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Mechanochemical–hydrothermal synthesis of hydroxyapatite from nonionic surfactant emulsion precursors
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Journal of Crystal Growth 270 (2004) 615–623 www.elsevier.com/locate/jcrysgro
Mechanochemical–hydrothermal synthesis of hydroxyapatite from nonionic surfactant emulsion precursors Chun-Wei Chena,, Richard E. Rimana, Kevor S. TenHuisenb,1, Kelly Brownb a
Department of Ceramic and Materials Engineering, Rutgers University, 607 Taylor Road, Piscataway, NJ 08854, USA b Center for Biomaterials & Advanced Technologies, Medical Devices Group, a Division of Ethicon, Inc., Rt. 22 W, P.O. Box 151, Somerville, NJ 08876, USA Received 7 May 2004; accepted 29 June 2004 Communicated by M. Schieber Available online 3 September 2004
Abstract Nanocrystalline hydroxyapatite [HA, Ca10 ðPO4 Þ6 ðOHÞ2 ] powders were synthesized by the mechanochemical–hydrothermal method using emulsion systems consisting of aqueous phase, petroleum ether (PE) as the oil phase and biodegradable Tomadol 23–6.5 as the nonionic surfactant. ðNH4 Þ2 HPO4 and CaðNO3 Þ2 or CaðOHÞ2 were used as the phosphorus and calcium sources, respectively. The calcium source and emulsion composition had significant effects on the stoichiometry, crystallinity, thermal stability, particle size and morphology of final products. Disperse HA crystals with a 160 nm length and aspect ratio of ca. 6 were formed in an emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant. The HA particles had needle morphology with a specific surface area of 190 m2 =g. With this technique, HA nanopowders with specific surface areas in the range of 72–231 m2 =g were produced. r 2004 Elsevier B.V. All rights reserved. PACS: 61.66.Fn; 61.82.Rx; 81.10.A Keywords: A1. Biomaterials; A1. Characterization; A1. Mechanochemical; A2. Hydrothermal; B1. Hydroxyapatite
1. Introduction Hydroxyapatite [HA, Ca10 ðPO4 Þ6 ðOHÞ2 ] is the main constituent of hard tissues such as bone, Corresponding author. Tel.: +1-732-445-7092; fax: +1-
732-445-3258. E-mail address: [email protected] (C.-W. Chen). 1 Now at Reconstructive Business, Stryker Howmedica Osteonics, 59 Rt. 17 South, Allendale, NJ 07401, USA.
dentin, and enamel. Its biocompatibility and osteoconductive properties make it desirable as an implant material and drug delivery agent [1–4]. Nevertheless, either dense or porous HA suffers from low fracture toughness (less than 1 MPa m1=2 ) with respect to human bone (2–12 MPa m1=2 ), which limits its application as a monolithic phase in artificial bone materials [5]. One solution to the problem is the incorporation
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.051
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of HA particles into a polymer matrix to produce HA/polymer composites. Composites for orthopedic applications have been designed with the aim of attempting to match the properties and structure of bone. Those HA/polymer composites with improved mechanical properties have been used as an alternative material for internal fixation of human fractures [6,7]. Expanded use of engineered materials incorporating HA may depend on the ability to synthesize HA particulates with well-controlled physicochemical characteristics (purity, stoichiometry, morphology, and crystallinity, etc.). In the literature, various synthesis methods to prepare HA powders have been reported, including solid-state reactions, precipitation from mixed aqueous solutions, hydrolysis of calcium phosphates, and sol–gel crystallization [8–12]. Most of the synthetic procedures followed until now led to the agglomerated HA particles. Agglomeration can cause lower sintered density as well as crack-like voids during densification [13,14]. Thus, because of these defects, the use of HA materials is limited by poor mechanical strength and toughness and resultantly finds utility mostly in non-load-bearing applications [15]. Recently, we reported the synthesis of phasepure and crystalline HA by heterogeneous reaction of CaðOHÞ2 powder and aqueous ðNH4 Þ2 HPO4 at room temperature using the mechanochemical– hydrothermal (M–H) method [16,17]. The main advantages of this synthesis method are simplicity and low cost since inexpensive milling equipment and starting materials can be used. Under these synthesis conditions, amorphous calcium phosphate (ACP) precipitates upon initial mixing of the precursor solutions. The ACP precursor phase crystallizes and transforms into HA during the M–H process. A nanophase HA material results, which consists of agglomerates of 20–30 nm HA crystals. The mass-weight mean agglomerate size is ca. 2:6 mm in deionized water with an average agglomeration number (AAN) of 4:5 106 [17]c. The objective of this work is to minimize the degree of agglomeration. To accomplish this, we focused on control of nucleation and growth of the ACP phase by the utilization of emulsions. An emulsion is a mixture of two immiscible liquids in
which one liquid is dispersed in the other with the aid of a surfactant. The droplet not only controls the physical powder characteristics but also limits chemical segregation to the length scale of the droplet diameter. Emulsion methods have been shown to be one of the techniques that are capable of preparing ceramic powders with controlled particle size and distribution, uniform shape, lowered degree of agglomeration, and a high degree of chemical purity [18]. However, limited studies have been reported on the development of HA powders by the emulsion technique [19–21]. Lim et al. prepared nanosized HA powders in the form of dendritic agglomerates by a unique oil-in-water emulsion route [19]. They found that specific surface area of the HA powders changed from 7 to 75 m2 =g depending on the petroleum ether content in the emulsion. Bose and Saha investigated the effects of emulsion composition and synthesis parameters on the formation of HA nanopowders and their morphology [20]. Layrolle et al. have reported the synthesis of HA powders with a specific surface area of 26 m2 =g and a strongly agglomerated morphology [21]. The present study is aimed at investigating the optimization of the M–H synthesis of HA powers with lowered degree of particle agglomeration from nonionic surfactant emulsion precursors. The effects of the calcium source and the emulsion composition on the stoichiometry, crystallinity, thermal stability and morphology of final products were studied.
2. Experimental methods A biodegradable nonionic surfactant, Tomadol 23–6.5 ðCn H2nþ1 OðCH2 CH2 OÞm H; n ¼ 12–13; m ¼ 6:5), was obtained from Tomah Products, Inc. A partial phase diagram (see Fig. 1) was established at room temperature for the emulsion system consisting of the following: an aqueous phase (either CaðNO3 Þ2 solution or CaðOHÞ2 suspension), petroleum ether and a nonionic surfactant, Tomadol 23–6.5. Specific reaction conditions are given in Table 1. Two different calcium sources, CaðOHÞ2 and CaðNO3 Þ2 , were
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evaluated. The calcium source was either dissolved ðCaðNO3 Þ2 Þ or dispersed ðCaðOHÞ2 Þ into deionized water. The surfactant followed by the petroleum ether was then mixed with the salt solution/ suspension to form an emulsion. For most emulsion systems, a clear gel was formed during mixing (see Table 1). Finally, a stoichiometric amount of ðNH4 Þ2 HPO4 solution was added to the emulsion system while vigorously stirring with a mechanical stirrer. The slurry was transferred into a laboratory-scale mill (Model Micros MIC-0, Nara Machinery CO.). Detailed mechanochemical–hydrothermal (M–H) synthesis and product
Fig. 1. Partial phase diagram for the ternary system water-PETomadol 23-6.5 at room temperature. PE: petroleum ether. Symbol ðÞ denotes the compositions selected for experimental work.
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treatment were described in our previous paper [17]c. The product was termed as ‘‘as-prepared DOL’’. XRD analyses were performed using a Kristalloflex D-500 powder diffractometer using Ni filtered Cu Ka radiation. Crystallographic identification of the as-prepared and heat-treated HA powders was accomplished by comparing the experimental XRD patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS; HA, card #09-0432; bTCP, #09-0169; CaO, #37-1497). Transmission electron microscopy (TEM) images were obtained with a high-resolution analytical electron microscope (Model EM-002B, International Scientific Instruments) at an acceleration voltage of 200 kV. Infrared spectra were recorded using a PerkinElmer 1720-X fourier transform infrared (FTIR) spectrometer. The specific surface area (SSA) of product was measured via BET analysis of nitrogen isotherms (Gemini III 2375 surface area analyzer). d BET and sv LBET are the diameter and length of a circular sv cylinder particle calculated from BET analysis of nitrogen adsorption isotherms. Assuming a circular cylinder morphology, LBET can be calculated as sv follows: LBET ¼ sv
2 þ 4a ; SSAr
Table 1 Emulsion systems of Water–PE–Tomadol 23-6.5 and characteristics of the HA powders Samplea
Point number
PE (wt%)
Water (wt%)
Surfactant (wt%)
Emulsion
SSAb ðm2 =gÞ
HA DOL0, ðCaðNO3 Þ2 Þ DOL1 DOL2 DOL3 DOL4 DOL5 DOL6 DOL7 DOL8 DOL9
— 1 1 2 3 4 5 6 7 8 9
— 10 10 10 10 10 20 40 52 55 80
100 87 87 80 60 40 60 40 25 5 10
— 3 3 10 30 50 20 20 23 40 10
Suspension 2L (water, PE) 2L (water, PE) Gel+ 1L (water) Gel Gel Gel+ 1 L (water) Gel+ 1 L (PE) Gel+ 1 L (PE) One phase (1L) Gel+ 1 L (PE)
175 137 139 231 190 213 179 149 72 95 85
a
CaðOHÞ2 was used as Ca source except for DOL0. Specific surface area is determined by BET method.
b
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o
(526)
(1511)
DOL0, heat-treated at 900 C, 1h β-TCP
(517)
In the present study, gel phase, one isotropic liquid phase and two isotropic liquid phase formed depending on the composition in the ternary system water-PE-Tomadol 23-6.5. Fig. 1 represents the partial phase diagram at room temperature for this system. Table 1 gives the compositions and characteristics of emulsions (Points 1–9) investigated for the HA synthesis. The terms 1L, 2L, Gel, and Gel+1L represent one-liquid, twoliquid, gel, and gel plus one-liquid phases, respectively. The phase changes were determined visually on the following criteria. Single isotropic phases (1L) were clear and of low viscosity. Gel phases were clear and of high viscosity. Systems containing two isotropic phases were turbid and of low viscosity. The gel-like material is not a single phase, but simply a mixture of phases with high concentration of droplets [22]. It may be a fine dispersion of either oil-in-water or water-in-oil emulsion type depending on the composition. When the PE concentration was 10 wt% and surfactant concentration was in the range of 30–50 wt%, gel emulsions of oil-in-water type were formed (Points 3 and 4). The two-phase region (Gel + 1L) resulted when the concentration of Tomadol 23-6.5 was 10–23 wt%. Points 2 and 5 represented a gel and a lower liquid phase that was mainly water. Emulsions of Points 6, 7 and 9 had a gel and an upper PE liquid phase. The system (Point 1) containing 3 wt% of Tomadol 23-6.5 and 10 wt% of PE separated into two liquids: an upper turbid PE phase and a lower clear water phase. As indicated in the partial phase diagram, one-liquid phase emulsion (Point 8) was formed when the
When CaðNO3 Þ2 was used as the calcium source, the pH of the system was adjusted to 9 by adding ammonium hydroxide prior to milling. Fig. 2 shows the XRD patterns of the as-prepared and heat-treated DOL0 powders. The as-prepared DOL0 powder derived from the emulsion (Point 1) containing 10 wt% PE, 87 wt% water and 3 wt% Tomadol 23-6.5 showed a much broadened X-ray diffraction trace, indicating its low degree of crystallinity. The DOL0 powder also exhibited low thermal stability and XRD showed only bCa3 ðPO4 Þ2 ðb-TCP) after heat-treatment at 900 C for 1 h. This indicates that HA powder formed under these conditions is calcium deficient with a Ca/P molar ratio close to 1.5 [8]. When CaðOHÞ2 was used as the calcium source, no additional alkaline mineralizer was needed as the pH of the precursor slurry was 9.3 owing to the inherent basicity of CaðOHÞ2 . Typical XRD patterns of as-prepared and heat-treated DOL3 samples derived from a gel emulsion (Point 3) containing 10 wt% PE, 60 wt% water and 30 wt%
(1010) (214) (0210) (128)) (220) (2110) (1211)) (1016) (404)) (3012) (0018) (048)) (2212)) (4010) (238)) (416) (0120) (054)) (2020) (508)
3.1. Phase diagram
3.2. HA crystallization
(300)
3. Results and discussion
concentrations of PE and surfactant were 55 and 40 wt%, respectively.
(024)
where SSA is the specific surface area, a is the BET aspect ratio (LBET sv =d sv , length/diameter of a circular cylinder), and r is particle density. When d BET and LBET were calculated, the a value sv sv was determined based on TEM observation. A density of 3:156 g=cm3 , which is the theoretical density of stoichiometric HA, was used in all calculations.
Intensity (a.u.)
618
DOL0 from Ca(NO3 )2
20
30
40
50
60
70
2Θ (deg)
Fig. 2. XRD patterns of DOL0 powders synthesized using CaðNO3 Þ2 as the calcium source and heat-treated in air at 900 C for 1 h.
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(420) (214) (502) (511)(304,323)
(322) (313)
(222) (312) (213) (321)(410) (402) (004)
(211)
o
DOL3, heat-treated at 900 C, 1h HA
(310) (311)
(202)
(002) (102) (210)
(200) (111)
Intensity (a.u.)
(300)(112)
C.-W. Chen et al. / Journal of Crystal Growth 270 (2004) 615–623
DOL3 from Ca(OH)2
20
30
40
50
60
70
2Θ (deg)
Fig. 3. XRD patterns of DOL3 powders synthesized using CaðOHÞ2 as the calcium source and heat-treated in air at 900 C for 1 h.
Tomadol 23-6.5 are shown in Fig. 3. The crystallinity of as-prepared DOL3 was similar to that of as-prepared DOL0. Upon heat-treatment at 900 C for 1 h, however, no b-TCP or CaO occurred and the level of crystallinity increased significantly as revealed by the sharpened and distinguished peaks over the entire 2y angle range. The XRD peaks were well defined and attributable only to HA according to the JCPCS card. This fact indicates that the mechanochemical–hydrothermal (M–H) treatment results in a nearly stoichiometric HA single phase from the emulsion system. We have reported that the phase-pure HA powder was also obtained from an aqueous suspension when CaðOHÞ2 and ðNH4 Þ2 HPO4 were used as the calcium and phosphorus sources, respectively [16]. The addition of ðNH4 Þ2 HPO4 solution into the CaðOHÞ2 suspension resulted in a fast precipitation of amorphous calcium phosphate (ACP). The ACP phase crystallized and transformed into HA during the M–H process. The nucleation and growth of HA proceeded in a more or less uncontrolled manner in the M–H reactor, resulting in large agglomerates of primary HA particles [17c]. In the case of emulsion system (Point 3), however, the CaðOHÞ2 suspension was mixed with PE and Tomadol 23-6.5 to form gel-like dispersion. As mentioned previously, the gel-like
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dispersion was an oil-in-water emulsion with high concentration of droplets. The hydrophobic portion of surfactant molecule resides in the hydrophobic cores of emulsion droplets and the ethylene oxide portion stretches into the aqueous phase. The ethylene oxide units of the surfactant molecules tend to form complexes with Ca2þ ions at the surfaces of emulsion droplets [23,24]. The Ca2þ ions are mostly released to react with phosphate ions during the M–H process. The diffusion of both calcium and phosphate ions may be hindered by the high viscosity gel emulsion, leading to the delayed nucleation and slow growth of the HA crystalline. In addition, the local stabilization of Ca2þ on the droplet surfaces due to complexation of Tomadol 23-6.5 molecules also favors the formation of disperse HA crystallites. All these factors make the emulsion system more suitable than the aqueous suspension for the formation of HA particles with less agglomeration. Lim et al. reported the formation of nanocrystalline HA via an oil-in-water emulsion processing route [19]. They suggested that the complexation of calcium ions by ethylene oxide groups of the nonionic KB6ZA surfactant on the surface of emulsion droplets constitutes the formation sites for nanosized HA powders with enhanced crystallinity and size uniformity. A significant advantage of the present M–H treatment is that the stoichiometry of resulting HA powders is not affected by the high viscosity of the gel-like emulsion. The multi-ring media mill MIC0 can mix the gel-like emulsion at a high rotation speed up to 2000 rpm. The reaction mixture is extensively homogenized, which provides favorable conditions for the formation of nearly stoichiometric HA from the high viscosity emulsion system. However, it has been reported that milling is also thought to cause deprotonation from HPO2 ions in monetite ðCaHPO4 ) or 4 brushite ðCaHPO4 2H2 OÞ that may coexist with HA during the M–H process [25]. Seven runs were performed on MIC-0 using the emulsion system denoted by point 3 in the ‘‘gel’’ region shown in Fig. 1 and produced phase-pure HA judged on the basis of XRD. Therefore, the synthesis of HA powders using the M–H treatment has very good reproducibility in terms of crystallinity and
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chemical composition and milling does not appear to degrade phase purity. 3.3. Chemical moieties FTIR spectra of the as-prepared and heattreated DOL3 samples are presented in Fig. 4, and are characteristic of hydroxyapatite. The DOL3 sample produced the characteristic phosphate and hydroxyl bands corresponding to synthetic hydroxyapatite [26]. Phosphate absorption bands at about 1044, 963, 603, 566, and 475 cm1 were observed on the FTIR spectra of the as-prepared and heat-treated DOL3 samples. The absorption band at 3572 and 633 cm1 , due to OH stretching and libration modes, respectively, were very weak in the spectrum of the as-prepared sample and likely resulted from the high surface area, low degree of crystallinity, and presence of adsorbed water [27]. The degree of crystallinity was improved and the adsorbed water was removed after calcination at 900 C for 1 h, resulting in an increase in intensity and a sharpening of the absorption bands due to OH groups. In addition to these phosphate and hydroxyl bands, the as-prepared DOL3 sample also produced carbonate bands around 1480–1410 cm1 for the n3 mode and at 870 cm1 for n2 mode. Using the data of LeGeros [28], this
ν1 PO4
Transmittance (a.u.)
OH- (stretch)
4000
DOL3, as-prepared
H2O CO3
H2O
ν2 PO4
CO3 OH(libr) ν4 PO4 ν3 PO4
DOL3, heat-treated at 900° C, 1h
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. FTIR spectra of as-prepared and heat-treated DOL3 powders. Heat-treatment condition: in air, 900 C, 1 h.
would mainly correspond to B-type carbonate 3 substitution) in the substitution ðCO2 3 -for-PO4 DOL3 particles. The level of carbonate substitution in DOL3 was higher than that found in the HA material prepared from aqueous suspensions. Apart from the absorption of atmospheric CO2 during the slurry preparation and the M–H process, the organic PE and surfactant phases used for creating the emulsion composition also contributed to carbonate substitution in the emulsion-derived hydroxyapatites [12,29]. These carbonates cannot be eliminated upon heat-treatment at 900 C for 1 h. The CO2 ion peaks at 3 1480–1420 and 870 cm1 are still notable for the heat-treated DOL3 powder. Biological apatites contain appreciable amounts of carbonates upto about 7.4 wt% [30]. Thus, the presence of carbonates in the emulsion-derived hydroxyapatites is desirable for many medical applications.
3.4. Morphology and SSA Fig. 5 shows TEM micrographs of the asprepared HA powders from aqueous suspension and emulsion systems with different compositions. In the absence of PE and surfactant, the HA crystals showed needle-like morphology with a mean length of 35 nm and mean diameter of ca. 8 nm (Fig. 5a). The aspect ratio was nearly 4.4. The HA particles exhibited significant agglomeration and broad particle/agglomerate size distribution. In the presence of PE and surfactant, the morphology of the HA particles was found to depend on the Ca source and the composition of the emulsion system. Needle shaped particles with a high aspect ratio of 10–15 were formed when CaðNO3 Þ2 was used as the Ca source, as shown in Fig. 5b (DOL0). The average length of the particles was in the range of 50–60 nm and the diameter was between 4 and 5 nm. In comparison, HA powders obtained using CaðOHÞ2 as the Ca source had particle sizes of 20–30 nm with a low aspect ratio of 2–3 (DOL1). HA material prepared from the gel-like emulsion containing 10 wt% PE, 60 wt% water and 30 wt% surfactant consisted of needle-like particles with an approximate length and diameter of 160 and 25 nm, respectively
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Fig. 5. TEM images of as-prepared HA powders: (a) HA from aqueous suspension; (b) DOL0 from CaðNO3 Þ2 ; (c) DOL3; (d) DOL4; (e) DOL7; and (f) DOL8.
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showed a similar morphology to DOL7. Needleshaped HA particles with a narrow size distribution were formed when the water concentration was 5 wt% in the one-liquid phase emulsion (DOL8), as shown in Fig. 5f. The average diameter of the particles was in the range of 10–20 nm and the length between 100 and 150 nm. Therefore, the morphology and particle size of the HA particles from the emulsion systems strongly depended upon the composition of the emulsion. Fig. 6 shows the variation in the SSA of the HA samples with the composition of the emulsion system. The SSA value of the HA sample prepared in aqueous suspension was 175 m2 =g. When we selected the aspect ratio of 4.4, the calculated d BET sv and LBET were 8.1 and 35.5 nm, respectively. The sv result was in good agreement with the TEM observation. As shown in Fig. 5, the composition of the emulsion system governs the morphology and size of emulsion-derived HA powders. This was reflected by the average SSA shown in Fig. 6. At lower PE concentrations, the particles had higher SSA in the range of 139–231 m2 =g. The DOL1 synthesized from the two-liquid phase emulsion exhibited an SSA as low as 139 m2 =g. The DOL3 particles showed a high SSA value of 190 m2 =g. If the aspect ratio was set to 6.4, the d BET and LBET sv sv were 7.2 and 46 nm, respectively. The values were far smaller than those observed by TEM. This can
300
(DOL3, Fig. 5c). The HA particles were very dispersible and had a uniform shape. With decreasing water/surfactant ratio from 2 to 0.8, the morphology of the resulting DOL4 powders changed significantly. The DOL4 nanopowders with lower aspect ratio were agglomerated at the water/surfactant ratio of 0.8 (Fig. 5d). TEM micrograph of HA powder obtained from emulsion system in the gel plus 1L region is shown in Fig. 5e (DOL7). The HA material showed a greater degree of agglomeration. The DOL9 particles obtained in the same gel plus 1L region
Specific surface area (m2/g)
PE = 10 wt%
PE > 50 wt%
250 200 150 100 50 0
HA DOL0 1
2
3
4
5
6
7
8
9
Fig. 6. Specific surface area for HA powders obtained from aqueous suspension and emulsion systems with different compositions.
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be explained by the microstructure of HA particles. The HA particles consisted of nanocrystallites and had a high porous surface, as revealed by TEM observation. The DOL4 sample obtained from a gel-like emulsion showed a much higher SSA value of 213 m2 =g. When the PE concentration was higher than 50 wt%, the resulting powders possessed low SSA values due to the greater degree of agglomeration. For example, the DOL7 powder obtained from the emulsion system containing 52 wt% PE, 25 wt% water and 23 wt% Tomadol 23-6.5 exhibited an average SSA of 72 m2 =g only. Disperse HA crystals with a low SSA of 95 m2 =g were obtained from the one-liquid phase emulsion (DOL8). When the aspect ratio was set to 8.3, the calculated d BET and LBET were 14.1 and 118 nm, respectively, sv sv which were close to the data from TEM observations. Thus, the low SSA was attributed to a large particle size with a smooth surface. Thus, it can be concluded that the SSA of the synthesized nanopowders is significantly affected by the PE concentration in the emulsion.
4. Conclusions Hydroxyapatite nanocrystals were synthesized by the mechanochemical–hydrothermal method in a unique emulsion system using a biodegradable nonionic surfactant, Tomadol 23-6.5. The composition of the emulsion system and calcium source had a significant effect on the formation of phasepure HA nanopowders and their morphology and surface area. The HA powder synthesized by using CaðNO3 Þ2 as the Ca source showed low thermal stability and transformed into b-TCP after heattreatment at 900 C for 1 h. No b-TCP was detected in the heat-treated HA powder prepared from the CaðOHÞ2 precursor. The emulsionderived HA material exhibited type B carbonate substitution, which could not be eliminated by heat-treatment at 900 C for 1 h. HA powders with specific surface area in the range from 72 to 231 m2 =g were prepared depending on the emulsion composition. Disperse HA crystals with a length of 160 nm and an aspect ratio of 6 were
formed in the emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant.
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Journal of Crystal Growth 270 (2004) 615–623 www.elsevier.com/locate/jcrysgro
Mechanochemical–hydrothermal synthesis of hydroxyapatite from nonionic surfactant emulsion precursors Chun-Wei Chena,, Richard E. Rimana, Kevor S. TenHuisenb,1, Kelly Brownb a
Department of Ceramic and Materials Engineering, Rutgers University, 607 Taylor Road, Piscataway, NJ 08854, USA b Center for Biomaterials & Advanced Technologies, Medical Devices Group, a Division of Ethicon, Inc., Rt. 22 W, P.O. Box 151, Somerville, NJ 08876, USA Received 7 May 2004; accepted 29 June 2004 Communicated by M. Schieber Available online 3 September 2004
Abstract Nanocrystalline hydroxyapatite [HA, Ca10 ðPO4 Þ6 ðOHÞ2 ] powders were synthesized by the mechanochemical–hydrothermal method using emulsion systems consisting of aqueous phase, petroleum ether (PE) as the oil phase and biodegradable Tomadol 23–6.5 as the nonionic surfactant. ðNH4 Þ2 HPO4 and CaðNO3 Þ2 or CaðOHÞ2 were used as the phosphorus and calcium sources, respectively. The calcium source and emulsion composition had significant effects on the stoichiometry, crystallinity, thermal stability, particle size and morphology of final products. Disperse HA crystals with a 160 nm length and aspect ratio of ca. 6 were formed in an emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant. The HA particles had needle morphology with a specific surface area of 190 m2 =g. With this technique, HA nanopowders with specific surface areas in the range of 72–231 m2 =g were produced. r 2004 Elsevier B.V. All rights reserved. PACS: 61.66.Fn; 61.82.Rx; 81.10.A Keywords: A1. Biomaterials; A1. Characterization; A1. Mechanochemical; A2. Hydrothermal; B1. Hydroxyapatite
1. Introduction Hydroxyapatite [HA, Ca10 ðPO4 Þ6 ðOHÞ2 ] is the main constituent of hard tissues such as bone, Corresponding author. Tel.: +1-732-445-7092; fax: +1-
732-445-3258. E-mail address: [email protected] (C.-W. Chen). 1 Now at Reconstructive Business, Stryker Howmedica Osteonics, 59 Rt. 17 South, Allendale, NJ 07401, USA.
dentin, and enamel. Its biocompatibility and osteoconductive properties make it desirable as an implant material and drug delivery agent [1–4]. Nevertheless, either dense or porous HA suffers from low fracture toughness (less than 1 MPa m1=2 ) with respect to human bone (2–12 MPa m1=2 ), which limits its application as a monolithic phase in artificial bone materials [5]. One solution to the problem is the incorporation
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.051
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of HA particles into a polymer matrix to produce HA/polymer composites. Composites for orthopedic applications have been designed with the aim of attempting to match the properties and structure of bone. Those HA/polymer composites with improved mechanical properties have been used as an alternative material for internal fixation of human fractures [6,7]. Expanded use of engineered materials incorporating HA may depend on the ability to synthesize HA particulates with well-controlled physicochemical characteristics (purity, stoichiometry, morphology, and crystallinity, etc.). In the literature, various synthesis methods to prepare HA powders have been reported, including solid-state reactions, precipitation from mixed aqueous solutions, hydrolysis of calcium phosphates, and sol–gel crystallization [8–12]. Most of the synthetic procedures followed until now led to the agglomerated HA particles. Agglomeration can cause lower sintered density as well as crack-like voids during densification [13,14]. Thus, because of these defects, the use of HA materials is limited by poor mechanical strength and toughness and resultantly finds utility mostly in non-load-bearing applications [15]. Recently, we reported the synthesis of phasepure and crystalline HA by heterogeneous reaction of CaðOHÞ2 powder and aqueous ðNH4 Þ2 HPO4 at room temperature using the mechanochemical– hydrothermal (M–H) method [16,17]. The main advantages of this synthesis method are simplicity and low cost since inexpensive milling equipment and starting materials can be used. Under these synthesis conditions, amorphous calcium phosphate (ACP) precipitates upon initial mixing of the precursor solutions. The ACP precursor phase crystallizes and transforms into HA during the M–H process. A nanophase HA material results, which consists of agglomerates of 20–30 nm HA crystals. The mass-weight mean agglomerate size is ca. 2:6 mm in deionized water with an average agglomeration number (AAN) of 4:5 106 [17]c. The objective of this work is to minimize the degree of agglomeration. To accomplish this, we focused on control of nucleation and growth of the ACP phase by the utilization of emulsions. An emulsion is a mixture of two immiscible liquids in
which one liquid is dispersed in the other with the aid of a surfactant. The droplet not only controls the physical powder characteristics but also limits chemical segregation to the length scale of the droplet diameter. Emulsion methods have been shown to be one of the techniques that are capable of preparing ceramic powders with controlled particle size and distribution, uniform shape, lowered degree of agglomeration, and a high degree of chemical purity [18]. However, limited studies have been reported on the development of HA powders by the emulsion technique [19–21]. Lim et al. prepared nanosized HA powders in the form of dendritic agglomerates by a unique oil-in-water emulsion route [19]. They found that specific surface area of the HA powders changed from 7 to 75 m2 =g depending on the petroleum ether content in the emulsion. Bose and Saha investigated the effects of emulsion composition and synthesis parameters on the formation of HA nanopowders and their morphology [20]. Layrolle et al. have reported the synthesis of HA powders with a specific surface area of 26 m2 =g and a strongly agglomerated morphology [21]. The present study is aimed at investigating the optimization of the M–H synthesis of HA powers with lowered degree of particle agglomeration from nonionic surfactant emulsion precursors. The effects of the calcium source and the emulsion composition on the stoichiometry, crystallinity, thermal stability and morphology of final products were studied.
2. Experimental methods A biodegradable nonionic surfactant, Tomadol 23–6.5 ðCn H2nþ1 OðCH2 CH2 OÞm H; n ¼ 12–13; m ¼ 6:5), was obtained from Tomah Products, Inc. A partial phase diagram (see Fig. 1) was established at room temperature for the emulsion system consisting of the following: an aqueous phase (either CaðNO3 Þ2 solution or CaðOHÞ2 suspension), petroleum ether and a nonionic surfactant, Tomadol 23–6.5. Specific reaction conditions are given in Table 1. Two different calcium sources, CaðOHÞ2 and CaðNO3 Þ2 , were
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evaluated. The calcium source was either dissolved ðCaðNO3 Þ2 Þ or dispersed ðCaðOHÞ2 Þ into deionized water. The surfactant followed by the petroleum ether was then mixed with the salt solution/ suspension to form an emulsion. For most emulsion systems, a clear gel was formed during mixing (see Table 1). Finally, a stoichiometric amount of ðNH4 Þ2 HPO4 solution was added to the emulsion system while vigorously stirring with a mechanical stirrer. The slurry was transferred into a laboratory-scale mill (Model Micros MIC-0, Nara Machinery CO.). Detailed mechanochemical–hydrothermal (M–H) synthesis and product
Fig. 1. Partial phase diagram for the ternary system water-PETomadol 23-6.5 at room temperature. PE: petroleum ether. Symbol ðÞ denotes the compositions selected for experimental work.
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treatment were described in our previous paper [17]c. The product was termed as ‘‘as-prepared DOL’’. XRD analyses were performed using a Kristalloflex D-500 powder diffractometer using Ni filtered Cu Ka radiation. Crystallographic identification of the as-prepared and heat-treated HA powders was accomplished by comparing the experimental XRD patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS; HA, card #09-0432; bTCP, #09-0169; CaO, #37-1497). Transmission electron microscopy (TEM) images were obtained with a high-resolution analytical electron microscope (Model EM-002B, International Scientific Instruments) at an acceleration voltage of 200 kV. Infrared spectra were recorded using a PerkinElmer 1720-X fourier transform infrared (FTIR) spectrometer. The specific surface area (SSA) of product was measured via BET analysis of nitrogen isotherms (Gemini III 2375 surface area analyzer). d BET and sv LBET are the diameter and length of a circular sv cylinder particle calculated from BET analysis of nitrogen adsorption isotherms. Assuming a circular cylinder morphology, LBET can be calculated as sv follows: LBET ¼ sv
2 þ 4a ; SSAr
Table 1 Emulsion systems of Water–PE–Tomadol 23-6.5 and characteristics of the HA powders Samplea
Point number
PE (wt%)
Water (wt%)
Surfactant (wt%)
Emulsion
SSAb ðm2 =gÞ
HA DOL0, ðCaðNO3 Þ2 Þ DOL1 DOL2 DOL3 DOL4 DOL5 DOL6 DOL7 DOL8 DOL9
— 1 1 2 3 4 5 6 7 8 9
— 10 10 10 10 10 20 40 52 55 80
100 87 87 80 60 40 60 40 25 5 10
— 3 3 10 30 50 20 20 23 40 10
Suspension 2L (water, PE) 2L (water, PE) Gel+ 1L (water) Gel Gel Gel+ 1 L (water) Gel+ 1 L (PE) Gel+ 1 L (PE) One phase (1L) Gel+ 1 L (PE)
175 137 139 231 190 213 179 149 72 95 85
a
CaðOHÞ2 was used as Ca source except for DOL0. Specific surface area is determined by BET method.
b
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o
(526)
(1511)
DOL0, heat-treated at 900 C, 1h β-TCP
(517)
In the present study, gel phase, one isotropic liquid phase and two isotropic liquid phase formed depending on the composition in the ternary system water-PE-Tomadol 23-6.5. Fig. 1 represents the partial phase diagram at room temperature for this system. Table 1 gives the compositions and characteristics of emulsions (Points 1–9) investigated for the HA synthesis. The terms 1L, 2L, Gel, and Gel+1L represent one-liquid, twoliquid, gel, and gel plus one-liquid phases, respectively. The phase changes were determined visually on the following criteria. Single isotropic phases (1L) were clear and of low viscosity. Gel phases were clear and of high viscosity. Systems containing two isotropic phases were turbid and of low viscosity. The gel-like material is not a single phase, but simply a mixture of phases with high concentration of droplets [22]. It may be a fine dispersion of either oil-in-water or water-in-oil emulsion type depending on the composition. When the PE concentration was 10 wt% and surfactant concentration was in the range of 30–50 wt%, gel emulsions of oil-in-water type were formed (Points 3 and 4). The two-phase region (Gel + 1L) resulted when the concentration of Tomadol 23-6.5 was 10–23 wt%. Points 2 and 5 represented a gel and a lower liquid phase that was mainly water. Emulsions of Points 6, 7 and 9 had a gel and an upper PE liquid phase. The system (Point 1) containing 3 wt% of Tomadol 23-6.5 and 10 wt% of PE separated into two liquids: an upper turbid PE phase and a lower clear water phase. As indicated in the partial phase diagram, one-liquid phase emulsion (Point 8) was formed when the
When CaðNO3 Þ2 was used as the calcium source, the pH of the system was adjusted to 9 by adding ammonium hydroxide prior to milling. Fig. 2 shows the XRD patterns of the as-prepared and heat-treated DOL0 powders. The as-prepared DOL0 powder derived from the emulsion (Point 1) containing 10 wt% PE, 87 wt% water and 3 wt% Tomadol 23-6.5 showed a much broadened X-ray diffraction trace, indicating its low degree of crystallinity. The DOL0 powder also exhibited low thermal stability and XRD showed only bCa3 ðPO4 Þ2 ðb-TCP) after heat-treatment at 900 C for 1 h. This indicates that HA powder formed under these conditions is calcium deficient with a Ca/P molar ratio close to 1.5 [8]. When CaðOHÞ2 was used as the calcium source, no additional alkaline mineralizer was needed as the pH of the precursor slurry was 9.3 owing to the inherent basicity of CaðOHÞ2 . Typical XRD patterns of as-prepared and heat-treated DOL3 samples derived from a gel emulsion (Point 3) containing 10 wt% PE, 60 wt% water and 30 wt%
(1010) (214) (0210) (128)) (220) (2110) (1211)) (1016) (404)) (3012) (0018) (048)) (2212)) (4010) (238)) (416) (0120) (054)) (2020) (508)
3.1. Phase diagram
3.2. HA crystallization
(300)
3. Results and discussion
concentrations of PE and surfactant were 55 and 40 wt%, respectively.
(024)
where SSA is the specific surface area, a is the BET aspect ratio (LBET sv =d sv , length/diameter of a circular cylinder), and r is particle density. When d BET and LBET were calculated, the a value sv sv was determined based on TEM observation. A density of 3:156 g=cm3 , which is the theoretical density of stoichiometric HA, was used in all calculations.
Intensity (a.u.)
618
DOL0 from Ca(NO3 )2
20
30
40
50
60
70
2Θ (deg)
Fig. 2. XRD patterns of DOL0 powders synthesized using CaðNO3 Þ2 as the calcium source and heat-treated in air at 900 C for 1 h.
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(420) (214) (502) (511)(304,323)
(322) (313)
(222) (312) (213) (321)(410) (402) (004)
(211)
o
DOL3, heat-treated at 900 C, 1h HA
(310) (311)
(202)
(002) (102) (210)
(200) (111)
Intensity (a.u.)
(300)(112)
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DOL3 from Ca(OH)2
20
30
40
50
60
70
2Θ (deg)
Fig. 3. XRD patterns of DOL3 powders synthesized using CaðOHÞ2 as the calcium source and heat-treated in air at 900 C for 1 h.
Tomadol 23-6.5 are shown in Fig. 3. The crystallinity of as-prepared DOL3 was similar to that of as-prepared DOL0. Upon heat-treatment at 900 C for 1 h, however, no b-TCP or CaO occurred and the level of crystallinity increased significantly as revealed by the sharpened and distinguished peaks over the entire 2y angle range. The XRD peaks were well defined and attributable only to HA according to the JCPCS card. This fact indicates that the mechanochemical–hydrothermal (M–H) treatment results in a nearly stoichiometric HA single phase from the emulsion system. We have reported that the phase-pure HA powder was also obtained from an aqueous suspension when CaðOHÞ2 and ðNH4 Þ2 HPO4 were used as the calcium and phosphorus sources, respectively [16]. The addition of ðNH4 Þ2 HPO4 solution into the CaðOHÞ2 suspension resulted in a fast precipitation of amorphous calcium phosphate (ACP). The ACP phase crystallized and transformed into HA during the M–H process. The nucleation and growth of HA proceeded in a more or less uncontrolled manner in the M–H reactor, resulting in large agglomerates of primary HA particles [17c]. In the case of emulsion system (Point 3), however, the CaðOHÞ2 suspension was mixed with PE and Tomadol 23-6.5 to form gel-like dispersion. As mentioned previously, the gel-like
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dispersion was an oil-in-water emulsion with high concentration of droplets. The hydrophobic portion of surfactant molecule resides in the hydrophobic cores of emulsion droplets and the ethylene oxide portion stretches into the aqueous phase. The ethylene oxide units of the surfactant molecules tend to form complexes with Ca2þ ions at the surfaces of emulsion droplets [23,24]. The Ca2þ ions are mostly released to react with phosphate ions during the M–H process. The diffusion of both calcium and phosphate ions may be hindered by the high viscosity gel emulsion, leading to the delayed nucleation and slow growth of the HA crystalline. In addition, the local stabilization of Ca2þ on the droplet surfaces due to complexation of Tomadol 23-6.5 molecules also favors the formation of disperse HA crystallites. All these factors make the emulsion system more suitable than the aqueous suspension for the formation of HA particles with less agglomeration. Lim et al. reported the formation of nanocrystalline HA via an oil-in-water emulsion processing route [19]. They suggested that the complexation of calcium ions by ethylene oxide groups of the nonionic KB6ZA surfactant on the surface of emulsion droplets constitutes the formation sites for nanosized HA powders with enhanced crystallinity and size uniformity. A significant advantage of the present M–H treatment is that the stoichiometry of resulting HA powders is not affected by the high viscosity of the gel-like emulsion. The multi-ring media mill MIC0 can mix the gel-like emulsion at a high rotation speed up to 2000 rpm. The reaction mixture is extensively homogenized, which provides favorable conditions for the formation of nearly stoichiometric HA from the high viscosity emulsion system. However, it has been reported that milling is also thought to cause deprotonation from HPO2 ions in monetite ðCaHPO4 ) or 4 brushite ðCaHPO4 2H2 OÞ that may coexist with HA during the M–H process [25]. Seven runs were performed on MIC-0 using the emulsion system denoted by point 3 in the ‘‘gel’’ region shown in Fig. 1 and produced phase-pure HA judged on the basis of XRD. Therefore, the synthesis of HA powders using the M–H treatment has very good reproducibility in terms of crystallinity and
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chemical composition and milling does not appear to degrade phase purity. 3.3. Chemical moieties FTIR spectra of the as-prepared and heattreated DOL3 samples are presented in Fig. 4, and are characteristic of hydroxyapatite. The DOL3 sample produced the characteristic phosphate and hydroxyl bands corresponding to synthetic hydroxyapatite [26]. Phosphate absorption bands at about 1044, 963, 603, 566, and 475 cm1 were observed on the FTIR spectra of the as-prepared and heat-treated DOL3 samples. The absorption band at 3572 and 633 cm1 , due to OH stretching and libration modes, respectively, were very weak in the spectrum of the as-prepared sample and likely resulted from the high surface area, low degree of crystallinity, and presence of adsorbed water [27]. The degree of crystallinity was improved and the adsorbed water was removed after calcination at 900 C for 1 h, resulting in an increase in intensity and a sharpening of the absorption bands due to OH groups. In addition to these phosphate and hydroxyl bands, the as-prepared DOL3 sample also produced carbonate bands around 1480–1410 cm1 for the n3 mode and at 870 cm1 for n2 mode. Using the data of LeGeros [28], this
ν1 PO4
Transmittance (a.u.)
OH- (stretch)
4000
DOL3, as-prepared
H2O CO3
H2O
ν2 PO4
CO3 OH(libr) ν4 PO4 ν3 PO4
DOL3, heat-treated at 900° C, 1h
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. FTIR spectra of as-prepared and heat-treated DOL3 powders. Heat-treatment condition: in air, 900 C, 1 h.
would mainly correspond to B-type carbonate 3 substitution) in the substitution ðCO2 3 -for-PO4 DOL3 particles. The level of carbonate substitution in DOL3 was higher than that found in the HA material prepared from aqueous suspensions. Apart from the absorption of atmospheric CO2 during the slurry preparation and the M–H process, the organic PE and surfactant phases used for creating the emulsion composition also contributed to carbonate substitution in the emulsion-derived hydroxyapatites [12,29]. These carbonates cannot be eliminated upon heat-treatment at 900 C for 1 h. The CO2 ion peaks at 3 1480–1420 and 870 cm1 are still notable for the heat-treated DOL3 powder. Biological apatites contain appreciable amounts of carbonates upto about 7.4 wt% [30]. Thus, the presence of carbonates in the emulsion-derived hydroxyapatites is desirable for many medical applications.
3.4. Morphology and SSA Fig. 5 shows TEM micrographs of the asprepared HA powders from aqueous suspension and emulsion systems with different compositions. In the absence of PE and surfactant, the HA crystals showed needle-like morphology with a mean length of 35 nm and mean diameter of ca. 8 nm (Fig. 5a). The aspect ratio was nearly 4.4. The HA particles exhibited significant agglomeration and broad particle/agglomerate size distribution. In the presence of PE and surfactant, the morphology of the HA particles was found to depend on the Ca source and the composition of the emulsion system. Needle shaped particles with a high aspect ratio of 10–15 were formed when CaðNO3 Þ2 was used as the Ca source, as shown in Fig. 5b (DOL0). The average length of the particles was in the range of 50–60 nm and the diameter was between 4 and 5 nm. In comparison, HA powders obtained using CaðOHÞ2 as the Ca source had particle sizes of 20–30 nm with a low aspect ratio of 2–3 (DOL1). HA material prepared from the gel-like emulsion containing 10 wt% PE, 60 wt% water and 30 wt% surfactant consisted of needle-like particles with an approximate length and diameter of 160 and 25 nm, respectively
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Fig. 5. TEM images of as-prepared HA powders: (a) HA from aqueous suspension; (b) DOL0 from CaðNO3 Þ2 ; (c) DOL3; (d) DOL4; (e) DOL7; and (f) DOL8.
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showed a similar morphology to DOL7. Needleshaped HA particles with a narrow size distribution were formed when the water concentration was 5 wt% in the one-liquid phase emulsion (DOL8), as shown in Fig. 5f. The average diameter of the particles was in the range of 10–20 nm and the length between 100 and 150 nm. Therefore, the morphology and particle size of the HA particles from the emulsion systems strongly depended upon the composition of the emulsion. Fig. 6 shows the variation in the SSA of the HA samples with the composition of the emulsion system. The SSA value of the HA sample prepared in aqueous suspension was 175 m2 =g. When we selected the aspect ratio of 4.4, the calculated d BET sv and LBET were 8.1 and 35.5 nm, respectively. The sv result was in good agreement with the TEM observation. As shown in Fig. 5, the composition of the emulsion system governs the morphology and size of emulsion-derived HA powders. This was reflected by the average SSA shown in Fig. 6. At lower PE concentrations, the particles had higher SSA in the range of 139–231 m2 =g. The DOL1 synthesized from the two-liquid phase emulsion exhibited an SSA as low as 139 m2 =g. The DOL3 particles showed a high SSA value of 190 m2 =g. If the aspect ratio was set to 6.4, the d BET and LBET sv sv were 7.2 and 46 nm, respectively. The values were far smaller than those observed by TEM. This can
300
(DOL3, Fig. 5c). The HA particles were very dispersible and had a uniform shape. With decreasing water/surfactant ratio from 2 to 0.8, the morphology of the resulting DOL4 powders changed significantly. The DOL4 nanopowders with lower aspect ratio were agglomerated at the water/surfactant ratio of 0.8 (Fig. 5d). TEM micrograph of HA powder obtained from emulsion system in the gel plus 1L region is shown in Fig. 5e (DOL7). The HA material showed a greater degree of agglomeration. The DOL9 particles obtained in the same gel plus 1L region
Specific surface area (m2/g)
PE = 10 wt%
PE > 50 wt%
250 200 150 100 50 0
HA DOL0 1
2
3
4
5
6
7
8
9
Fig. 6. Specific surface area for HA powders obtained from aqueous suspension and emulsion systems with different compositions.
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be explained by the microstructure of HA particles. The HA particles consisted of nanocrystallites and had a high porous surface, as revealed by TEM observation. The DOL4 sample obtained from a gel-like emulsion showed a much higher SSA value of 213 m2 =g. When the PE concentration was higher than 50 wt%, the resulting powders possessed low SSA values due to the greater degree of agglomeration. For example, the DOL7 powder obtained from the emulsion system containing 52 wt% PE, 25 wt% water and 23 wt% Tomadol 23-6.5 exhibited an average SSA of 72 m2 =g only. Disperse HA crystals with a low SSA of 95 m2 =g were obtained from the one-liquid phase emulsion (DOL8). When the aspect ratio was set to 8.3, the calculated d BET and LBET were 14.1 and 118 nm, respectively, sv sv which were close to the data from TEM observations. Thus, the low SSA was attributed to a large particle size with a smooth surface. Thus, it can be concluded that the SSA of the synthesized nanopowders is significantly affected by the PE concentration in the emulsion.
4. Conclusions Hydroxyapatite nanocrystals were synthesized by the mechanochemical–hydrothermal method in a unique emulsion system using a biodegradable nonionic surfactant, Tomadol 23-6.5. The composition of the emulsion system and calcium source had a significant effect on the formation of phasepure HA nanopowders and their morphology and surface area. The HA powder synthesized by using CaðNO3 Þ2 as the Ca source showed low thermal stability and transformed into b-TCP after heattreatment at 900 C for 1 h. No b-TCP was detected in the heat-treated HA powder prepared from the CaðOHÞ2 precursor. The emulsionderived HA material exhibited type B carbonate substitution, which could not be eliminated by heat-treatment at 900 C for 1 h. HA powders with specific surface area in the range from 72 to 231 m2 =g were prepared depending on the emulsion composition. Disperse HA crystals with a length of 160 nm and an aspect ratio of 6 were
formed in the emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant.
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