Synthetic apatites containing Na, Mg, and CO3 and their
comparison with tooth enamel mineral
Citation for published version (APA):
Featherstone, J. D. B., Mayer, I., Driessens, F. C. M., Verbeeck, R. M. H., & Heijligers, H. J. M. (1983). Synthetic
apatites containing Na, Mg, and CO3 and their comparison with tooth enamel mineral. Calcified Tissue
International, 35(1), 169-171. https://doi.org/10.1007/BF02405026
DOI:
10.1007/BF02405026
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Published: 01/01/1983
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Calcif Tissue Int (1983) 35:169- 171
Calcified Tissue
International
9 1983 by Springer-Verlag
Synthetic Apatites Containing Na, Mg, and C03 and Their Comparison with
Tooth Enamel Mineral
J. D. B. F e a t h e r s t o n e , I. M a y e r , * " F. C. M. D r i e s s e n s , 2 R. M. H. V e r b e e c k , **'3 a n d H. J. M. H e i j l i g e r s 4 'Eastman Dental Center, Rochester, New York 14620, USA; 2Institute of Dental Material Science, Catholic University, 6500 HB Nijmegen, The Netherlands; 3Laboratory for Analytical Chemistry, State University Gent. 9000 Gent, Belgium: ~Laboratory for Physical Chemistry, University of Technology, Eindhoven. The Netherlands
Summary.
A p a t i t e s c o n t a i n i n g N a , Mg, a n d CO3 in a m o u n t s o c c u r r i n g in t o o t h e n a m e l m i n e r a l w e r e s y n t h e s i z e d b y p r e c i p i t a t i o n f r o m a q u e o u s s o l u - t i o n s . T h e X - r a y d i f f r a c t i o n p a t t e r n s s h o w e d o n l y apatitic r e f l e c t i o n s , w h i c h w e r e s o m e w h a t b r o a d e r t h a n t h o s e o f e n a m e l . X - r a y G u i n i e r p h o t o g r a p h s r e s u l t e d in d i f f u s e r e f l e c t i o n s f r o m w h i c h the l a t t i c e p a r a m e t e r s c o u l d n o t be d e t e r m i n e d . T h e i n f r a r e d (IR) a b s o r p t i o n s p e c t r u m o f the s y n t h e t i c s a m p l e s w a s p r a c t i c a l l y i d e n t i c a l with that o f t o o t h e n a m e l . H o w e v e r , after h e a t i n g the s a m p l e s f o r V2 h at a t e m p e r a t u r e b e t w e e n 300 a n d 600~ n o a b s o r p t i o n p e a k w a s f o u n d at 2340 c m -1 as in t h e I R s p e c t r u m o f t o o t h e n a m e l a f t e r h e a t i n g .Key words:
N a , Mg, a n d CO3 - - C a r b o n a t e d a p a t i t e - - T o o t h e n a m e l - - I R s p e c t r u m . M a n y i n v e s t i g a t o r s t h i n k o f t o o t h e n a m e l m i n e r a l as h y d r o x y a p a t i t e , w h i c h is a r e a s o n a b l e s t a r t i n g p o i n t . E n a m e l is b e t t e r t h o u g h t o f as a c a r b o n a t e d a p a t i t e a n d s e v e r a l i n v e s t i g a t o r s h a v e c o n s i d e r e d it as h y d r o x y a p a t i t e c o n t a i n i n g CO3, N a , a n d Mg as its m a i n i m p u r i t i e s , a n d m a n y o t h e r less i m p o r t a n t c o n s t i t u e n t s as its m i n o r i m p u r i t i e s [1, 2, 3]. A c - c o r d i n g to v a r i o u s l i t e r a t u r e d a t a c o m p i l e d b y D r i e s s e n s [4], t h e CO3 c o n t e n t c a n v a r y f r o m 2 . 7 - 5 . 0 % , N a c o n t e n t f r o m 0 . 2 - 0 . 9 % , a n d Mg c o n t e n t f r o m 0 . 2 - 0 . 6 % . L e G e r o s [5] w a s t h e first to s h o w t h a t a p a t i t e s c o n t a i n i n g the r e l e v a n t a m o u n t s o f CO3, N a , a n d Mg c a n be s y n t h e s i z e d b y p r e c i p i - t a t i o n f r o m a q u e o u s s o l u t i o n s . I n the p r e s e n t s t u d y , t h e m e t h o d to s y n t h e s i z e s u c h a p a t i t e s w a s ex-* P r e s e n t a d d r e s s : Department of Inorganic and Analytical Chemistry, Hebrew University of Jerusalem, Jerusalem, Israel. ** Research Associate NFSR (Belgium)
t e n d e d a n d r e v i s e d , a n d s o m e o f t h e r e s u l t i n g a p a - tites w e r e c o m p a r e d w i t h t o o t h e n a m e l m i n e r a l . T h i s c o m p a r i s o n w a s f o c u s e d o n c h e m i c a l c o m p o - sition, lattice p a r a m e t e r s , a n d I R a b s o r p t i o n s p e c t r a . T h e latter were d e t e r m i n e d n o t o n l y o f s a m p l e s as s y n t h e s i z e d , b u t also after heat t r e a t m e n t at t e m p e r - a t u r e s b e t w e e n 200 a n d 900~ s i n c e s u c h heat treat- m e n t s r e s u l t in a s p e c i f i c a b s o r p t i o n at a w a v e - n u m b e r o f 2340 c m ' f o r t o o t h e n a m e l [4, 6].
Materials and Methods
The method of LeGeros (1967) for synthesis of the apatites was used with modifications as follows. Synthetic carbonated- apatites were prepared by precipitation under conditions of closely controlled temperature, time, and pH [7]. Sodium phos- phate solution was dripped into stirred calcium nitrate solution under a reflux condenser at 92 _+ I~ and pH 9-10 during 2 h. Carbonate was added to both solutions at 5-25 mmol/liter. Mag- nesium was likewise added to each solution at 0.33 or 1.37 retool/liter as magnesium nitrate. After the 2-hour reaction, the temperature was raised to 100~ and the system refluxed for 2 hours, then the precipitate was washed and dried. Carbonate- only apatites were also produced using the same procedure.
Chemical analysis was carried out with the following methods. The apatites were analyzed for Ca, Na, and Mg by atomic ab- sorption using a Perkin Elmer 403 spectrophotometer and appro- priate precautions against interference. Phosphate was analyzed for by the molybdate method and fluoride by specific ion elec- trode. Carbonate was estimated from the IR spectra using the ratio of the extinctions of the carbonate band at about 1415 cm 1 (E,4,.0 to the phosphate band at about 575 cm ' (E57~), and com- paring this with a standard curve derived from known carbonated-apatites, previously analyzed by precise (_+2%) gas solid chromatography [8]. This IR method gives carbonate values with an accuracy of better than _+5% and is based on similar methods used by LeGeros [5] and Arends and Davidson [9]. Details are given elsewhere ll0].
The phase composition of the samples was checked with a General Electric X-ray powder diffractometer and subsequently
170 J . D . B . F e a t h e r s t o n e et al.: Apatites Containing Na. Mg, and C Q in T o o t h E n a m e l
on a Philips p o w d e r d i f f r a c t o m e t e r using a Ge internal standard. This m e t h o d was also suitable to determine relative shifts in the lattice p a r a m e t e r s by m e a s u r i n g shifts in the position of the re- flections for the lattice p a r a m e t e r a and the reflections for the lattice p a r a m e t e r c. G o o d diffraction patterns were obtained using a well c o m p a c t e d thick layer, w h i c h p r o d u c e d sharp peaks f r o m which lattice p a r a m e t e r s could be calculated.
The s a m p l e s were also s u b j e c t e d to X-ray diffraction in a Philips Guinier c a m e r a (type X D L 700) to obtain accurate values for the lattice p a r a m e t e r s . Cu K,~ radiation was u s e d for an expo- sure time o f 8 h on single-coated film. C o r u n d u m (~-AI~O~) was u s e d as an internal s t a n d a r d so that the c a m e r a c o n s t a n t could be calculated per sample. T h e position of distinct reflections could be d e t e r m i n e d from the d e n s i t o m e t e r tracing of the X-ray film. A l e a s t - s q u a r e s calculation on t h e s e positions p r o d u c e s the best fitting values for the lattice p a r a m e t e r s a a n d c.
Infrared a b s o r p t i o n s p e c t r a were d e t e r m i n e d with the K B r pellet t e c h n i q u e in a Perkin E l m e r Infrared S p e c t r o p h o t o m e t e r type 457. Spectra were obtained from the samples as s y n t h e s i z e d and after heating for ~/2 h in air at a t e m p e r a t u r e o f either 200, 300, 400, 500, 600, 700, 800, or 900~ Special attention was given to a b s o r p t i o n s at 2340 c m -~ (origin u n k n o w n , see discussion), 628, and 3575 c m - ' (OH), 880, 1450, a n d 1545 c m 1 (A type carbonate) and 873, 1412, and 1465 c m l (B type carbonate). Similar IR spectra were obtained f r o m h u m a n tooth enamel.
Results
Samples used in the present study were c h o s e n
from a c o m p r e h e n s i v e series of c a r b o n a t e d apatites
precipitated in the p r e s e n c e o f magnesium, reported
e l s e w h e r e by M a y e r and F e a t h e r s t o n e [11]. The
Mg, Na, and CO3 contents were chosen to be in the
ranges found for these in dental enamel. Table 1
s u m m a r i z e s t h e i r c h e m i c a l c o m p o s i t i o n . T h e s e
samples were used for c o n s e c u t i v e investigations.
X-ray diffraction patterns obtained with the X-ray
p o w d e r diffractometer showed only apatitic reflec-
tions, which were relatively broad c o m p a r e d with
those o f apatite samples synthesized by solid-state
reactions at high t e m p e r a t u r e s [12, 13] and even
c o m p a r e d to tooth enamel [14, 15], but were typical
o f c a r b o n a t e d apatites p r o d u c e d by aqueous pre-
cipitation [7, 8]. The lattice parameters a and c as
derived from calculations from these patterns are
given in Table 1.
Attempts to determine accurate values for these
lattice parameters with the Guinier camera failed
completely. The apatite patterns were so diffuse
that no distinct reflections could be observed, al-
though the pattern of c o r u n d u m used as the internal
standard was very sharp, as usual [12, 13].
In the IR absorption spectra of the synthetic sam-
ples, the peaks at 628 and 3575 cm -~ were weak as
in tooth enamel, whereas the pattern o f CO3 peaks
was also very similar to that of tooth enamel: A-
type carbonate was weak and B-type very strong.
After heating in the range from 300-600~
A-type
c a r b o n a t e i n c r e a s e d a n d B - t y p e c a r b o n a t e de-
Table 1. Chemical c o m p o s i t i o n a n d lattice p a r a m e t e r s of the synthetic apatites in the p r e s e n t s t u d y containing CO3, Na, a n d Mg. Lattice p a r a m e t e r s were c a l c u l a t e d f r o m X-ray p o w d e r diffraction patterns p r o d u c e d with a Ge standard. C o m p a r e den- tal e n a m e l sample with 4.1% c a r b o n a t e [8] which had a- and c- axis 9.447 and 6.882 A, respectively
C h e m i c a l c o m p o s i t i o n , % wt/wt
Lattice p a r a m e t e r s
Sample a-axis c-axis
no. Ca P Na CO,3 Mg A A 154 37.2 17.7 0.26 2.2 0 9.417 6.869 169 37.5 17.0 0.47 5.7 0 9.386 6.873 148 35.5 17.4 0.66 2.3 0.11 9.428 6.876 166 35.2 17.3 0.47 2.8 0.10 9.414 6.878 155 37.7 17.9 0.26 2.6 0.39 9.401 6.862 164 35.4 18.0 0.30 3.9 0.31 9.407 6.889
creased somewhat. Heating at higher temperatures
resulted in decreases o f both A and B type carbon-
ate so that after heating at 900~
very little of the
carbonate remained. In that same range, the O H
peaks at 628 and 3575 cm ~ increased dramatically,
whereas in the samples no. 155 and 164 (high Mg
content) there was also a t e n d e n c y of the broad
P O 4absorption pattern between 1000 and 1150 cm -~ to
split up, indicating the formation of a fi-TCP (ter-
tiary calcium phosphatej. This could be confirmed
with X-ray diffraction. The IR absorption behavior
of tooth enamel after heat treatment was very simi-
lar e x c e p t for the thct that the IR spectra of tooth
enamel heated at temperatures o f 300-700~
con-
tained a clearly observable peak at 2340 cm 1. The
spectra of the synthetic samples did not show any
sign of an absorption peak there.
Discussion
The 1R spectrum of the synthetic samples at room
t e m p e r a t u r e and after heating was qualitatively in-
distinguishable from that o f tooth enamel except for
the absorption peak at 2340 cm -1 after heating tooth
enamel in the range o f 300-600~
This peak has
also been o b s e r v e d by H o l c o m b and Young [6] who
interpreted it as an absorption by intermediary mo-
lecular CO2, f o r m e d w h e n B - t y p e c a r b o n a t e is
t r a n s f o r m e d into A-type carbonate. T h e y derive
this interpretation from the work o f D o w k e r and
Elliott [16], w h o f o u n d a similar a b s o r p t i o n in
heated samples o f synthetic c a r b o n a t e d apatites.
H o w e v e r , as mentioned by Driessens and Verbeeck
[4], this interpretation may not be correct for tooth
enamel, as Santos [17] found this same peak in the
IR spectrum o f heated samples of carbonate-free
synthetic whitlockite.
J. D. B. Featherstone et al.: Apatites Containing Na, Mg, and CO3 in Tooth Enamel 171 T h e d i f f r a c t o m e t e r t r a c i n g s o f the s y n t h e t i c s a m - ples w e r e a l s o v e r y s i m i l a r to t h o s e o f t o o t h e n a m e l , t h o u g h t h e y h a d s o m e w h a t m o r e b r o a d e n e d a n d w e a k e r p e a k s . T h i s m i g h t be d u e to a s m a l l e r parti- cle s i z e , b u t c o u l d a d d i t i o n a l l y be c a u s e d b y intra- c r y s t a l l i n e s t r a i n s . T h e l a t t i c e p a r a m e t e r s a a n d c c a l c u l a t e d f r o m t h e s e d i f f r a c t o m e t e r t r a c i n g s a n d m e n t i o n e d in T a b l e 1 s h o w d i f f e r e n c e s w i t h s i m i l a r v a l u e s d e r i v e d f r o m d i f f r a c t o m e t e r t r a c i n g s o f t o o t h e n a m e l . T h e s e d i f f e r e n c e s m a y be r e l a t e d to t h e f a c t t h a t t h e CI ions c o n t a i n e d in t o o t h e n a m e l a n d n o t in the s y n t h e t i c s a m p l e s h a v e a m a r k e d influ- e n c e o n the l a t t i c e p a r a m e t e r s , e s p e c i a l l y t h e a - a x i s p a r a m e t e r [18, 19]. T h e X - r a y d i f f r a c t i o n p a t t e r n s o b t a i n e d f r o m the s y n t h e t i c s a m p l e s w e r e s i m i l a r to t h o s e o f s a m p l e s t a k e n f r o m t h e d e e p e r l a y e r s o f t o o t h e n a m e l f r o m f r e s h l y e r u p t e d t e e t h [14]. R e f l e c t i o n s in t h e G u i n i e r c a m e r a w e r e v e r y b r o a d a n d n o t d i s t i n c t so t h a t t h e i r p o s i t i o n c o u l d n o t be e s t i m a t e d . H o w e v e r , s a m p l e s o f t o o t h e n a m e l t a k e n f r o m o l d e r t e e t h g i v e d i s t i n c t r e f l e c t i o n s w i t h t h e G u i n i e r t e c h n i q u e [15] a n d t h u s a r e n o t c o m p a r a b l e w i t h t h e s y n t h e t i c s a m p l e s o f the p r e s e n t s t u d y . T h e d i f f u s e n e s s o f the G u i n i e r p a t t e r n f o r t h e s e l a t t e r s a m p l e s is p r o b a b l y d u e n o t so m u c h to a s m a l l p a r t i c l e s i z e , but to i n h o m o g e n e i t i e s a n d r e s u l t i n g s t r a i n s w i t h i n t h e p a r t i c l e s . S u c h i n h o m o g e n e i t i e s w e r e s h o w n b y N e l s o n [20], u s i n g h i g h r e s o l u t i o n t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , to o c c u r in N a - a n d Co:j-con- t a i n i n g a p a t i t e s p r e p a r e d in a s i m i l a r w a y . F o r this r e a s o n , it is n o t c l e a r w h e t h e r t h e y a r e s i n g l e - p h a s e a p a t i t e s o r e p i t a c t i c a l m i x t u r e s o f m o r e t h a n o n e a p a t i t e . S i m i l a r s u g g e s t i o n s w e r e m a d e f o r the min- e r a l o f t o o t h e n a m e l b y D r i e s s e n s [21] a n d D r i e s - s e n s a n d V e r b e e c k [4]. T h i s s t u d y s u p p o r t s t h e h y p o t h e s i s t h a t d e n t a l e n a m e l m i n e r a l c o n t a i n s not o n l y N a a n d CO3 but a l s o M g i o n s w i t h i n its a p a t i t e - l i k e s t r u c t u r e . H o w - e v e r , it d o e s n o t d i s t i n g u i s h b e t w e e n t h e p o s - sibilities t h a t t h e m i n e r a l c o n s i s t s e i t h e r o f a single p h a s e o r o f 2 o r m o r e a p a t i t e p h a s e s in an e p i t a c t i c a l m i x t u r e .
Acknowledgment.
This study was supported in part by NIH/ NIDR Grant number DE 05510.References
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