show clearly that all the values are comprised between ∼13 and ∼46 nm size for all the powders. These results show that although pure HAp has been obtained irrespective of the kind of added surfactant, the crystallite size and the intensity of diffraction peaks are dependent on type of surfactant used. The crystallite size scale reasonable well with the molecular size of the surfactants/micelles. But the XRD peaks are sharpener in the case n-HAp-Triton X-100 powder (Fig. 1(c)), followed by that prepared in the presence of cationic surfactant (CTAB, Fig. 1(a)). Accordingly, the XRD peaks are broadest for the HAp-SDS sample (Fig. 1(b)). The morphological features of calcined hydroxyapatite nanopowders are revealed in the Transmission Electron Microscopy (TEM) micrographs shown in Figure 3. It can be seen that the average dimensions of calcined n-HAp-CTAB particles (Fig. 3(a)) are about 15 nm diameter and 75 nm length, corresponding to an aspect ratio (i.e., length/diameter) of about 4.8. The n-HAp-SDS particles are much thinner and longer (Fig. 3(b)) with average diameter and length of about 11 nm and 137 nm, respectively, and an aspect ratio of about 12.4. The morphology of the particles formed in the presence of Triton X-100 (Fig. 3(c)) vary between the sphere-like shape with dimensions in the range of 19–30 nm and the needle shape with the lengths and widths of 79 nm and 21 nm, respectively. The aspect ratio is about 4.4 for the needle shaped particles. This less distinctive morphology might be attributed to the fact that the non-ionic surfactant is less prone to form an ordered structure in the solution to template the synthesized inorganic particles. These results are roughly consistent with those obtained from the XRD data. The corresponding crystallite size distributions are also shown in Figure 4. In the TEM measurements (using “Image J” software) more than 20 particles were randomly chosen in different areas in each powder sample. Two main features emerge from this figure: (i) the XRD size distributions determined along the directions 002 and 300 are
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Fig. 3. TEM micrographs and their respective EDAX patterns of n-HAp nanopowders prepared in the presence of different surfactants: (a) CTAB, (b) SDS, (c) Triton X-100.
very different, accounting for the anisotropy nature of the particles as observed by TEM; (ii) there is an excellent qualitative agreement between the distributions determined by XRD and TEM. The quantitative differences between the X-ray and the TEM data are probably due to the fact that the particles might not be monocrystalline but instead result from the coalescence of some growing nuclei. While TEM images reflect the size of the polycrystalline entities, XRD is able to discriminate the features of the composing individual crystallites. 4
4. DISCUSSION The surface active agents, also called surfactants, are generally referred to as being amphiphilic. They usually consist of a hydrophilic polar head with high affinity for water and a hydrophobic tail with a completely opposite behaviour towards water, as schematized in Figure 5(a). Their first property is the ability bridge two immiscible phases. For example, in an aqueous solution, surfactants have the tendency to migrate to the interfaces, particularly J. Nanosci. Nanotechnol. 12, 1–8, 2012
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160 n-HAp CTAB n-HAp SDS n-HAp Triton X-100
140
Fraction
120 100 80 60 40 20 0 Lm
Dm
L/D TEM
TEM
D002
D300
XRD
Fig. 4. Bar diagram of the size distributions obtained from XRD and TEM micrograph. For each sample more than 20 particles were counted. Lm is the average length. Dm is the average diameter.
Hydrophobic tail Hydrophilic head group
(a)
In aqueous
Phase2: liquid or vapor
surfacatnt solution
Phase1: aqueous
This is the case of synthetic products like betaines or sulfobetaines and natural substances such as aminoacids and phospholipids. For the present study we selected the cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS), and Triton X-100 as cationic, anionic and non-ionic surfactants, respectively, in an attempt to shed further light on the mechanisms involved in the formation of HAp nanoparticles. Their structures are schematised in Figure 6. Surfactants have been used as additives to template the crystallization processes, but their mechanisms of action are still poorly understood.44 45 The presence of surfactants tends to exert an overall inhibitory action on the overall crystallization process, with specific effects depending on the type and concentration of surfactant. Within the pH values of interest in the present work, CTAB molecules dissociate completely in the aqueous solution, forming cationic CTAB+ species. Above CMC, these species tend to self-assemble in direct cylindrical micelles exposing the positive shells to water molecules, as schematized in Figure 7(a). Therefore, when the PO3− solution is 4 mixed with the CTAB solution, the PO3− ions are read4 ily attracted towards ionized heads forming CTAB+ –PO3− 4 complexes (which might be mono-, bi-, multi-dentate) onto the surface of the cylindrical micelles and decreasing the total charge and the freedom of the PO3− 4 ions (Fig. 7(b)). Upon the subsequent addition of the Ca2+ containing solution, Ca9 (PO4 6 clusters are likely formed, driven by (a) N+ Br–
(b)
O Micellization
(c) (c)
Fig. 5. Schematic representation of a surfactant molecule (a), its behaviour in aqueous solution (b) to minimize the interfacial tension, and the self-assembly after reaching the critical micelle concentration (CMC) (c).
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O
O
(b)
O
S
O– Na+
H O n
Fig. 6. Surfactant structures: (a) Cationic (CTAB), (b) anionic (SDS), (c) non-ionic (Triton X-100).
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the air-liquid interface, Figure 5(b).42 As a consequence, the interfacial tension decreases and interfacial area might be easily increased through the formation of air bubbles, justifying their use as detergents, foaming agents and froth flotation additives in mineral separation. At low surfactant concentrations, the molecules are present in solution as separate species. Beyond a critical concentration, the individualised molecules start to selforganize to form molecular aggregates called micelles, measuring between 1 nm and 1 m, to minimize the contacts between hydrophobic parts of surfactant molecules and water molecules, Figure 4(c). The concentration at which the molecules begin to form these aggregates is defined as the critical micelle concentration (CMC). “Direct” micelles are formed in water with the surfactant polar heads facing outwards and the hydrophobic chains remain on the inside of the micelle. Conversely, assemblies of micelles called “reverse,” are formed in an organic medium with the hydrophilic polar head inside the micelle and the hydrocarbon chains forming a hydrophobic outer layer in contact with the organic medium.43
Surfactants are broadly classified into the following four groups based on the ionic charges they carry on the hydrophilic head group: (i) Anionic, with the hydrophilic group carrying a negative charge, such as carboxyl (RCOO− M+ ) or sulphate + (ROSO− 3 M ; (ii) Cationic, with the hydrophilic group bearing a positive charge, as for example, the quaternary ammonium salts (R4 N+ X− ) or amine salts; (iii) Amphoteric and zwitterionic, in which the surface— active position bears both positive and negative charges; (iv) Non-ionic, where the hydrophilic group has no net electrical charge.
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Fig. 7. Scheme of the formation process of crystalline HAp nanorods in the presence of CTAB.
the remaining electrostatic interactions, thus leading to the nucleating and growth of rod-like n-HAp crystals (Fig. 7(c)). The formation of ionic-surfactant complexes means that the effectiveness of the collisions between the hydroxyapatite forming ions decreases, explaining their inhibitory action on the overall crystallization process.44 45 The cylindrical micelles configuration formation is thus expected to exacerbate the well-known tendency for the nanocrystalline HAp particles to grow up along c-axis44 giving raise to the formation of HAp nanorods.45 The formation of ionic-surfactant complexes and the consequent decreased efficiency of the relevant collisions towards the formation of hydroxyapatite mean that ions are added to the surfaces of n-HAp particles more slowly but in a more ordered manner, resulting in crystalline powders (Fig. 1). The addition of ions to the crystal surfaces is helped by the high temperature/pressure used in the hydrothermal treatment. The n-HAp-CTAB complexes might coalesce and contribute to control the size and morphology of the obtained nanoparticles and account for their polycrystalline nature.34 45 This coalescence would explain the quantitative differences observed in Figure 4 between the size distributions data determined from XRD and TEM. A similar reasoning could be argued to interpret the anisometric growth of n-HAp particles in the presence of the other surfactants. The only difference expected when using SDS is the early formation of SDS− –Ca2+ complexes that precedes the formation of Ca9 (PO4 6 clusters upon mixing the two HAp precursor solutions. It is likely that the specific interactions between the surfactant molecules and the different crystal surfaces, which might differ especially among the basal plans (perpendicular to c-axis) and the lateral plans (parallel to c-axis), also contribute in different extents to the anisometric growth of n-HAp particles.46 These effects related to the adsorption of surfactant molecules onto crystal faces are thermodynamically 6
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driven and may greatly govern the crystallization and crystal growth.4950 The molecular structure of Triton X-100 (Fig. 6(c)) consists of a hydrophilic polyethylene oxide group (head) and a hydrophobic or lipophilic hydrocarbon (tail). Being a non-ionic surfactant, the possibility of electrostatic interactions may be eliminated. Above the CMC, it forms an isotropic colloidal mixture of spherical micelles and hexagonally ordered cylinders, with the polyethylene oxide group located at the outer shell and the hydrophobic tail at the micelle core.47 Lee coworkers48 suggested that the polyethylene oxide group could coordinate with Ca2+ ions through hydrogen bonds, forming calcium complexes that mediate the reaction with the PO3− solution. These 4 intermolecular interactions would enable nucleation and particle growth to proceed, upon mixing the reagent solutions, according to the template micelles. These theoretical arguments are in good agreement with our experimental TEM observations that reveal a mixed morphology for the n-HAp particles from spherical to rod-like.47 Concerning the size of the produced nanoparticles, we perceive that this factor is strongly dependant on the surfactant used in the experiment. Each surfactant has its sharp and characteristic critical micelle concentration above which the dissolved species combine to form micelles.42 51 The structure of these micelles was assumed to be an aggregate of from 50–100 molecules with a radius approximately equal to the length of the hydrocarbon chain of the surfactant. Thus, for ionic surfactants, the average size of CTAB micelles is expected to be bigger than that of SDS micelles. Further, the CMC effectively decreases with increasing alkyl chain length. These different features explain why nanoparticles with smaller diameters were obtained in the presence of SDS. In the case of the nonionic surfactant (Triton X-100), its lower solubility in comparison to the ionic ones makes the CMC to decrease (Table II). The shape and the structure of the resulting micelles are not so quite defined and their average size increases. The space between micelles also increases, thus inducing a steric hindrance that further contributes to increase the size of the produced particles (Fig. 8).51 The expected stronger interactions between the ionic surfactants and the HAp-forming ions, in comparison to those involving the non-ionic surfactants, mean that the Ca2+ and PO3− ions are likely to form from mono- to 4 multi-dentate complexes. This stronger sequestering effect Table II. Typical CMC values of the surfactants used 42 and the respective crystallite sizes of the n-HAp powders derived thereof. Surfactant CMC (mM) Length of synthesized n-HAp (nm) Diameter of synthesized n-HAp (nm) Surfactant length (Å)a
CTAB
SDS
Triton X-100
0.92–1.0 75 15 22.2
8.5 137 11 18.4
0.55 79 21 59.9
Note: a Calculation was done by Chem 3D Pro software.
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Surfactant-Assisted Hydrothermal Synthesis of Hydroxyapatite Nanopowders
the different crystal surfaces tend to hinder atomic arrangement and lead to decreased crystallinity. The results from the different characterization techniques used (FTIR, XRD and TEM) show good consistency. The size and shape of the obtained self-assembled rod-like nano-crystalline HAp powders are similar to those of the hydroxyapatite mineral existing in human bones, making them good candidate materials for tissue engineering applications and drug delivery systems.
(a)
(b)
(c)
Acknowledgments: This work was funded through by MASCIR—Moroccan Foundation for Advanced Science, Innovation and Research. The support from CICECO is also greatly appreciated.
References and Notes
Fig. 8. Shematic presentation of the cross-sectional view of cylindrical micelles: (a) CTAB, (b) SDS, (c) Triton X-100.
5. CONCLUSIONS Rod-like nano-crystalline HAp particles have been successfully prepared by hydrothermal synthesis in the presence of different surface active agents. The obtained results show that the ionic surfactants are better suited to promote and control the anisometric growth of n-HAp particles. The strong interactions between the dissolved CTAB and SDS species and water molecules lead to the formation of well-defined cylindrical micelles that act as templates for the nucleation and growth of rod-like n-HAp particles. The low solubility of the smaller and non-ionic Triton X-100 molecules stimulates them to self-organize but not in a so quite defined manner leading to the formation of smaller sized micelles and resulting particles. Stronger specific interactions between the surfactant molecules and J. Nanosci. Nanotechnol. 12, 1–8, 2012
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exerted by the ionic surfactants might hinder the immediate release of the surfactants species after an effective collision between two HAp-forming ions. Shall this happens, the adsorbed surfactants species will disturb the atomic arrangement upon building the HAp crystal lattice. Therefore, a less ordered structure is expected to form in the presence of ionic surfactants. Although the as adsorbed surfactant species will be thermally degraded upon calcination, explaining why no surfactant molecules were detected in the samples heat treated at 550 C for 6, Figure 2, it is likely that this disturbing role in the atomic arrangement will have reflexes in the degree of crystallinity obtained after being treated at this moderate temperature. This explains why the sharpness of XRD peaks of the powders n-HAp-CTAB (Fig. 1(a)) and n-HAp-SDS (Fig. 1(b)) is less in comparison to that n-HAp-Triton X-100 (Fig. 1(c)).
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Received: 26 June 2012. Accepted: 18 July 2012.
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