Langmuir 2000, 16, 10221-10226
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Fourier Transform Infrared Spectroscopy Study of Deuteration of Calcium Hydroxyapatite Particles Tatsuo Ishikawa,* Akihiro Teramachi, Hidekazu Tanaka,† Akemi Yasukawa, and Kazuhiko Kandori School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan Received March 31, 2000. In Final Form: October 3, 2000 The deuteration of synthetic calcium hydroxyapatite (CaHap) and the effect of fluoridation on the deuteration were investigated by in situ Fourier transform infrared spectroscopy to elucidate the surface structure of CaHap particles. The deuteration of column OH- ions of CaHap was reduced by the fluoridation, indicating that the deuteration does not happen in the columns of which the entrance is closed with Fions replacing OH- ions. The 2680-cm-1 band observed on the deuterated CaHap was assigned to the surface OD- ions which react with CO2 molecules. The 2633-cm-1 band due to the column OD- ions was not influenced by CO2 adsorption. The fluoridated CaHap did not give rise to the surface OD- band, because the surface OH- ions were replaced with F-.
Introduction Calcium hydroxyapatite (Ca10(PO4)6(OH)2 designated as CaHap), which is a principal component of hard tissues, has been of interest in dental and medical fields. Colloidal particles of synthetic CaHap find various usages such as bioceramics, adsorbents, catalysts, and so on. The OHions of CaHap crystals play important roles in anionexchange, surface modification, ionic conductance, solubility among others. The state of the OH- ions in the crystals has been investigated by IR1-7 and NMR8,9 spectroscopy. According to the crystal structure of CaHap determined by X-ray and neutron diffraction,10 the OHions align in columns along the c-axis of the crystals. Information on the surface structure of CaHap particles is desirable in various applications, because the interaction of the particle surface to various substances is fundamentally important in the usage. The synthetic CaHap usually consists of fine particles, and their agglomerates exhibiting high specific surface area and PO43- and OHions are conceived to be exposed on the particle surface. In addition to these ions, H2O molecules must be strongly adsorbed by coordination to Ca2+ ions on the surface of the particles prepared by precipitation in aqueous system. Cant et al. found a weak IR band at 3660 cm-1 and assigned this band as a surface OH group by its exchange behavior with D2O and back exchange with NH3, and they presumed that the surface OH group is OH- ions located where the OH- columns are terminated by the crystal surface or the P-OH groups of surface HPO42- ions.11 In our previous
study, three absorption bands at 3680, 3670, and 3656 cm-1, which slightly shift depending on the samples, were assigned to the OH stretching vibration of the P-OH groups of surface HPO42- and/or H2PO4- ions produced by the protonation of PO43- ions to balance the surface charge.12,13 However, to our knowledge, the satisfactory IR bands of the surface OH- ions are still not detected, though these OH- ions must govern various surface functions of the CaHap particle in addition to the surface P-OH groups. We have found that CaHap adsorbed CO2 irreversibly and inferred the adsorption sites to be the surface OH- ions from the Fourier transform infrared (FTIR) results though there was no definitive evidence for this.14 It is the purpose of the present study to increase insight into surface OH species of CaHap particles, such as P-OH groups, OH- ions, and strongly adsorbed H2O molecules, by means of deuteration. Since the OH- ions of CaHap have been well established to be exchanged with F- ions,15,16 the effect of fluoridation on the deuteration was examined to gain information on the surface OHions. The surface structure of fluoridated CaHap is of interest in the dental field,17 but the details are still not thoroughly clarified. The interaction of CO2 molecules to the deuterated CaHap particles was investigated by in situ IR spectroscopy not only to assign the bands of the surface species but also to elucidate the mechanism of CO2 adsorption on CaHap, which is concerned with the incorporation and storage of CO2 molecules into bones and the catalytic nature of CaHap. Experimental Section
* To whom correspondence should be addressed. Fax: +81-72978-3394. E-mail:
[email protected]. † Research Fellow of the Japan Society for the Promotion of Science. (1) Engel, G.; Klee, W. E. J. Solid State Chem. 1972, 5, 28. (2) Joris, S. J.; Amberg, C. H. J. Phys. Chem. 1971, 75, 3172. (3) Fowler, B. O. Inorg. Chem. 1974, 13, 194. (4) Fowler, B. O. Inorg. Chem. 1974, 13, 207. (5) Gonzalez-Diaz, P. F.; Santos, M. J. Solid State Chem. 1978, 23, 265. (6) Reisner, I.; Klee, W. E. Spectrochim. Acta 1982, 38A, 899. (7) Menzel, B.; Amberg, C. H. J. Colloid Interface Sci. 1972, 38, 256. (8) Young, R. A.; van der Lugt, W.; Elliott, J. C. Nature 1969, 223, 729. (9) Yesinowski, J. P.; Eckert, H. J. Am. Chem. Soc. 1987, 106, 6274. (10) Kay, M. I.; Young, R. A.; Posner, A. S. Nature 1964, 204, 1050.
Materials. Colloidal CaHap particles were prepared by the following precipitation method. A 0.405-mol portion of Ca(OH)2 (11) Cant, N.; Bett, J. A. S.; Wilson, G. R.; Hall, W. K. Spectrochim. Acta 1971, 27A, 425. (12) Ishikawa, T.; Wakamura, M.; Kondo, S. Langmuir 1989, 5, 140. (13) Ishikawa, T. In Adsorption on New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Elsevier: Amsterdam, 1996. (14) Chen, Z. H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Langmuir 1998, 14, 6618. (15) Young, R. A.; Elliott, J. C. Arch. Oral Biol. 1966, 11, 699. (16) Amberg, C. H.; Lik, H. C.; Wagstaff, K. P. Can. J. Chem. 1974, 52, 4001. (17) Elliott, J. C. Structure and Chemistry of the Apatites and other Calcium Orthophosphates; Elsevier: Amsterdam, 1994.
10.1021/la0004855 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/2000
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Table 1. Composition, specific surface area (Sn) and mean particle size of Hap and Fap ICP, mmol g-1 sample
Ca
P
Ca/P atomic ratio
Hap Fap
9.31 9.32
5.97 5.86
1.56 ( 0.02 1.58 ( 0.02
XPS, atomic % O
Ca
P
F
Ca/P atomic ratio
Sn, m2 g-1
length, nm
width, nm
63.4 61.0
20.3 20.4
16.3 15.9
0 2.7
1.25 1.28
85 94
65 60
18 18
Figure 1. IR spectra in vacuo of Hap (a) and Fap (b) pretreated by outgassing at 300 °C for 2 h. The dashed line is the spectrum obtained by subtracting (a) from (b). was dissolved in 20 dm3 deionized, distilled water free from CO2 in a Teflon vessel under an N2 atmosphere by stirring for 24 h at room temperature. To the Ca(OH)2 solution, 0.236 mol of H3PO4 was added and then the resulting precipitation was aged in the capped vessel at 100 °C for 48 h. After the aging process, the precipitates were filtered, rinsed with 10 dm3 of water, and dried at 70 °C for 16 h. The obtained material was called Hap. Fluoridation. The Hap particles (1.5 g) were immersed in 250 cm3 of 0.5 mol dm-3 NaF aqueous solution at 25 °C for 48 h. The solution pH was increased from 8.74 to 9.32 by the treatment, signifying the exchange of OH- ions with F-. Following the immersion, the particles were separated from the solution, washed with 1.5 dm3 water, and finally dried under the same condition as Hap. The fluoridated Hap is designated as Fap. Characterization. The Hap and Fap particles thus prepared were characterized by the following complementary techniques. Powder X-ray diffraction (XRD) patterns were taken by a Rigaku diffractometer. The particle morphology was observed by a JEOL transmission electron microscope (TEM). The specific surface area of the particles was determined by the BET method from N2 adsorption isotherms. The chemical composition of the particles was determined by a Seiko induced coupled plasma (ICP) spectrometer. The samples for the ICP analysis were dissolved in a 1.0 mol dm-3 HNO3 solution. The surface composition of the particles was determined by a Shimadzu X-ray photoelectron spectroscopy (XPS). IR spectra were in situ recorded by a Nicolet Fourier transformed infrared (FTIR) spectrometer using a vacuum cell capable of controlling temperature and introducing gas at a desired pressure. The self-supporting sample disks were made by pressing the samples (30 mg) into a disk with 10 mm diameter under 572 kg cm-2. Deuteration. The deuteration reaction of Hap particles with D2O was examined by FTIR. The sample disks in the IR vacuum cell were outgassed at 300 °C for 2 h unless otherwise noted. After the pretreatment D2O vapor was introduced into the cell and then kept at different temperatures up to 300 °C for 10 min. The sample cell was subsequently evacuated for 10 min at the same temperature as the exposure to D2O, and then the IR spectra were taken at room temperature. The deuteration was performed by repeating the exposure and evacuation of D2O.
Results and Discussion Compositions of Hap and Fap. The XRD patterns of Hap and Fap were characteristic of CaHap (JCPDS9-432). The crystallinity and lattice parameter were not influenced by the fluoridation. Table 1 lists the contents of Ca and P, Ca/P ratio, and specific surface areas of Hap and Fap particles. The Ca/P ratios are the mean of five values. The Ca/P ratios of both materials are less than the stoichiometric ratio of 1.67, which means that they are nonstoichiometric and have a Ca deficiency. The surface composition of Hap and Fap particles determined by XPS is also given in Table 1. The Ca/P ratios of both the materials were less than those determined by ICP, indicating that the surface phase of the particles is more deficient in Ca2+ than the bulk phase. TEM observation shows that Hap and Fap particles are short rods with mean lengths of 65 and 60 nm, respectively, and the same mean width of 18 nm, the Hap particles being a little longer than the Fap ones. The specific surface areas of Hap are slightly less than that of Fap. It appears therefore that the particle morphology of these materials is essentially not influenced by the fluoridation besides the slight decrease of particle length. The Ca/P ratios of Hap and Fap determined by ICP in Table 1 agree within the experimental error, which reveals that PO43- ions are not exchanged with F- ions. As described in the Experimental Section the solution pH increases from 8.74 to 9.32 by the fluoridation, supporting the exchange of OH- with F-. IR Spectra of Hap and Fap. For the characterization of Hap before and after fluoridation the IR spectra of Hap and Fap in vacuo were taken on the sample disks outgassed at 300 °C for 2 h. They are compared in Figure 1. Spectrum a of Hap shows a saturated strong band at ca. 3570 cm-1, a weak band at ca. 3670 cm-1, and a broad band centered at ca. 3200 cm-1. The former saturated band is due to
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Figure 2. IR spectra of Hap deuterated at 150 °C for varied periods: (a) before deuteration; (b) 10 min; (c) 30 min; (d) 50 min; (e) 80 min.
OH- ions in the crystals, and the broad band is due to the hydrogen-bonding OH species, such as surface OH- ions and strongly adsorbed H2O molecules. The enlarged spectra on the left side of Figure 1 show that the weak 3670-cm-1 band is divided into three peaks at 3656, 3671, and 3680 cm-1. These bands have been assigned to the stretching vibration of OH of the surface P-OH groups by various techniques including molecular adsorption, deuteration, and cation exchange in our previous study.13 The surface P-OH groups are formed by protonation of PO43- ions to balance the surface charge and to compensate the Ca2+ deficiency as aforementioned. On the other hand, spectrum b of Fap gives rise to a band at 3715 cm-1 besides the surface P-OH bands at 3658, 3667, and 3680 cm-1, which are a little different from the wavenumbers of the surface P-OH bands of Hap. To know the influence of fluoridation on the surface P-OH bands, the spectrum (dashed line) obtained by subtracting spectrum a from b is shown in Figure 1. In the subtracting spectrum, five weak bands appear at 3640, 3660, 3672, 3687, and 3715 cm-1. The last band corresponds to the 3715-cm-1 band before deuteration. The surface P-OH bands are slightly intensified in the high wavenumber side by the fluoridation. Since the OH- bands at ca. 3570 cm-1 of Hap and Fap are saturated as is seen in Figure 1, the influence of fluoridation on the OH- band cannot be followed up; however, the OH- band has a faint shoulder in the right side. It is well-known that the fluoridated CaHap shows a band at 3540 cm-1 from an O-H‚ ‚ ‚F hydrogen bond.7,9,11 The weak shoulder observed in the spectrum of Fap appears to correspond to the 3540-cm-1 band, indicating the fluoridation of OH- ions takes place. Although the assignment of the 3715-cm-1 band remains unclear, at least the band seems to be due to OH groups because this band disappears with deuteration as described below. The broad band at ca. 3200 cm-1 of Fap was weaker than those of Hap, because the strongly adsorbed H2O on the former was less than that on the latter. The bands due to PO43- ions below 2000 cm-1 in the spectra traced on the samples embedded in KBr disks were substantially not influenced by the fluoridation. Deuteration of Hap. Figure 2 shows IR spectra of Hap deuterated at 150 °C during varied periods. In spectrum b traced after 10 min of deuteration, the surface
Figure 3. IR spectra of Hap deuterated at different temperatures for 10 min: dashed line, before deuteration; (a) 100 °C, (b) 150 °C, and (c) 200 °C.
P-OH bands and broad bands disappear and new bands appear at 2709, 2680, and 2633 cm-1. A shoulder band is detected at 2736 cm-1 as more clearly seen in the magnified spectra in Figure 3. On comparison of spectrum a in Figure 1 and spectrum c in Figure 3, the wavenumber ratios of three bands except for the 2680-cm-1 band are 3680/ 2736 ) 1.345, 3656/2709 ) 1.350, and 3570/2633 ) 1.356, close to the theoretical isotope ratio (ν(OH)/ν(OD)) of 1.374, which allows us to assign these bands to an OH stretching vibration; the 3570-cm-1 band is assigned to the column OH- ions,3 and the other bands are assigned to the surface P-OH groups;12 the 2736- and 2709-cm-1 bands are assignable to the surface P-OD bands, and the 2633cm-1 band is assignable to the OD- band of the column OH- ions. On comparison of Figures 1 and 3, the surface P-OH and P-OD bands are different in shape. This suggests that the surface P-OH groups have different bonding modes, though the detailed configurations of these P-OH groups are not clarified at the moment. The assignment of the 2680-cm-1 band will be discussed later. The OD- band intensifies with extending the deuteration period, while the intensities of the surface P-OD bands and the 2680-cm-1 band are saturated with deuteration for 10 min. These results show that the surface P-OH groups are immediately deuterated while the deuteration of the column OH- ions gradually progresses. The deuteration mechanism of the OH- ions will be described later. The hydrogen bonding OH species showing the broad 3200cm-1 band are also easily deuterated, inferring that these OH species exist in the surface phase of the particles. Deuteration of Fap. To know the influence of fluoridation on deuteration, the spectra of Fap were obtained after deuteration under the same conditions as Hap, whose the spectra are shown in Figure 2, and they are shown in
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Figure 4. IR spectra of Fap deuterated at 150 °C for varied periods: (a) before deuteration; (b) 10 min; (c) 30 min; (d) 50 min; (e) 80 min.
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Figure 6. Plots of area intensity of OD- band vs root of deuteration period for Hap (open symbols) and Fap (solid symbols). The deuteration temperatures were 100 °C (O, b), 150 °C (4, 2), and 200 °C (0, 9).
intensifies the column OD- band at 2633 cm-1 as well as the case of Hap. Deuteration of Column OH- Ions. Figure 3 shows the spectra of Hap deuterated at different temperatures for 10 min. It is seen that the column OD- band at 2633 cm-1 steeply grows as the deuteration temperature is elevated, while the surface P-OD bands and the 2680cm-1 band are substantially not influenced. Figure 5 shows the spectra of Fap deuterated in the same manner as Hap of which the spectra are given in Figure 3. The OD- band at 2633 cm-1 grows with the rise of deuteration temperature as well as the same band of Hap. The OD- bands at 2633 cm-1 of Hap and Fap grow by the deuteration as seen in Figures 2 and 4. In Figure 6 the area intensity of the OD- band is plotted against the square root of the deuteration period. All the plots are linear passing the origin. The slopes of the lines increase with raising the deuteration temperature, and the slopes for Hap (open symbols) are larger than those for Fap (solid symbols). This reveals that the deuteration of OH- ions is more easy at higher temperature and it is depressed by the fluoridation. The lines in Figure 6 can be formulated by eq 1
X ) Dt1/2
Figure 5. IR spectra of Fap deuterated at different temperatures for 10 min: dashed line, before deuteration; (a) 100 °C, (b) 150 °C, and (c) 200 °C.
Figure 4. In the spectra of deuterated Fap the surface P-OD bands are detected at 2736 and 2703 cm-1, and the OD- band is detected at 2633 cm-1. As seen from the enlarged spectra in Figure 5 the surface P-OD bands are resolved to three bands at 2736, 2710, and 2703 cm-1. Similar to the deuteration of Hap, the surface P-OH and P-OD bands of Fap show distinct profiles as is seen from Figures 1 and 5. It should be noted that the band at 2680 cm-1 detected on Hap is not observed on Fap. The reason for this will be explained later. Continuing the deuteration
(1)
where X, t, and D are degree of deuteration, deuteration period, and constant, respectively. A relation such as eq 1 is generally known as the parabolic law, which is often encountered in self-diffusion of ions in solids.18 Figure 7 plots the D constant for Hap and Fap against the deuteration temperature. The D constants for Hap (open circles) and Fap (solid circles) increase above 100 °C and the D constant for Hap is larger than that for Fap when compared at the same deuteration temperature. As described in the Introduction, the OH- ions align in the columns along the c-axis corresponding to the long axis of the rod-shaped Hap particles. As can be estimated the mean particle sizes in Table 1, 10-20% of the particle surface is the (0001) plane where the terminals of OH(18) Jander, W. Z. Anorg. Chem. 1927, 168, 113.
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Figure 7. Plots of constant D vs deuteration temperature for Hap (O) and Fap (b).
columns lie. The other part (80-90%) of the particle surface is the (11h 00) and (101h 0) planes with the same structure having OH- in addition to PO43- ions. The deuteration of the column OH- ions is perceived to take place at first by D-H exchange between adsorbed D2O molecules and the OH- ions in the entrance of columns (D2O + OH- f DHO + OD-), followed by the deuteration of the OH- ions in the inside of the columns by H-D exchange between adjacent OD- and OH- ions (OD- + OH- f OH- + OD-). Fowler has confirmed diffusion of O18 in the OH- columns at 900 °C.4 Later, Young and Holcomb has adopted D2O, H2O, and probably HDO as the diffusing species at 110 °C.19 However, the diffusion of H2O or D2O molecules in the OH- columns appears to be difficult at low temperature below 200 °C. Therefore, it seems reasonable to consider that the deuteration in the present study progresses by diffusion of D+ and H+ by hopping on O atoms of OHions. Since the OH- ions in the entrance of columns of Fap are replaced by F- ions with the radius of 0.133 nm similar to 0.137 nm of OH- ions,20 the deuteration of the inner column OH- ions would be inhibited by closure of their entrance with F- ions. To corroborate the inhibitory effect of F- ions on deuteration, the activation energy (Ea) of deuteration was evaluated from the temperature dependence of constant D using the following equation
Ea ) -R d ln D/d(1/T)
(2)
where R and T are gas constant and deuteration temperature. The Ea values obtained for Hap and Fap are 25.9 ( 1.5 and 24 ( 1.4 kJ mol-1, respectively, which is almost the same. The fact that the deuteration of column OH- ions occurs on Fap indicates the existence of the columns with an open entrance not stopped with F- ions. The similar Ea values for Hap and Fap are able to be explained by a model where the deuteration of OH- ions in the open columns of Fap not closed with F- ions proceeds with the same activation energy as that of Hap whereas the deuteration in the closed columns of Fap is completely impeded. Such a restriction of deuteration by fluoridation is a kinetic rather than thermodynamic process. The (19) Young, R. A.; Holcomb, D. W. Calcif. Tissue Int. 1982, 34, S17. (20) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
Figure 8. Change of IR spectra of deuterated Hap with CO2 adsorption: dashed line, before deuteration; (a) before CO2 adsorption, (b) CO2 adsorption for 10 min, (c) 30 min, (d) 70 min, (e) 130 min, (f) outgassed at room temperature after taking spectrum (e), and (g) outgassed at 100 °C after taking spectrum (f).
interference of H+ diffusion by fluoridation can be related to the well-known matter that small amounts of F- ions contained in CaHap decrease its solubility in acid. Assignment of the 2680-cm-1 Band by CO2 Adsorption. As is seen from Figures 3 and 5 the 2680-cm-1 band was detected on Hap but not on Fap. For assignment of this band, the influence of CO2 adsorption on the spectra of deuterated Hap was examined. The spectra of Hap deuterated at 100 °C and exposed to CO2 for varied periods at room temperature are shown in Figure 8. With CO2 adsorption the intensity of the 2680-cm-1 band slightly increases accompanied with a red shift to 2675 cm-1 (spectra b and c) and then diminishes with the elapsing of an adsorption period (spectra c-e). The baselines of spectra a-e are different from those of the other spectra because of the broad band centered around 2400 cm-1 due to the strongly adsorbed D2O. The surface P-OD bands are reduced by physisorption of CO2 though the column OD- band at 2633 cm-1 is not influenced, distinctly revealing that the 2633-cm-1 band is due not to the surface OD- ions but to the bulk ones. Therefore, the intense band at 3570 cm-1 due to the column OH- ions does not involve the surface OH- band. The 2680-cm-1 band was not detected on Fap, on whose surface the OH- ions were replaced by F- ions as mentioned, and it diminished by CO2 adsorption. This finding implies that the 2680-cm-1 band can be ascribed to surface OD- ions. However, we did not succeed in specifying the surface OH- band in the subtracted spectrum of Figure 1, though the wavenumber (2680 cm-1) of the OD- band suggests that the surface OH- band appears at 3700-3600 cm-1 around the surface P-OH bands. This would be because the surface P-OH
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by the reaction with surface OD-
OD- + CO2 f DCO3-
(3)
The intermediate DCO3- ions are consumed by the reaction
OD- + DCO3- f CO32- + D2O
(4)
The overall reaction is shown for the adsorption of CO2 on the duterated CaHap as follows
2OD- + CO2 f CO32- + D2O
(5)
This supports the mechanism already proposed for the adsorption of CO2 on Hap.14 As is seen from spectrum g in Figures 8 and 9, the 2680cm-1 band of the surface OD- band revives in part on evacuating at 100 °C and the 1595-cm-1 band disappears, implying that reaction 5 is reversible. Conclusions
Figure 9. Change of IR spectra of deuterated Hap with CO2 adsorption: dashed line, before deuteration; (a) before CO2 adsorption, (b) CO2 adsorption for 10 min, (c) 30 min, (d) 70 min, (e) 130 min, (f) outgassed at room temperature after taking spectrum (e), and (g) outgassed at 100 °C after taking spectrum (f).
bands undergo a complicated interaction with the surface F- ions as speculated from the different profiles of P-OH bands of Hap and Fap in Figure 1 and the band corresponding to the surface OH- ions exchanged with Fions is hard to search in the subtracted spectrum. Figure 9 shows the low wavenumber part (1800-1500 cm-1) of the spectra in Figure 8. When CO2 is introduced, the bands appear at 1708 and 1595 cm-1, and the 1708cm-1 band disappears with elapsing of time as well as the 2675-cm-1 band in Figure 8. The 1595-cm-1 band is ascribed to CO32- and the 1708-cm-1 band to DCO3- formed
From the information presented in this publication, the following conclusions can be drawn. Fluoridation affects the surface structure of Hap. The weak bands at around 3670 cm-1 did not disappear with fluoridation, strongly supporting the assignment of these bands to the surface P-OH groups but not to the surface OH- ions as we proposed in the previous papers.12-14 The diffusion of H+ through the OH- columns was interfered with by the fluoridation, which is of interest concerning with the wellknown phenomenon that the solubility of CaHAp in acid is lowered by fluoridation. The 2680-cm-1 band was detected on the deuterated Hap but was not detected on Fap, and this band disappeared by CO2 adsorption. These findings allow us to assign the 2680-cm-1 band to the surface OD- ions. However, the surface OH- band could not be specified in the spectra before deuteration, because of overlapping with the surface P-OH bands. Acknowledgment. The authors thank Mr. Masao Fukusumi of Osaka Municipal Technical Research Institute for help with the TEM observations. This study was made possible in part by a Grant-in-Aid for Scientific Research (C) of the Ministry of Education, Science, Sports and Culture, Japanese Government. LA0004855
