Thermal Conversion of Octacalcium Phosphate Into Hydroxyapatite A. Bigi, G. Cojazzi, A&
AR.
MG, NR. Ztaly
M. Gazzano, A. Ripamonti,
and N. Roveri
Dipartimento di Chimica “G. Ciamician’, Universitd di Bologna, Italy.-GC, Centro di Studio per la Fisica delle Macromolecole, Universitd di Bologna,
ABSTRACT The thermal conversion of octacalcium phosphate into hydroxyapatite has been investigated by a crystallographic, thermogravimetric, and calorimetric study. The conversion of octacalcium phosphate takes place through the remotion of three of its five water molecules and yields a poor crystalline apatitic phase. The three water molecules are lost in two steps. The first one, which is reversible, corresponds to the remotion of one water molecule and induces a slight contraction of the unit cell of OCP. The successive remotion of two water molecules, which provokes the structural conversion of OCP into apatite, is in irreversible process. The mechanism of the water loss of OCP is explained in terms of its crystal structure.
INTRODUCTION Octacalcium phosphate, CasHz(POd)h .SHzO (OCP) has been reported to be present in some pathological calcifications such as dental and renal calculi, usually coexistent
with hydroxyapatite [l-3]. Furthermore, its possible role as precursor of biological apatites is widely recognized on the basis of chemical and structural observations. The similarity between the crystal morphology of mineral deposits in calcified tissues and OCP at the electron microscope has been utilized to suggest that in biological calcification OCP is nucleated first during mineralization and transformed later by topotactical reactions into apatite or it serves as a nucleus for a successive ongrowth of apatite [4]. Solubility and kinetic dissolution data of the mineral fraction of cardiovascular deposits compared with the corresponding data of OCP indicate that the mineral deposits, represented by a series of carbonate-substituted apatites, arise from varying degrees of OCP hydrolyzation [5]. The numerous studies carried out on the mechanism of transformation of OCP into apatite in vitro and on the factors controlling this transformation contribute to confirm that OCP occurs as a precursor in the formation of many biominerals [6-lo]. The theory that OCP is a precursor for bioAddress reprint requests to: Dr. A. Bigi, Dipartimento Bologna, via Selmi 2, Bologna 40126, Italy.
di Chimica,
Journal of Inorganic Biochemistry, 40, 293-299 (1990) @ 1990 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas,
“G. Ciamician”,
Universiti
di
293 NY, NY 10010
0162-0134/90/$3.50
294
A. Bigi et al.
logical apatites is supported by the striking structural similarities between OCP and calcium hydroxyapatite. OCP crystallizes in the space group pi I11 ] and is constituted of “apatitic layers” interlayered to “hydrated layers”. The “apatitic layer” is made up of PO43 - and Ca2+ ions which fit the same positions as in hydroxyapatite structure, while in the hydrated layers POb”- groups and Ca” + are more widely spaced with interdispersed Hz0 [12]. The hydrated layer is almost certainly hydrophilic and it has been suggested [I31 that an approximately half unit cell of OCP may act as the transition layer between the aqueous phase and the apatitic lattice. In order to get more information on the mechanism of conversion in the solid state we have carried out a crystallographic, thermogravimetric, and calorimetric study on the thermal conversion of octacalcium phosphate into hydroxyapatite EXPERIMENTAL Materials Crystalline octacalcium phosphate was prepared by dropwise addition of calcium acetate solution into a stirring phosphate solution at 60°C according to LeGeros et al. [S]. The product was filtered, washed with bidistilled water, and dried at 2S’C on P205 overnight. Methods X-ray diffraction analysis was carried out by means of a Philips powder diffractometer equipped with an Anton Paar HTK 10 high temperature attachment, using Ni filtered CuK radiation. The 20 ranges covered were from 3--65- at a scanning speed of O.S”/min. The lattice constants were determined by least square refinements. Powder x-ray diffraction patterns have been calculated hy means %>.rf ;I modificd version of the PREFIN program [ 14 J For ir absorption analysis, 1 mg of the powered sample was carefully mixed with 300 mg of KBr (infrared grade) and pelletized under vacuum. The pellets were analyzed using a Perkin Elmer 380 ir Grating Spectrophotometer, range 4000-400 cm--‘. normal slit and scanning speed of 72 cm- ’ min ’ 1 Thermograms were obtained using a Perkin Elmer DSC-7 calorimeter equipped with a DELTA series computer. The measurements were carried out in the temperature range 20-270°C at a scanning rate of S’Cimin using Al pans. Thermogravimetric analysis was carried out using a Perkin Elmer TG-7 equipped with a DELTA series computer. Heating was performed in nitrogen flow (20 cm”/min) using a platinum crucible with a rate of S”C/min up to 900°C. Isothermal curves were recorded in air at different temperatures up to 200°C using a heating rate of 80”Cimin. The weight of the sample was in the range 3--S mg. RESULTS Thermogravimetric
Analysis
The TG-DTG traces obtained from octacalcium phosphate dried on P205 are characterized by two thermal processes. The DTG plots show two well resolved peaks. the first of which, at Tl == 75 & 1“C, corresponds to a weight loss of WI = 1.9 i 0.4 ‘Z and the second one, at Tz = 146 i 1°C. corresponds to a weight loss of W: z~-3.5 Z+Z 0.4 ‘%. A typical TG-DTG plot is reported in Figure 1. Some samples have been tested up to 9OO”C, giving a total weight loss of about 10’1: werght.
THERMAL
CONVERSION
OF OCTACALCIUM
PHOSPHATE
295
95
80
J
‘.
40
FIGURE
70
1.
(-1
100
130
160 190 220 Temperature ("C)
250
280
310
TG plot and (- -) DTG curve of OCP at heating rate of 5O/min in nitrogen
(20 cm3 /min) .
In order to test the reversibility of the two thermal processes, some samples have been retested after cooling and storing in air or in water for 1 hr. The TG-DTG traces of the samples cooled after heating only to 104°C (just before the beginning of the second weight loss) are very close to that reported in Figure 1. The TG-DTG traces of the samples cooled after TG treatment to 220°C do not show any thermal process. The isothermal curves recorded at temperatures lower than Ti display a weight loss corresponding roughly to Wi %. A weight loss corresponding roughly to (WI + W2) % is obtained when the isothermal treatment is carried out at temperatures higher than 95100°C.
DSC Analysis Figure 2 displays a typical DSC thermogram of octacalcium phosphate where it is possible to observe two endothermic broad peaks. The second peak is characterized by an evident change of the slope of the baseline. The onset temperature, T, , and the peak temperature, T,, are 47 f 1°C and 66 f 1°C for the first peak and 121 f 1°C and 137 f 1°C for the second one. The enthalpy change AHI of the first peak is 21 fl J.g-‘. The thermograms recorded from samples cooled after DSC treatment to about lOO”C, that is after the first endothermic peak, and stored in air for 1 hr display both the endothermic transition at Tp = 66°C and at T, = 137°C. No peak at all can be observed when the thermogram is recorded from samples cooled after DSC treatment up to 200°C.
X-ray Diffraction Analysis The powder x-ray diffraction pattern of OCP is reported in Figure 3a. A similar pattern is obtained from OCP sample previously submitted to thermogravimetric analysis up to 104°C (Fig. 3b). Differences in the values of the cell parameters are scarcely significant (Table 1). Moreover, the powder x-ray diffraction pattern calculated after the remotion of one molecule of crystallization water from OCP structure does not show any appreciable difference from that calculated for OCP and is very close to the
296
A. Bigi et al.
5
..
r----
FIGURE 2.
Plot of DSC curve of OCP, scanning rate iiC!rnin
m nitrogen (20 cm3hnini.
experimental one reported in Figure 3a. OCP samples examined after TG treatment up to 200°C display the diffraction maxima characteristic of a poor crystalline apatitic phase (Fig. 3~). the cell parameters are reported in Table 1. Powder x-ray patterns have been recorded from OCP samples during heating at increasing temperatures from 25-.25O”C, with a heating rate of O.S”C/min. The results indicate that the structural conversion of OCP into apatite takes place at 140- 1S@‘c’ This apatitic phase converts completely into 8-triealcium phosphate Ifl-TCP) by heat treatment at temperatures higher than 900°C. Infrared Absorption
Analysis
No appreciable differences in the ir absorption spectrum of OCP can bc observed after heat treatment up to 104°C. However, the ir spectrum obtained from OCP samples heat-treated at 200°C displays an evident weakening of the absorption bands at 1290, 630, and 5.50 cm I, the disappearance of the bands at ? 190, 920. and 87(; cm ‘. and the presence of a new broad band at about 900 cm ’ //
1 FIGURE 3. POW&~x-ray diffraction I patterns of a) OCP. b) OCP after TG I treatment tip 10 104°C. and c) OCP after TG
trcatrncnt
up
to
‘00°C.
THERMAL
CONVERSION
TABLE 1. Unit Cell Parameters
PHOSPHATE
297
of Samples Heated at Different Temperatures
a(& OCP OCP after TG treatment up to 104°C Apatitic phase obtained after TG treatment up to 200°C
OF OCTACALCIUM
&
66
4)
P(“)
-0
19.03(4) 18.99(2)
9.56(2) 9.51(l)
6.84(l) 6.830(8)
90.1(l) 90.20(5)
92.4(l) 92.6(l)
100.1(l) 100.2(l)
9.48(2)
9.48(2)
6.90(l)
90
90
120
$3.3) 1224(5) 1213(2) 536(2)
DISCUSSION The results of this study reveal that OCP converts into a poor crystalline apatitic phase through the remotion of only a part of its crystallization water, corresponding to about three water molecules per OCP unit formula. TG and DSC analyses reveal that the weight loss takes place in two steps, the first of which reaches the maximum rate at 75°C and corresponds to the remotion of one water molecule with a AH of 215 . g-l. This water loss, which is reversible, induces only a slight contraction of the unit cell of OCP. Furthermore, the relative calculated x-ray pattern does not display any appreciable structural modifications with respect to that of pure OCP. The successive weight loss takes place with a maximum rate at 146°C and corresponds to the remotion of two water molecules per OCP molecule. This water remotion induces the structural conversion of OCP into a poor crystalline apatitic phase as evidenced by x-ray diffraction analysis. This result is coherent with the change in the slope of the baseline during the second endothermic process present in the DSC thermogram. This last water loss is an irreversible process, as expected on the basis of the observed simultaneous structural conversion. The complete remotion of the five structural water molecules of OCP can be obtained by heat treatment at 900°C. In isothermal condition, treatment at 95-100°C is enough to yield the conversion of OCP into apatite through the remotion of three water molecules. The poor thermal stability of the apatitic phase evidenced by its complete conversion into /3-TCP by heat treatment at temperatures higher than 9OO”C, is to be related to its nonstoichiometry and low degree of crystallinity. The nonstoichiometry and poor crystallinity of the obtained apatite together with the presence in its ir absorption spectrum of the band at about 900 cm-’ characteristic of HPOd*- groups, suggest that the thermal conversion of OCP into apatite takes place through the reaction CasH2(P04)6
.5H20 ---t HzOT + CasHz(PO4)6.4HzO +
2H201
+
Ca&dP0ddOH)2.
The first stage of water remotion can be explained in terms of the crystal structure of OCP determined by Brown [ 111. The water molecule which can be removed from OCP structure by heat treatment at 104”C, or even at temperatures lower than 75°C working in isothermal conditions, is probably that labeled 43 in Figure 4. As Fan be observed in Table 2, which reports the interatomic distances shorter than 3.0 A, this water molecule has no contact with calcium ions while its strongest contact is with the water molecule O(43)’ related to 0(43) by a center of symmetry, and with two other water molecules, namely O(40) (2.833 A) and 0(41) (2.351 A). Furthermore the water molecules 0(43) and 0(43)‘, being located in a channel, can easily be lost without collapse of the lattice.
298
A. Bigi et al.
FIGURE 4. A view of OCP structure along [OOl] direction. Dotted lines evidence the interatomic distances in Table 2. Atomic coordinates and numbering are those reported by Brown [L)l. The identification of the two water molecules which are lost during the second step is undoubtedly more difficult, due also to the structural conversion caused by this further remotion. However, as can be observed in Table 2, molecules O(40) and 0(42) display two short contacts with calcium ions, while both O(39) and 0(41) have only one short contact with calcium ions. On this basis. the remotion of molecules O(39) and O(41) during the second stage of the process appears most likely.
The authors are grateful to Consiglio Nazionale delle Kicherche and Minister0 della Pubblica Istruzione, Italy, for financial support. They also wish m thank Mr ,?;I Gandolf; for excellent technical assistance. TABLE 2. Interatomic Distances (Shorter than 3& Involving Water Oxygens in OCP. Atomic Coordinates and Nomenclature are Those Reported by Brown i 1962) [ 11)
(A, O(39) - Ca(3)
__-l_l_
O(24) 0(36) O(33) Ca(4) O(37)
2.476 2.700 2.924 2.955 3.510 2.548
O(42)
2.772
- O(38) -- O(43) O(43) - O(43) - O(40) -0(41)
2.839 2.951 2.564 2.833 2.951
O(41) --
(A)
-----.-____ O(40) --~Cu(7) --Ca(3!~ -0(19) - O(37) - O(43) 0(42) - Cd(5) -. Ca(7) -- 0(41! --O(371
2.488 2.676 2.676 2.784 2.833 2.501
2.522 2.772 2.804
THERMAL
CONVERSION
OF OCTACALCIUM
PHOSPHATE
299
REFERENCES 1. R. A. Young and W. E. Brown, in Biological Mineralization and Demineralization, G. H. Nancollas, Ed., Dahalem Konferenzen, Springer-Verlag, Berlin, 1982, pp. 101-141. 2. R. Z. LeGeros, J. Dent. Res. 53, 45 (1974). 3. H. Schroeher, Formation and Inhibition of Dental Calculus, Hans Huber Pub., Vienna, 1969. 4. F. C. M. Driessens, R. A. Terpstra, P. Bemtema, J. H. M. Woltagens, and R. M. H. Verbeeck, J. Naturforsch. C. Biosci. 42, 916 (1987). 5. B. B. Tomazic, E. S. Etz, and W. E. Brown, Scanning Microscopy 1, 95 (1987). 6. N. Eidelman, L. C. Chow, and W. E. Brown, Calcif. Tissue Znt. 41, 18 (1987). 7. P. T. Cheng, Calcif. Tissue Znt. 40, 339 (1987). 8. R. Z. LeGeros, R. Kijkowslm, and J. P. LeGeros, Scanning Electron A4icroscopy 4,
1771 (1984). 9. A. Bigi, M. Gazzano, A. Ripamonti, and N. Roveri, J. Znorg. Biochem. 32, 25 (1988). 10. L. J. Shyu, L. Perez, S. J. Zawacki, J. C. Heughebaert, and G. H. Nancollas, J. Dent. Res. 62, 398 (1983). 11. W. E. Brown, Nature 196, 1048 (1962). 12. W. E. Brown, L. R. W. Schroeder, and J. S. Ferris, J. Phys. Chem. 83, 1385 (1979). 13. W. E. Brown, Clin. Orthop. 44, 205 (1966). 14. A. Immirzi, Acta Cryst. B36, 2578 (1986). Received November 22, 1989; accepted January IO, 1990
AR.
MG, NR. Ztaly
M. Gazzano, A. Ripamonti,
and N. Roveri
Dipartimento di Chimica “G. Ciamician’, Universitd di Bologna, Italy.-GC, Centro di Studio per la Fisica delle Macromolecole, Universitd di Bologna,
ABSTRACT The thermal conversion of octacalcium phosphate into hydroxyapatite has been investigated by a crystallographic, thermogravimetric, and calorimetric study. The conversion of octacalcium phosphate takes place through the remotion of three of its five water molecules and yields a poor crystalline apatitic phase. The three water molecules are lost in two steps. The first one, which is reversible, corresponds to the remotion of one water molecule and induces a slight contraction of the unit cell of OCP. The successive remotion of two water molecules, which provokes the structural conversion of OCP into apatite, is in irreversible process. The mechanism of the water loss of OCP is explained in terms of its crystal structure.
INTRODUCTION Octacalcium phosphate, CasHz(POd)h .SHzO (OCP) has been reported to be present in some pathological calcifications such as dental and renal calculi, usually coexistent
with hydroxyapatite [l-3]. Furthermore, its possible role as precursor of biological apatites is widely recognized on the basis of chemical and structural observations. The similarity between the crystal morphology of mineral deposits in calcified tissues and OCP at the electron microscope has been utilized to suggest that in biological calcification OCP is nucleated first during mineralization and transformed later by topotactical reactions into apatite or it serves as a nucleus for a successive ongrowth of apatite [4]. Solubility and kinetic dissolution data of the mineral fraction of cardiovascular deposits compared with the corresponding data of OCP indicate that the mineral deposits, represented by a series of carbonate-substituted apatites, arise from varying degrees of OCP hydrolyzation [5]. The numerous studies carried out on the mechanism of transformation of OCP into apatite in vitro and on the factors controlling this transformation contribute to confirm that OCP occurs as a precursor in the formation of many biominerals [6-lo]. The theory that OCP is a precursor for bioAddress reprint requests to: Dr. A. Bigi, Dipartimento Bologna, via Selmi 2, Bologna 40126, Italy.
di Chimica,
Journal of Inorganic Biochemistry, 40, 293-299 (1990) @ 1990 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas,
“G. Ciamician”,
Universiti
di
293 NY, NY 10010
0162-0134/90/$3.50
294
A. Bigi et al.
logical apatites is supported by the striking structural similarities between OCP and calcium hydroxyapatite. OCP crystallizes in the space group pi I11 ] and is constituted of “apatitic layers” interlayered to “hydrated layers”. The “apatitic layer” is made up of PO43 - and Ca2+ ions which fit the same positions as in hydroxyapatite structure, while in the hydrated layers POb”- groups and Ca” + are more widely spaced with interdispersed Hz0 [12]. The hydrated layer is almost certainly hydrophilic and it has been suggested [I31 that an approximately half unit cell of OCP may act as the transition layer between the aqueous phase and the apatitic lattice. In order to get more information on the mechanism of conversion in the solid state we have carried out a crystallographic, thermogravimetric, and calorimetric study on the thermal conversion of octacalcium phosphate into hydroxyapatite EXPERIMENTAL Materials Crystalline octacalcium phosphate was prepared by dropwise addition of calcium acetate solution into a stirring phosphate solution at 60°C according to LeGeros et al. [S]. The product was filtered, washed with bidistilled water, and dried at 2S’C on P205 overnight. Methods X-ray diffraction analysis was carried out by means of a Philips powder diffractometer equipped with an Anton Paar HTK 10 high temperature attachment, using Ni filtered CuK radiation. The 20 ranges covered were from 3--65- at a scanning speed of O.S”/min. The lattice constants were determined by least square refinements. Powder x-ray diffraction patterns have been calculated hy means %>.rf ;I modificd version of the PREFIN program [ 14 J For ir absorption analysis, 1 mg of the powered sample was carefully mixed with 300 mg of KBr (infrared grade) and pelletized under vacuum. The pellets were analyzed using a Perkin Elmer 380 ir Grating Spectrophotometer, range 4000-400 cm--‘. normal slit and scanning speed of 72 cm- ’ min ’ 1 Thermograms were obtained using a Perkin Elmer DSC-7 calorimeter equipped with a DELTA series computer. The measurements were carried out in the temperature range 20-270°C at a scanning rate of S’Cimin using Al pans. Thermogravimetric analysis was carried out using a Perkin Elmer TG-7 equipped with a DELTA series computer. Heating was performed in nitrogen flow (20 cm”/min) using a platinum crucible with a rate of S”C/min up to 900°C. Isothermal curves were recorded in air at different temperatures up to 200°C using a heating rate of 80”Cimin. The weight of the sample was in the range 3--S mg. RESULTS Thermogravimetric
Analysis
The TG-DTG traces obtained from octacalcium phosphate dried on P205 are characterized by two thermal processes. The DTG plots show two well resolved peaks. the first of which, at Tl == 75 & 1“C, corresponds to a weight loss of WI = 1.9 i 0.4 ‘Z and the second one, at Tz = 146 i 1°C. corresponds to a weight loss of W: z~-3.5 Z+Z 0.4 ‘%. A typical TG-DTG plot is reported in Figure 1. Some samples have been tested up to 9OO”C, giving a total weight loss of about 10’1: werght.
THERMAL
CONVERSION
OF OCTACALCIUM
PHOSPHATE
295
95
80
J
‘.
40
FIGURE
70
1.
(-1
100
130
160 190 220 Temperature ("C)
250
280
310
TG plot and (- -) DTG curve of OCP at heating rate of 5O/min in nitrogen
(20 cm3 /min) .
In order to test the reversibility of the two thermal processes, some samples have been retested after cooling and storing in air or in water for 1 hr. The TG-DTG traces of the samples cooled after heating only to 104°C (just before the beginning of the second weight loss) are very close to that reported in Figure 1. The TG-DTG traces of the samples cooled after TG treatment to 220°C do not show any thermal process. The isothermal curves recorded at temperatures lower than Ti display a weight loss corresponding roughly to Wi %. A weight loss corresponding roughly to (WI + W2) % is obtained when the isothermal treatment is carried out at temperatures higher than 95100°C.
DSC Analysis Figure 2 displays a typical DSC thermogram of octacalcium phosphate where it is possible to observe two endothermic broad peaks. The second peak is characterized by an evident change of the slope of the baseline. The onset temperature, T, , and the peak temperature, T,, are 47 f 1°C and 66 f 1°C for the first peak and 121 f 1°C and 137 f 1°C for the second one. The enthalpy change AHI of the first peak is 21 fl J.g-‘. The thermograms recorded from samples cooled after DSC treatment to about lOO”C, that is after the first endothermic peak, and stored in air for 1 hr display both the endothermic transition at Tp = 66°C and at T, = 137°C. No peak at all can be observed when the thermogram is recorded from samples cooled after DSC treatment up to 200°C.
X-ray Diffraction Analysis The powder x-ray diffraction pattern of OCP is reported in Figure 3a. A similar pattern is obtained from OCP sample previously submitted to thermogravimetric analysis up to 104°C (Fig. 3b). Differences in the values of the cell parameters are scarcely significant (Table 1). Moreover, the powder x-ray diffraction pattern calculated after the remotion of one molecule of crystallization water from OCP structure does not show any appreciable difference from that calculated for OCP and is very close to the
296
A. Bigi et al.
5
..
r----
FIGURE 2.
Plot of DSC curve of OCP, scanning rate iiC!rnin
m nitrogen (20 cm3hnini.
experimental one reported in Figure 3a. OCP samples examined after TG treatment up to 200°C display the diffraction maxima characteristic of a poor crystalline apatitic phase (Fig. 3~). the cell parameters are reported in Table 1. Powder x-ray patterns have been recorded from OCP samples during heating at increasing temperatures from 25-.25O”C, with a heating rate of O.S”C/min. The results indicate that the structural conversion of OCP into apatite takes place at 140- 1S@‘c’ This apatitic phase converts completely into 8-triealcium phosphate Ifl-TCP) by heat treatment at temperatures higher than 900°C. Infrared Absorption
Analysis
No appreciable differences in the ir absorption spectrum of OCP can bc observed after heat treatment up to 104°C. However, the ir spectrum obtained from OCP samples heat-treated at 200°C displays an evident weakening of the absorption bands at 1290, 630, and 5.50 cm I, the disappearance of the bands at ? 190, 920. and 87(; cm ‘. and the presence of a new broad band at about 900 cm ’ //
1 FIGURE 3. POW&~x-ray diffraction I patterns of a) OCP. b) OCP after TG I treatment tip 10 104°C. and c) OCP after TG
trcatrncnt
up
to
‘00°C.
THERMAL
CONVERSION
TABLE 1. Unit Cell Parameters
PHOSPHATE
297
of Samples Heated at Different Temperatures
a(& OCP OCP after TG treatment up to 104°C Apatitic phase obtained after TG treatment up to 200°C
OF OCTACALCIUM
&
66
4)
P(“)
-0
19.03(4) 18.99(2)
9.56(2) 9.51(l)
6.84(l) 6.830(8)
90.1(l) 90.20(5)
92.4(l) 92.6(l)
100.1(l) 100.2(l)
9.48(2)
9.48(2)
6.90(l)
90
90
120
$3.3) 1224(5) 1213(2) 536(2)
DISCUSSION The results of this study reveal that OCP converts into a poor crystalline apatitic phase through the remotion of only a part of its crystallization water, corresponding to about three water molecules per OCP unit formula. TG and DSC analyses reveal that the weight loss takes place in two steps, the first of which reaches the maximum rate at 75°C and corresponds to the remotion of one water molecule with a AH of 215 . g-l. This water loss, which is reversible, induces only a slight contraction of the unit cell of OCP. Furthermore, the relative calculated x-ray pattern does not display any appreciable structural modifications with respect to that of pure OCP. The successive weight loss takes place with a maximum rate at 146°C and corresponds to the remotion of two water molecules per OCP molecule. This water remotion induces the structural conversion of OCP into a poor crystalline apatitic phase as evidenced by x-ray diffraction analysis. This result is coherent with the change in the slope of the baseline during the second endothermic process present in the DSC thermogram. This last water loss is an irreversible process, as expected on the basis of the observed simultaneous structural conversion. The complete remotion of the five structural water molecules of OCP can be obtained by heat treatment at 900°C. In isothermal condition, treatment at 95-100°C is enough to yield the conversion of OCP into apatite through the remotion of three water molecules. The poor thermal stability of the apatitic phase evidenced by its complete conversion into /3-TCP by heat treatment at temperatures higher than 9OO”C, is to be related to its nonstoichiometry and low degree of crystallinity. The nonstoichiometry and poor crystallinity of the obtained apatite together with the presence in its ir absorption spectrum of the band at about 900 cm-’ characteristic of HPOd*- groups, suggest that the thermal conversion of OCP into apatite takes place through the reaction CasH2(P04)6
.5H20 ---t HzOT + CasHz(PO4)6.4HzO +
2H201
+
Ca&dP0ddOH)2.
The first stage of water remotion can be explained in terms of the crystal structure of OCP determined by Brown [ 111. The water molecule which can be removed from OCP structure by heat treatment at 104”C, or even at temperatures lower than 75°C working in isothermal conditions, is probably that labeled 43 in Figure 4. As Fan be observed in Table 2, which reports the interatomic distances shorter than 3.0 A, this water molecule has no contact with calcium ions while its strongest contact is with the water molecule O(43)’ related to 0(43) by a center of symmetry, and with two other water molecules, namely O(40) (2.833 A) and 0(41) (2.351 A). Furthermore the water molecules 0(43) and 0(43)‘, being located in a channel, can easily be lost without collapse of the lattice.
298
A. Bigi et al.
FIGURE 4. A view of OCP structure along [OOl] direction. Dotted lines evidence the interatomic distances in Table 2. Atomic coordinates and numbering are those reported by Brown [L)l. The identification of the two water molecules which are lost during the second step is undoubtedly more difficult, due also to the structural conversion caused by this further remotion. However, as can be observed in Table 2, molecules O(40) and 0(42) display two short contacts with calcium ions, while both O(39) and 0(41) have only one short contact with calcium ions. On this basis. the remotion of molecules O(39) and O(41) during the second stage of the process appears most likely.
The authors are grateful to Consiglio Nazionale delle Kicherche and Minister0 della Pubblica Istruzione, Italy, for financial support. They also wish m thank Mr ,?;I Gandolf; for excellent technical assistance. TABLE 2. Interatomic Distances (Shorter than 3& Involving Water Oxygens in OCP. Atomic Coordinates and Nomenclature are Those Reported by Brown i 1962) [ 11)
(A, O(39) - Ca(3)
__-l_l_
O(24) 0(36) O(33) Ca(4) O(37)
2.476 2.700 2.924 2.955 3.510 2.548
O(42)
2.772
- O(38) -- O(43) O(43) - O(43) - O(40) -0(41)
2.839 2.951 2.564 2.833 2.951
O(41) --
(A)
-----.-____ O(40) --~Cu(7) --Ca(3!~ -0(19) - O(37) - O(43) 0(42) - Cd(5) -. Ca(7) -- 0(41! --O(371
2.488 2.676 2.676 2.784 2.833 2.501
2.522 2.772 2.804
THERMAL
CONVERSION
OF OCTACALCIUM
PHOSPHATE
299
REFERENCES 1. R. A. Young and W. E. Brown, in Biological Mineralization and Demineralization, G. H. Nancollas, Ed., Dahalem Konferenzen, Springer-Verlag, Berlin, 1982, pp. 101-141. 2. R. Z. LeGeros, J. Dent. Res. 53, 45 (1974). 3. H. Schroeher, Formation and Inhibition of Dental Calculus, Hans Huber Pub., Vienna, 1969. 4. F. C. M. Driessens, R. A. Terpstra, P. Bemtema, J. H. M. Woltagens, and R. M. H. Verbeeck, J. Naturforsch. C. Biosci. 42, 916 (1987). 5. B. B. Tomazic, E. S. Etz, and W. E. Brown, Scanning Microscopy 1, 95 (1987). 6. N. Eidelman, L. C. Chow, and W. E. Brown, Calcif. Tissue Znt. 41, 18 (1987). 7. P. T. Cheng, Calcif. Tissue Znt. 40, 339 (1987). 8. R. Z. LeGeros, R. Kijkowslm, and J. P. LeGeros, Scanning Electron A4icroscopy 4,
1771 (1984). 9. A. Bigi, M. Gazzano, A. Ripamonti, and N. Roveri, J. Znorg. Biochem. 32, 25 (1988). 10. L. J. Shyu, L. Perez, S. J. Zawacki, J. C. Heughebaert, and G. H. Nancollas, J. Dent. Res. 62, 398 (1983). 11. W. E. Brown, Nature 196, 1048 (1962). 12. W. E. Brown, L. R. W. Schroeder, and J. S. Ferris, J. Phys. Chem. 83, 1385 (1979). 13. W. E. Brown, Clin. Orthop. 44, 205 (1966). 14. A. Immirzi, Acta Cryst. B36, 2578 (1986). Received November 22, 1989; accepted January IO, 1990
