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Formation of Nanocrystalline Hydroxyapatite in Nonionic Surfactant Emulsions G. K. Lim,† J. Wang,*,† S. C. Ng,‡ and L. M. Gan§ Department of Materials Science, Department of Physics, and Institute of Materials Research and Engineering/Department of Chemistry, National University of Singapore, Singapore 119260 Received November 30, 1998. In Final Form: June 16, 1999 Nanosized hydroxyapatite (HA) powders were synthesized via a unique oil-in-water emulsion processing route using petroleum ether as the oil phase, KB6ZA as the nonionic surfactant phase, and aqueous CaCl2 solution as the water phase. An (NH4)2HPO4 solution was added to the oil-in-water emulsions to form hydroxyapatite. The resulting HA powders as examined by transmission electron microscopy are nanosized and crystalline. Very little growth in particle size of HA is observed upon calcination at 650 °C for 6 h. In contrast, the HA powders prepared by directly reacting (NH4)2HPO4 with CaCl2 in an aqueous solution and in a micellar system containing 3.2 wt % KB6ZA nonionic surfactant are much less crystalline than those formed in the oil-in-water emulsions. Moreover, the HA powders underwent extensive growth in crystallite and particle sizes when calcined at 650 °C for 6 h. A mechanism for the formation of nanocrystalline HA is proposed for the oil-in-water emulsions containing 2-11 wt % petroleum ether. The complexation of Ca2+ ions by oxyethylene groups of the nonionic KB6ZA surfactant on the surface of emulsion droplets constitutes numerous sites for forming nanosize HA crystallites. This unique oil-in-water emulsion process can produce not only nanocrystalline HA powders with high yield, but also uses a much smaller amount of oil and surfactant phases than that required by the water-in-oil microemulsion process.
Introduction Hydroxyapatite (HA), with the structural formula of Ca10(PO4)6(OH)2, is the major constituent of human bone and teeth.1-3 Synthetic HA has excellent biocompatibility and bioactivity and is widely used in many biomedical applications such as implants and coatings onto prostheses.4,5 It is also sought after in several multidisciplinary applications such as filters for heavy metals from aqueous solutions,6 in HPLC for the separation of proteins and nuclei acids,7 and in gas sensors as well as in catalysis.8 The function of HA in almost all of these applications is largely determined by its specific surface areas, and therefore a nanocrystalline HA powder is required. It is also well established that an HA powder of large particle size and wide particle size distribution is not sinteringreactive.9,10 Nanosized HA powders exhibit a sintering * To whom correspondence should be addressed. † Department of Materials Science. ‡ Department of Physics. § Institute of Materials Research and Engineering/Department of Chemistry. (1) Hulbert, S. F.; Bokros, J. C.; Hench, L. L.; Wilson, J.; Heimke, G. In Ceramics in clinical applications: past, present and future, High Tech Ceramics; Vincenzini, P., Ed.; Elsevier: Amsterdam, 1987; pp 189-213. (2) Currey, J. D. In The Mechanical Adaptation of Bones; Princeton University Press: Princeton, NJ, 1984. (3) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487. (4) Geesink, R. G. T.; Manley, M. T. In Hydroxyapatite Coatings in Orthopaedic Surgery; Raven Press: New York, 1995. (5) Takaoka, T.; Okumura, M.; Ohgushi, H.; Inone, K.; Takakura, Y.; Tamai, S. Biomaterials 1996, 17, 1499. (6) Reichert, J.; Binner, J. G. P. J. Mater. Sci. 1996, 31, 1231. (7) Nviagu, J. O.; Moore, P. B. In Phosphate Minerals; SpringerVerlag: Berlin Heidelberg, 1984; Chapter 11. (8) Kanazs, T. In Materials Science Monographs: Inorganic Phosphate Materials; Elsevier: New York, 1989; Chapter 2. (9) Murray, M. G. S.; Wang, J.; Ponton, C. B.; Marquis, P. M. J. Mater. Sci. 1995, 30, 3061. (10) Best, S.; Bonfield, W.; Doyle, A. In Bioceramics, Proceedings of the 1st International Bioceramics Symposium; Oonishi, H., Aoki, H., Sawai, K., Eds.; Ishiyaku-Euroamerica: St. Louis, MO, 1989; p 68.
temperature of ∼200 °C lower than that of the HA with particles in the micron or submicron range.11 The highly densified and refined microstructure derived from a nanosized HA powder can lead to a significant improvement in mechanical properties of sintered HA, and therefore widens its applications as load-bearing implants.12,13 An emulsion is a heterogeneous system consisting of at least one immiscible liquid dispersed in another in the form of droplets with the diameter generally exceeding 0.1 µm. It possesses limited stability, although it may be accentuated by such additives as surface active agents and finely divided solids.14 In contrast, a microemulsion is a thermodynamically stable, isotropic dispersion of two immiscible liquids stabilized by the presence of surfactant/ cosurfactant. The dispersed phase is confined in nanometer regimes of spherical droplet or continuous channel. It is thus capable of delivering nanosized particles of organic and inorganic composition when the reaction is confined in the nanosized domains. However, one of the main disadvantages associated with the microemulsion processing is the usage of a large amount of oil and surfactant phases, although it has been established for synthesizing several types of inorganic and organic particles.15-18 (11) Lim, G. K.; Wang, J.; Ng, S. C.; Chew, C. H.; Gan, L. M. Biomaterials 1997, 18, 1433. (12) Lange, F. F. J. Am. Ceram. Soc. 1989, 72, 3. (13) Ruys, A. J.; Wei, M.; Sorrell, C. C.; Dickson, M. R.; Brandwood, A.; Milthorpe, B. K. Biomaterials 1995, 16, 409. (14) Becher, P. Emulsion: Theory and Practice, 2nd ed.; Reinhold: New York, 1966; p 2. (15) Gobe, M.; Kon-no, K.; Kandori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (16) Gan, L. M.; Chan, H. S. O.; Zhang, L. H.; Chew, C. H.; Loo, B. H. Mater. Chem. Phys. 1994, 37, 263. (17) Burgard, D.; Kropf, C.; Nass, R.; Schmidt, H. Mater. Res. Soc. Symp. Proc. 1994, 346, 101. (18) Hingorani, S.; Shah, D. O.; Multani, M. S. J. Mater. Res. 1995, 10, 461.
10.1021/la981659+ CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
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Recently, fine HA powders were synthesized in waterin-oil microemulsions consisting of cyclohexane as the oil phase, a mixed poly(oxyethylene)5 nonyl phenol ether (NP5) and poly(oxyethylene)9 nonyl phenol ether (NP9) as the surfactant phase, and 1.0 M CaCl2 solution as the aqueous phase.11,19 Nanosized HA particles were formed in the aqueous domains dispersed in a cyclohexane matrix, when the reaction between CaCl2 and (NH4)2HPO4 took place in the microemulsion. The potential of microemulsion processing is, however, limited by the considerably low production yield when the large amount of oil and surfactant phases is not recycled. In comparison, the aqueous phase is the major phase in an oil-in-water emulsion, which can be formed by a small amount of oil and surfactant phases. One of the very apparent advantages offered by the oil-in-water emulsion over a waterin-oil microemulsion is the high production yield at the expense of a small amount of oil and surfactant phases. In this paper, we describe a unique oil-in-water emulsion route, in which nanocrystalline HA particles are synthesized in the continuous aqueous phase in oil-in-water emulsions. This route is capable of producing nanocrystalline HA powders at the expense of only a small fraction of surfactant and oil phases as compared to that required by a water-in-oil microemulsion. Experimental Section Chemicals. The chemicals used were CaCl2, NH4OH, and H3PO4 (all from Merck Co.); petroleum ether (PE) with a boiling point of 120-160 °C (BDH, U.K.); Empilan KB6ZA (Albright and Wilson Asia Ptd. Ltd., Singapore) and distilled ethanol. Synthesis of HA Powders. HA powders were synthesized by reacting (NH4)2HPO4 with CaCl2 in three reaction systems: a conventional aqueous solution, a micellar system containing 3.2 wt % nonionic surfactant in 1.0 M CaCl2 aqueous solution, and oil-in-water emulsions containing nonionic surfactant and varying amounts of petroleum ether in 1.0 M CaCl2 aqueous solution. The nonionic surfactant used is KB6ZA, which is a lauryl alcohol condensed with an average of 6 mol of oxyethylene oxide with the molecular formula C12H25(OCH2CH2)6OH. An aqueous solution containing 1.0 M CaCl2 was first prepared. Appropriate amounts of KB6ZA without and with petroleum ether were then mixed with 1.0 M CaCl2 aqueous solutions to form a micellar solution of KB6ZA and emulsions containing various amounts of petroleum ether from 2.6 to 10.8 wt %. A 0.6 M (NH4)2HPO4 solution was prepared by mixing stoichiometric amounts of NH4OH and H3PO4. For the formation of HA in the conventional aqueous solution, the 0.6 M (NH4)2HPO4 solution was titrated into the 1.0 M CaCl2 solution at a rate of approximately 12 drops/ min while being stirred continuously at 34.5 °C. Similarly, the 0.6 M (NH4)2HPO4 solution was titrated very slowly to the micellar system containing the CaCl2 aqueous solution and emulsions of varying amounts of petroleum ether, respectively. The reacted mixtures were aged for 24 h and the resulting precipitates were washed with ethanol and dried in an oven. They were then calcined at 650 °C in an electric furnace for 6 h at a heating and cooling rate of 10 °C/min. Characterization of HA Powders. A transmission electron microscope (TEM, JEOL 100CX) was used to characterize the microstructure of both the as-precipitated and calcined HA powders. Their phases were analyzed using a Philips X-ray diffractometer (model PW1729). The powders were also characterized for their specific surface area using a BrunauerEmmett-Teller (BET) specific surface area analyzer (Quantachrome, NOVA2000).
Results Morphology and Specific Surface Area of HA Powders. Figures 1(a,b,c) are three bright field TEM (19) Lim, G. K.; Wang, J.; Ng, S. C.; Gan, L. M. Mater. Lett. 1996, 28, 431.
Figure 1. TEM micrographs and the associated electron diffraction patterns of as-synthesized HA powders derived from (a) conventional aqueous solution; (b) micellar system; and (c) emulsion system containing 7 wt % PE and 3 wt % KB6ZA.
micrographs together with the selected area electron diffraction (SAD) patterns, for the as-synthesized HA powders from aqueous solution, micellar system, and emulsion containing 7.0 wt % PE and 3.0 wt % KB6ZA, respectively. The conventional precipitation in the aqueous phase results in a gel-like hydroxyapatite precursor which exhibits a considerably low degree of crystallinity as revealed by the much-diffused rings in the SAD pattern. The precipitation of HA precursor in the micellar system containing 3.2 wt % KB6ZA produced a precursor powder of similar morphology to that synthesized in the aqueous solution. However, its overall crystallinity is apparently improved as revealed by the existence of dispersed HA nanocrystallites and the slightly sharpened SAD rings. In contrast, the HA powder formed in the emulsion containing 7.0 wt % PE and 3.0 wt % KB6ZA consists of nanocrystallites in the form of dendritic agglomerates. Its SAD pattern exhibits multispotty rings indicating a well-defined crystallinity of HA. The three as-synthesized HA powders as shown in Figure 1 were then calcined at 650 °C for 6 h to investigate their phase development and change in crystallinity. Figures 2(a,b,c) are three bright field TEM micrographs, together with the associated SAD patterns for the three calcined HA powders. The calcined HA powder derived from both the aqueous solution and micellar system exhibits rather irregular particle shapes and sizes in the submicron range. The very coarse nature of these HA particles is also indicated by the occurrence of rather irregular spots in the diffraction patterns observed after calcination at 650 °C. In contrast, the HA powder derived from the oil-in-water emulsion containing 7.0 wt % PE and 3.0 wt % KB6ZA consists of nanosized particles, the
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Figure 4. TEM micrographs of the calcined HA powders derived from the emulsion systems containing (a) 2.6 wt % PE and (b) 10.8 wt % PE. Both powders were calcined at 650 °C for 6 h.
Figure 2. TEM micrographs and the associated electron diffraction patterns of the powders derived from (a) conventional aqueous solution; (b) micellar system; and (c) emulsion system containing 7 wt % PE and 3 wt % KB6ZA. All three HA powders were calcined at 650 °C for 6 h.
Figure 3. TEM micrographs of the as-dried powders derived from the emulsion systems containing (a) 2.6 wt % PE and (b) 10.8 wt % PE.
nanocrystallinity of which is revealed by the spotty diffraction rings, as shown in Figure 2(c). These nanosized HA particles underwent little crystal growth and particle coarsening at the calcination temperature, and therefore their dendritic morphology was largely retained. The main difference in composition between the micellar system and emulsion is that the latter contains a 7.0 wt % oil phase. It is remarkable that the presence of such a small amount of oil phase in the emulsion has yielded such a tremendous difference in the characteristics of HA particles formed. It is thus of interest to investigate the effect of varying amounts of oil phase in the emulsion on the characteristics of the resulting HA powders. Figures 3(a,b) show the TEM micrographs for the as-dried HA powders derived from the emulsions containing 2.6 and 10.8 wt % PE, respectively. Figures 4(a,b) show the two powders upon calcination at 650 °C for 6 h. The emulsion containing 2.6 wt % PE yielded an HA precursor that is quite similar to that derived from the micellar system. Calcination at 650 °C led to a considerable degree of growth in crystallite size and particle coarsening and therefore
the resulting powder is irregular in both particle size and morphology. In contrast, the powder derived from the emulsion containing 10.8 wt % PE exhibits a nanocrystalline feature. It underwent little crystal growth and coarsening upon calcination at 650 °C for 6 h. N2 adsorption-desorption isotherm (BET) study was carried out for each of these calcined HA powders. The two HA powders derived from the aqueous solution and micellar system exhibit 0.59 and 2.16 m2/g in specific surface area, respectively. In contrast, a much higher specific surface area is obtained for each of the three HA powders derived from the emulsions as shown in Table 1. Furthermore, the specific surface area increases sharply from ∼7 to ∼75 m2/g when the PE content in emulsion is increased from 2.6 to 7.0 wt %. Further increasing the PE content in emulsion does not affect the specific surface area significantly. Crystallinity of HA Powders. Figures 5(a,b) show the X-ray diffraction patterns for the three HA powders derived from aqueous solution, micellar system, and emulsion containing 7.0 wt % PE and 3.0 wt % KB6ZA, before and after calcination at 650 °C for 6 h, respectively. The as-synthesized HA powder derived from the aqueous solution exhibits a much-broadened X-ray diffraction (XRD) trace, which indicates its low degree of crystallinity. Upon calcination at 650 °C, the level of crystallinity increased dramatically as revealed by the sharpened and distinct peaks over the 2θ angle range of 30-35°. A small amount of tricalcium phosphate (β-TCP) was also observed in the powder, indicating that the precipitation in the conventional aqueous phase did not result in a singlephase HA when calcined at 650 °C. A similar trend is observed for the HA powder derived from the micellar solution before and after the calcination at 650 °C for 6 h, although β-TCP was not observed. The emulsion-derived HA powder, on the other hand, is very different from the other two. The splitting of the broadened peaks in the 2θ angle region of 31-33° for the powder before calcination indicates that it possesses a higher level of crystallinity and a larger average crystallite size than those of the other two. This agrees with the TEM observation as shown in Figure 1(c). Upon calcination at 650 °C for 6 h, the broadened peaks in the 2θ angle range of 31-33° were further split into the three principal peaks (31.8, 32.2, and 32.8°) of HA. This is an indication of the increased crystallite size after calcination at 650 °C for 6 h, as affirmed by the TEM observation in Figure 2(c). In addition, Figure 6 shows the XRD patterns for the calcined HA powders derived from the emulsions containing 2.6 and 10.8 wt % PE, respectively. Apparently, there is a much higher degree of peak broadening in the X-ray trace for the HA powder derived from the emulsion containing 10.8 wt % PE as compared to that containing 2.6 wt % PE. This clearly indicates a refinement in the crystallite size in the former.
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Table 1. Effect of Petroleum Ether (bp 120-160 °C) Concentration in Emulsion on the Specific Surface Areas of HA Powders Calcined at 650 °C for 6 h emulsion systemsa PE
KB6ZA
1.0 M CaCl2 aqueous solution
sample no.
(g)
(wt %)
(g)
(wt %)
(g)
(wt %)
HA powders specific surface area (m2/g)
E1 E2 E3
0.28 0.78 1.25
2.6 7.0 10.8
0.33 0.33 0.33
3.1 3.0 2.8
10 10 10
94.3 90.0 86.4
7.20 74.57 71.39
a
A stoichiometric amount of 0.6 M (NH4)2HPO4 is slowly added to each emulsion to precipitate the HA precursors.
Figure 6. The X-ray diffraction patterns for the calcined HA powders derived from the emulsion systems containing 2.6 and 10.8 wt % PE.
Figure 5. The X-ray diffraction patterns for the HA powders derived from aqueous solution, micellar solution, and emulsion consisting of 7 wt % PE and 3 wt % KB6ZA: (a) before calcination and (b) after calcination at 650 °C for 6 h.
Discussion Dropwise addition of 0.6 M (NH4)2HPO4 solution into 1.0 M CaCl2 solution resulted in a fast precipitation of HA particles of irregular sizes. The reaction proceeded in a more or less uncontrolled manner as the Ca2+ ions were not confined in any particular arrangement in the aqueous solution. During the calcination at 650 °C, heterogeneous crystallization and particle coarsening led to a low specific surface area of the resulting HA powder. The process is similar to the crystallization and particle coarsening phenomena observed in many precipitated/coprecipitated oxide ceramic precursors.20,21 We propose here a mechanism for the formation of HA nanoparticles in the compositions containing the nonionic surfactant. Earlier studies on polyoxyalkylenes demonstrated that the ethylene oxide groups are able to complex calcium ions present in an aqueous solution.22,23 As (20) Lee, W. E.; Rainforth, W. M. In Ceramic Microstructures: Property Control by Processing; Chapman & Hall: New York, 1994; Chapter 1. (21) Li, H. P.; Wang, J.; Stevens, R. J. Mater. Sci. 1993, 28, 553.
Figure 7. The electrical conductivity curves for the 1.0 M CaCl2surfactant (KB6ZA) aqueous solutions.
analogies can be drawn between polyoxyethylene complexes and the so-called crown complexes, complexation has also been observed between calcium ions and crown ether.24,25 The trapping of calcium ions by nonionic surfactant KB6ZA in the micellar and emulsion systems is demonstrated by the dependence of electrical conductivity of 1.0 M calcium chloride solution on the amount of surfactant added as shown in Figure 7. The nonionic surfactant drastically lowers the conductivity over the composition range from 0 to 10 wt % KB6ZA. For example, the conductivity of 1.0 M CaCl2 solution is reduced from (22) Doscher, T. M.; Myers, G. E.; Attkin, D. C. J. Colloid Interface Sci. 1951, 6, 223. (23) Cross, J. In Nonionic Surfactants: Chemical Analysis; Marcel Dekker Inc: New York, 1987; Chapter 2. (24) Balasubramanian, B.; Chandani, B. J. Chem. Edu. 1983, 60, 73. (25) Darwish, I. A.; Uchegbu, I. F. Intl. J. Pharm. 1997, 159, 207.
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Figure 8. Schematic representation of micelle and emulsion droplets in CaCl2 aqueous solutions: Calcium-rich shell in adsorbed state occurring at (a) the micelle-water interface, and (b) emulsion droplet surface. The surfactant molecule is represented by
113.9 to 85.3 mS/cm at 25.3 °C and from 147 to 65.8 mS/ cm at 34.5 °C when 3.0 wt % KB6ZA is added to the solution. With the addition of 10 wt % KB6ZA, the conductivities are further reduced to 45.7 and 16.0 mS/ cm at 25.3 and 34.5 °C, respectively. It is apparent that temperature also significantly affects the conductivity of the system for a given KB6ZA addition. The reduction in conductivity of 1.0 M CaCl2 solution upon the addition of KB6ZA is due to the formation of Ca-complexes with the ethylene oxide units of the surfactant moelcules.22,23 With a cloud point at 36 °C, KB6ZA molecules are less hydrated in the aqueous solution at 34.5 °C than that at 25.3 °C. Thus, more active ethylene oxide units may form complexes with Ca2+ ions at high temperatures leading to the enhanced decrease in conductivity of the solution. Micelles with an interior hydrophobic microenvironment and a polar sphere at the micelle-water interface are generated in the KB6ZA-aqueous solution (above the critical micelle concentration, CMC). When a small amount of PE is added to the micellar solution, it forms emulsion droplets with PE embedded in the hydrophobic cores. With the presence of 1.0 M CaCl2 in the aqueous phase, there will be a calcium-rich shell due to Ca-complexation at the interfaces of micelle and emulsion droplets. The process is illustrated by the schematic diagram shown in Figure 8. The Ca binding to nonionic surfactant is further enhanced by an increase in temperature near the cloud
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point (36 °C) of KB6ZA. This is due to the diminishing hydrogen bonding between water and oxyethylene groups that facilitates the complexation between Ca2+ ion and oxyethylene units. Surfactant molecules in micelles or emulsion droplets interact with Ca2+ ions to form “cagelike” structures (see Figure 8). These numerous calcium-rich domains lead to the fast formation of HA particles upon contact with phosphate ions in the aqueous phase. The reaction between (NH4)2HPO4 and CaCl2 at the interfacial shells is deemed to be rapid because of the localized Ca2+-concentration effect. As numerous nucleation sites are formed on the surfaces of these micelles and emulsion droplets, spontaneous nucleation of forming HA takes place simultaneously. In addition, the positional stabilization of Ca2+ ions within each cagelike structure as a result of the complexation effect by KB6ZA molecules favors the formation of ordered HA crystallites. The continuous aqueous phase then acts as a reservoir of reactants to constantly supply the reactive sites, resulting in the simultaneous growth of HA crystallites. The surfactant molecules in the micellar system are aggregated in more or less spherical micelles, which are less than 10 nm in size. They are very dynamic in nature and undergo rapid breakup and coalescence. Their lifespans, as indicated by various analyses, are in the range of 10-5 and 10-3 s.26,27 Thus, the ability of the micellar system to facilitate the formation of crystalline HA particles through the caged Ca-complexation may be limited. The large number of Ca2+ ions in the aqueous phase may simply react with (NH4)2HPO4 in a manner similar to that in the conventional aqueous phase. Thus, the resulting HA precursors are of considerably low crystallinity. The calcination of the precursors at 650 °C leads to a rapid growth of HA crystallites and particle coarsening. As for the emulsion system, oil phase residing in the hydrophobic cores of oil-swollen micelles leads to a large expansion in the micellar’s dimension in the form of emulsion droplets. This undoubtedly increases the reaction sites between CaCl2 and (NH4)HPO4 at the interfaces. Moreover, the emulsion droplets are more stable than the surfactant micelles. This is attributed to the interaction between the hydrophobic portion of surfactant molecules and oil in the emulsion cores that possess a certain degree of viscoelasticity to retard the rapid disintegration and coalescence of emulsion droplets.28 The average life-span of an emulsion droplet is known to be much longer than that of a micelle.29 The formation of solid particles, such as HA on the surface of emulsion droplets, can further strengthen the droplet integrity.30 All of these factors make an emulsion system much more suitable than the micellar system for forming HA particles of high crystallinity. As a result of the existence of numerous nucleation sites created by the Ca-complexation for reaction between Ca2+ and phosphate at the droplet surface, nanocrystallites of HA are formed in emulsions containing the enlarged droplets. Because of the size uniformity of these nano(26) Isrealachvili, J. N. In Intermolecular and Surface Forces with Application to Colloidal and Biological Systems; Academic Press: London, 1985; Chapter 15. (27) Fendler, J. H. In Membrane Mimetic Chemistry; John Wiley & Sons: Canada, 1982; Chapter 2. (28) Myers, D. In Surfactant Science and Technology; VCH Publishers: New York, 1988; Chapter 6. (29) Evans, D. F.; Weinerstrom, H. In The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet; VCH: New York, 1994; Chapter 11. (30) Sjoblom, J. In Emulsions and Emulsion Stability; Marcel Dekker: New York, 1996; Chapter 4.
Formation of Nanocrystalline Hydroxyapatite
crystallites, the calcination at 650 °C for 6 h does not result in an extensive growth in both the crystallite and particle sizes. The enhanced crystallinity of HA derived from the emulsion compositions lies in the complexation of Ca2+ by oxyethylene groups in the nonionic surfactant. This has been further supported by the effects of reducing the degree of Ca-surfactant binding on the characteristics of resulting HA powder. As discussed earlier, temperature has a large impact on the electrical conductivity of 1.0 M calcium chloride at a given amount of KB6ZA added. At 3.0 wt % KB6ZA, the electrical conductivity at 25.3 °C is much higher than that at 34.5 °C, due to the strengthened hydrogen bonding between water molecules and oxyethylene groups. Therefore, the degree of Ca binding to KB6ZA is lower at 25.3 °C than that at 34.5 °C. We performed an experiment to synthesize HA in the emulsion containing 7.0 wt % PE and 3.0 wt % KB6ZA at 25.3 °C, followed by calcination of the precursor at 650 °C. The precursor was highly amorphous, although there were a few nanocrystallites of HA in the largely amorphous matrix as observed using a transmission electron microscope. It is therefore similar in particle characteristics to the precursor derived from the micellar system at 34.5 °C. The precursor synthesized in emulsion at 25.3 °C underwent a steady coarsening in both particle and crystallite sizes when calcined at 650 °C for 6 h, leading to an HA powder irregular in both particle size and particle morphology. As shown in Table 1, PE content in the emulsion has a significant effect on the specific surface area of the resulting HA powders. As the amount of oil phase in emulsion is increased, the formation of nanocrystalline
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HA particles is greatly facilitated at the enlarged droplet surface. The emulsion containing 7.0 wt % PE resulted in an HA powder of ∼75m2/g in specific surface area; it is therefore much more efficient in forming an HA powder of refined particle size as compared to that containing 2.6 wt % PE. The increase in the number of nucleation sites in the former is responsible for the enlarged specific surface area of resulting HA powder. Conclusions A unique oil-in-water emulsion route capable of producing nanosized HA powders using a small amount of nonionic surfactant (KB6ZA) and oil (PE) phases is demonstrated. This is shown by the remarkable difference in characteristics of the HA precursors and powders derived from the reaction between 1.0 M CaCl2 and 0.6 M (NH4)2HPO4 carried out in three different media. A precursor of considerably low crystallinity is obtained from both the aqueous solution and the micellar composition containing 3.2 wt % KB6ZA. Nanosized HA crystallites are formed in the oil-in-water emulsions containing ∼3 wt % of KB6ZA and PE ranging from 2.6 to 10.8 wt %. The complexation of Ca2+ ions by the oxyethylene groups of nonionic KB6ZA on the surface of emulsion droplets constitutes the formation sites for HA nanocrystallites. The resulting HA precursor powders undergo little growth in crystallite and particle sizes upon calcination at 650 °C for 6 h. A sufficient amount of oil (PE) phase, e.g., 7.0 wt %, is required for the oil-in-water emulsion to be efficient in producing an HA powder of nanoparticles. LA981659+
