Influence of strontium ion on the crystallization of hydroxyapatite from


Chem. 1981,85, 2403-2408. 2403 concentrations of di-ferf-butyl peroxide, an H-atom ab- straction adduct, IV, could be detected (3X3 lines,. = Z-Bu—N...
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J. Phys. Chem. 1981, 85,2403-2408

concentrations of di-tert-butyl peroxide, an H-atom abstraction adduct, IV, could be detected (3 X 3 lines, U N = f-Bu-N-CH2-R

? IV

13.76 G, U H = 4.75 G). Finally, the photolysis of tert-butyl perbenzoate generated copious amounts of the phenyl spin adduct. The photolysis of tert-butyl hydroperoxide yielded radical 111, and large amounts of the methyl radical spin adduct, even in dry dimethyl sulfoxide (3 X 4 lines, uN = 15.75 G, U H = 12.25 G), probably by the reaction of the solvent with the hydroxyl radical17split off from the hydroperoxide. In the presence of a suitable dipeptide, a radical generated by H-atom abstraction could be detected. Thus, glycylglycine exhibited a 3 X 4 lines spectrum (15.50

G (1 N) X 2.25 G (1 N, 1 H of equal splittings)), and alanylalanine a triplet of three poorly resolved triplets spectrum (15.01 G (1 N) X 3.56 G (1 N)) suggestive of radicals at the C(a) position. Since tert-butoxy radical induces the decarboxylation of peptides as shown by ditert-butyl peroxide experiments (vide supra), it seems logical that the H-atom abstraction radicals generated with this peroxide were due to the photolytically produced hydroxyl radical.28 While the present study disclosed a new mode of action of some photoexcited peroxides, acting as electron acceptors with amino acids, preliminary results indicate the generality of this process for carboxylic acids and pyrimidine bases as well as for alcohols and sugars. (28)Joshi, A.; Rustgi, S.; Moss, H.; Riesz, P. Int. J.Radiat. Biol.Rel. Stud. Phys., Chem. Med. 1978,33,205.

Influence of Strontium Ion on the Crystallization of Hydroxylapatite from Aqueous Solution P. G. Koutsoukos and G. H. Nancollas’ Chemistry Department, State [email protected] of New York at Buffalo, Buffalo, New York 14214 (Received:January 6, 1981)

The growth of hydroxylapatite seed crystals has been studied at very low supersaturation in the presence of strontium ions by using a constant solution concentration method. The Sr2+ion is incorporated into the apatite lattice to yield solid phases with considerably lower Sr/Ca molar ratios than those in the supersaturated solutions. Changes in the unit cell lattice parameters and infrared spectra are linearly related to the strontium content of the precipitated solid phases. The presence of strontium in the crystallizing solutions also markedly reduces the size of the developing Ca-Sr-hydroxylapatite crystallites.

Introduction In recent years the role of strontium in biological systems has received considerable attention. In contrast to the variable levels of this element in bones, the Y3r deposited in teeth varies little in concentration.l Experiments in rats have shown that the lack of strontium in the diet causes a high incidence of dental caries and poor growth conditions.2 Addition of strontium to the diet tends to reverse these changes but results in rickets accompanied by stimulation of the activity of periosteal and endosteal cells and by interference with the calcification mechan i s m ~ . ~More . ~ recently, a cariostatic activity of strontium has been r e p ~ r t e d , ~and - ~ it has been suggested that strontium has a beneficial action in the mineralization of depleted bones in persons suffering from disturbances of bone m e t a b ~ l i s m . ~ (1)G.N.Jenkins, “The Physiology and Biochemistry of the Mouth”, 4th ed., Blackwell, Oxford, 1978,p 80. (2)0. Rygh, Bull. SOC.Chim. Biol., 31, 1052 (194). (3)P.C. Shipley, E. A. Park, E. V. McCollun, S. N. Simmond, and E. M. Kinney, Bull. Johns Hopkins Hosp., 33, 216 (1922). (4)J. C.Bartley and E. F. Reber, J. Nutr., 75, 21 (1961). (5)M.E.J. Curzon, B. L. Adkins, B. G . Bibby, and F. L. Losee, J. Dent. Res., 49,526 (1970). (6)I. Gedalia, J. Annaise, and E. Latner, J.Dent. Res., 54, 1240 (1975). (7)C. Meyerowitz, M. F. Little, and M. E. J. Curzon, J.Dent. Res., 5SB, 126 (1976). (8)P. C. Spector and M. E. J. Curzon, J. Dent. Res., 57, 55 (1978).

It is believed that strontium is incorporated into the lattice of hydroxylapatite (Cas(P04)30H,HAP) because of the similarity of its ionic radius (1.12 A) with that of calcium (0.99 A).loJ1 A complete series of solid solutions of strontium with HAP was prepared by Collins12 and Hayek and Petter13 with unit cell lattice parameters linearly dependent on the extent of strontium incorporation into the apatite lattice. The molar Sr/Ca ratios in the solid, however, were smaller than those in the solution phase. These results were confirmed by the synthesis of a continuous series of solid solutions of strontium and HAP by reaction in the solid state.14 It was suggested that there was a slight preference of strontium for the 6-fold axis and that the solutions behaved ideally. At higher strontium contents, however, there was evidence for another phase coexisting with the solid solution of Ca-Sr-apatite. Hitherto, the influence of strontium on the precipitation of calcium phosphate has been studied at relatively high supersaturation where other phases may be formed and redissolved during the overall precipitation reaction. Under such conditions, it is difficult to distinguish between ~~~~

(9)E.Shorr and A. C. Carter, Bull. Hosp. J. Dis.,13,59 (1952). (10)J. C. Elliot, Clin. Orthop., 93,313 (1973). (11)R. A. Young, Collop. Int. C.N.R.S., No. 230,21 (1973). (12)R. L. Collin, J. A m . Chem. Soc., 81,5275 (1959). (13)E. Hayek and H. Petter, Monatsch. Chem., 91,356 (1960). (14)H. J. M. Heijligers, F. C. M. Driessens, and R. M. H. Verbeeck, Calcif. Tissue Int., 29,127 (1979).

0022-3654/81/2085-2403$01.25/00 1981 American Chemical Society

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Koutsoukos and Nancollas

1

1.01

5.01

L

DCPD

\ 10 o/

1

110 30

J 4.0

50

PH

60

70

80

9 0

Figure 1. Plots of log ( Tc, X Tp)against pH calculated for the system CaCI,-KH,PO,-KOH at 37 OC.

the influence of the added alkaline earth ion on the formation of each of the calcium phosphate phases which, in order of increasing solubility, are HAP, 0-tricalcium phosphate (/3-(Ca3(P04)2,TCP), octacalcium phosphate (Ca8Hz(P04)6-5Hz0, OCP), and dicalcium phosphate dihydrate (CaHP04.2Hz0,DCPD). The solubility isotherms, plotted as the logarithm of the product of total calcium, Tca,and total phosphate, Tp,concentration as a function of pH are shown in Figure 1. I t can be seen that at pH 7.40 and relatively high supersaturation (A, Figure 1) DCPD may be formed which rapidly hydrolyzes to OCP and thence to HAP.15 At slightly lower supersaturation (B, Figure 1) the formation of OCP has also been suggested as a precursor phase from seeded growth experiments on the basis of measurements of the Tea/ Tpmolar ratio during the course of the precipitation reaction.16 However, as discussed previously," the relatively small changes in concentrations during the experiments make it difficult to achieve the analytical precision required to differentiate between the different phases forming and redissolving during the crystallization reactions in which the calcium and phosphate concentrations decrease appreciably. Since it has been suggested that the interaction of strontium with bone is similar to that with synthetic HAP,lsJs the present work is concerned with the influence of strontium on the direct growth of HAP from aqueous solutions at physiological pH. The formation of other precursor phases is excluded by working at low, constant supersaturation (point C, Figure l),thus eliminating the transient phase problems associated with reactions at high supersaturations.2021 Not only the influence of strontium (15)P.Koutsoukos,Z.Amjad, and G. H. Nancollas, J.Dent. Res., 58A, 167 (1979). (16)G.H.Nancollas and B. Tomazic,J. Phys. Chem., 78,2218(1974). (17)G.H.Nanco1las;J. Dent. Res., 58B,861 (1979). (18)N. S.McDonald, F. Ezmirlian, P. Spain, and C. McArthur, J.Biol. Chem., 189, 387 (1951). (19)W. E. Neuman, B. Bjonerstedt, and B. J. Mulryan, Arch. Biochem. Biophys. 101,215 (1963). (20)M. B. Tomson and G. H. Nancollas, Science, 200, 1059 (1978). (21)P.Koutsoukos,Z.Amjad, M. B. Tomson, and G . H. Nancollas, J. Am. Chem. Soc., 102,1553 (1980).

ion on the rate of HAP formation has been studied, but also the degree to which this ion is incorporated into the apatite lattice during the crystal growth reactions. Experimental Section Grade A glassware and analytical reagent grade chemicals were used. Calcium chloride stock solutions, prepared from the dihydrate salt (J. T. Baker, Co.) recrystallized from distilled water, were standardized by passing aliquots through a cation exchange resin (Dowex-50W-X8)in the hydrogen form and titrating the eluted acid with standard potassium hydroxide. Solutions of ultrapure potassium dihydrogen phosphate (J. T. Baker Co., Ultrex) were standardized potentiometrically by titration with dilute potassium hydroxide. HAP seed crystals were prepared by the method of Nancollas and MohanZ2using calcium chloride and potassium dihydrogen phosphate with potassium hydroxide for the control of pH. They were characterized by infrared spectroscopy (Perkin-Elmer grating infrared spectrometer Model 467), X-ray powder diffraction (Phillips XRG-3000 X-ray diffractomer, Cu KCY radiation Ni filter, 1" incoming and 4' outgoing slits, scan rate (1/4)(28) deg min-l), specific surface area (SSA) determination (single point BET method using a 30:70 mixture of H e N z ; Quantasorb, Quantachrome, Greenvale, NY) and by scanning electron microscopy, SEM (IS1 scanning electron microscope, Model Super 11). Infrared spectra and the powder diffraction data were in excellent agreement with published values for stoichiometric Chemical analysis of the solid gave a molar ratio of Ca/P = 1.67 f 0.02; the SSA was 36.1 m2 g-l. The crystallization experiments were made at 37 "C in a water-thermostated, double-walled vessel in metastable supersaturated solutions of calcium phosphate. Nitrogen gas, presaturated with water vapor at 37 "C, was bubbled through the solution which was stirred with a Tefloncoated magnetic stirring bar at 300 rpm. The solutions were brought to the desired pH by the addition of 0.10 M COz-freepotassium hydroxide (J. T. Baker, Dilut-it). The pH of the solutions was measured by a glass/saturated calomel electrode pair, standardized before and after each experiment with NBS standard buffer solutions.25 The stability of the supersaturated solutions was verified by the constancy of the pH for a period of at least 4 h. Following the introduction of HAP seed, crystallization started immediately and a decrease in pH of as little as 0.003, resulting from the formation of HAP, triggered the addition of titrants from three mechanically coupled automatic burets controlled by a pH-stat (Metrohm-Herisau, Model 3D,combititrator). The three titrants consisted of calcium chloride, potassium phosphate + potassium hydroxide, and strontium chloride. Their concentrations, dictated by the stoichiometry of the precipitating phase, were as follows: CaC12, C1 + xTc,; KH2PO4, C2 + xTp; KOH, C3 2Cz xC4; and SrCIz, xTsr,where C1:C2:C3 = 5:3:1. x is the number of burets used ( x = 3), C4 the concentration of base required to bring the pH of the solution to the desired value, and Ts,the total strontium concentration. The values of C1, C2,and C3 were determined from a series of preliminary experiments. Samples were withdrawn from time to time and filtered (0.22-c~m filters, Millipore, Bedford, MA), and the filtrates analyzed for total calcium and total phosphate by a combined

+

+

(22)G. H.Nancollas and M. S. Mohan, Arch. Oral. Biol., 15, 731 (1970). (23)K. B. Baddiel and E. E. Berry, Spectrochim. Acta, 22, 1407 (1966). (24)ASTM, X-ray powder diffraction file No. 9-77. (25)R.G.Bates, "pH Determination", Wiley, New York, 1973.

The Journal of Physical Chemistty, Vol. 85, No. 16, 198 1 2405

Sr Ion and Crystallization of Hydroxylapatite

TABLE I: Strontium-Phosphate Ion-Pair Formation Constants equilibrium Si'* + SrZt+ Sr2++ SrZt+

H,PO,- 2 SrH,PO,' HP0,'- Z SrHPO,' P o d 3 -Z SrP0,OH- Z SrOH'

K , dm3 mol-' 1.59 16.22

1.51 X 10" 7.24

ref 49 49 49 50

I

2

02

I

04

1

I

OS

0 8

I

IO

1

12

I

(Sr'Ca).olu+lon

Flgure 3. Plot of the Sr/Ca ratio in the Precipitated HAP in the presence of Sr2+ ions as a function of the SrlCa ratio in the crystallization solution. Flgure 2. Crystallization of HAP in the presence of strontium: pH 7.40, 37 'C. (Numbers on the curves refer to experiments in Table 11.)

spectrophotometric method26and for strontium by atomic absorption spectrophotometry (Perkin-Elmer atomic absorption spectrophotometer Model 503). The solid phases were also separated at known times and examined by infrared and powder X-ray diffraction spectroscopy, SSA, SEM, and chemical analysis following dissolution in 0.1 M hydrochloric acid.

Results and Discussion The activities of the ionic species in solutions were calculated by successive approximations for the ionic strength, from phosphate protonation and calcium phosphate ion-pair equilibrium constants together with massbalance and electroneutrality expressions, as described previ~usly.~' In addition, complex formation between strontium and phosphate ions was taken into account by using the data summarized in Table I. HAP crystallization experiments in the presence of strontium, summarized in Table 11, were made at a supersaturation corresponding to point C (experiment 110) in Figure 1. Although the solutions were supersaturated with respect to both TCP and HAP, the formation of the former phase has never been convincingly demonstrated at ambient temperatures.% In addition, X-ray powder diffraction examination of the solid phase precipitated in experiments 250 and 251 (Table 11) confirmed the absence of Sr3(P0.J2even though the solutions were supersaturated with respect to this phase. The marked reduction in the rate of crystallization of HAP at low supersaturation in the presence of Sr is shown in Figure 2. The results are in agreement with those for calcium phosphate precipitation from highly supersaturated solutions.m In Table I11 it can be seen that, during (26)M. B.Tomson, J. P. Barone, and G. H. Nancollas, At. Absorpt. Newsl., 16, 117 (1977). (27) G. H.Nancollas, Z. Amjad, and P. Koutsoukas in "Chemical Modeling in Aqueous Systems", E. A. Jenne, Ed., American Chemical Society, Washington, DC, 1979. (28) R. W. Mooney and M. A. Aia, Chem. Reo., 61,433 (1961).

the experiments, the calcium concentrations in solutions increased while that of the strontium ion decreased. The effect was more marked at higher strontium concentrations, and the deviation from constancy of calcium phosphate supersaturation was reflected in the decrease rate of HAP crystallization shown in Figure 2 (experiments 249-251). It is interesting to note that the decrease in strontium concentration was always accompanied by an increase in calcium concentration, indicating that strontium ions were incorporated into the HAP lattice. The phosphate concentration remained almost constant during the experiments. The chemical analyses of the solids sampled during the crystal growth experiments, summarized in Table IV, indicate that the Ca/P molar ratio decreased from 1.66 to 1.48 at a Tsr of 7.0 X mol dm-3. I t can be seen in Table IV and Figure 3 that the precipitated solids had a considerably lower Sr/Ca ratio than that in the solution phase. This is in agreement with earlier suggestion^'^^^^^^' of discrimination between calcium and strontium so that the composition of the solution was continuously changing as crystallization proceeded. X-ray powder diffraction studies have shown that both a. and co axes increase with increasing strontium incorporation in the ~ o l i d . ' ~ JSuch ~ an effect cannot be accounted for by surface a d ~ o r p t i o nand , ~ ~it~ has ~ ~ been suggested'* that strontium shows a preference for the 6-fold position in the HAP lattice. The proportionality between unit cell lattice parameters and the strontium content of the solid, shown in Figure 4, suggests that solid solutions are formed during crystallization reactions rather than discrete calcium-strontium apatites. The straight lines in Figure 4 are drawn between the values corresponding to pure HAP and pure strontium apatite (ao= 9.760 A, co = 7.284 A).12 Baud and Very3 measured the unit cell lattice (29)B. N. Bachra, 0. R. Trautz, and S. L. Simon, Arch. Oral. Biol., 10.731 (1965). '(30)R.C. Likins, H. G. McCann, A. S. Posner, and D. B. Scott, J. Biol. Chem., 235, 2152 (1960). (31)H.C. Hodge, E. Gavett, and J. Thomas, J. Biol. Chem., 163, 1 f194fil. ~ - ._,. (32)C. Lagergren and D. Calstrom, Acta Chem. Scand., 11,545(1957). (33)C. A. Baud and J. M. Very, Colloq. Int. C.N.R.S., No.230, 405 (1973).

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TABLE 11: Crystallization of HAP o n HAP Seed Crystals in the Presence of S i Z +Ion a t yH 7.40 and 37 "Ca

110 248 249 250 251 titrants:

a

0.00 7.97 1.70 0.04 1.00 8.12 1.74 0.05 3.00 8.72 1.77 0.07 5.00 9.32 1.79 0.09 7.00 9.92 1.82 0.11 (1)CaCl,, 6.60 X M ( 2 ) KH,PO,, 3.96 x 10-3 M ; KOH, 7.92 ( 3 ) SrCl,, 3 X Ts,M

TCa= 5.50

mol dmF3,Tp = 3.30 X

X

-2.32 -2.25 -2.22 -2.20 -2.18

-6.44 -6.12 -6.09 -6.08 -6.06

6.10 3.32 2.06 1.24

5.39 2.60 1.34 0.52

8.28 7.13 4.31 2.85 2.24

11.78 3.41 - 0.39 -2.81

x 10-3 M mol dm'3, [KCl] = 5.50

mol dm-j.

X

TABLE 111: HAP Crystallization in t h e Presence of Strontium a t pH 7.4 and 37 "C sampling time, min

lO4Tc,, mol dm-3

104Tp, mol dm-3

104Ts,, mol dm-3

growth,' %

0 120 420

Experiment 248 5.500 3.300 1.000 3.240 0.983 5.500 5.523 3.280 0.966

0.0 20.1 47.1

0 105 410

Experiment 249 5.500 3.300 3.000 3.180 2.800 5.850 5.865 3.250 2.820

0.0 12.2 32.6

0 210 925 1890

5.500 5.287 5.845 5.980

Experiment 250 3.300 5.000 3.280 4.689 3.295 4.512 3.268 4.520

0.0 13.4 33.4 52.3

0 255 2270 3655

Experiment 251 3.300 7.000 5.500 6.574 5 082 3.302 6.132 5.822 3.182 6.128 5.885 3.100

0.0 14.8 62.1 111.0

a Percentage growth expressed as % of original amount of seed.

parameters of the inorganic component of the bones of mice that had been fed a strontium-rich diet and showed that the higher doses of strontium resulted in an increase in the a. axis. These results, together with those for experiments 248-251 (Table II), are plotted in Figure 5, from which it can be seen that bone mineral and synthetic HAP behaved similarly in their incorporation of strontium ions. The exclusion of calcium ions from the HAP lattice results in changes in the crystal field environment acting upon the phosphate ions. Extensive infrared studies on calcium-deficient apatites3440have shown that the spectroscopic changes result from calcium ions missing from the HAP lattice or replacement of these ions by others of similar size. Examination of the solids precipitated in experiments 248-251 revealed a band at 870 cm-', which increased as the calcium content of the solid decreased. The change in u,, of this band as a function of the molar (34)E. E. Berry, J.h o g . Nucl. Chem., 29, 1585 (1967). (35)J. A. S.Bett, L. G. Christner, and W. K. Hall, J.Am. Chen. SOC., 89,5535 (1967). (36)L.Winand, M.J. Dallemagne, and G. Duyckaerts, Nature (London), 190,164 (1961). (37)E. E. Berry, J. Inorg. Nucl. Chem., 29, 317 (1967). (38)S.J. Joris and C. H. Amberg, J. Phys. Chem., 76, 3172 (1971). (39)S.J. Joris and C. H. Amberg, J. Phys. Chem., 75, 3167 (1971). (40)L. Winand and G. Duyeckaerts, Bull. SOC.Chim. Belg., 71,142 (1962).

,a'

0 05-

Aa.

(a)

/ I

O//

t

,o/ /

/ I

0'04

/

P/

0.031

I0

I

2

I

I

e

4

I

8

I

10

X Sr

Figure 5. Plot of the change, Aao, of the a. unit cell lattice parameter of HAP as a function at the strontium incorporations: (0)ref 33; (A) present work.

Ca/P ratio in the solid is shown in Figure 6. A similar trend was observed by other workers.%,@By analogy with

The Journal of Physical Chemistry, Vola85, No. 10, 1981 2407

Sr Ion and Crystallization of Hydroxylapatite

TABLE IV: Cnemical and X-ray Analysis of HAP Precipitated in the Presence of Strontium IO2(Sr)/(Ca) ( Sr2 )/( CaZ ) expt no. Sr % solid a,, '4 co, a solid soln solid-phase stoichiometry +

110 248 249 250 251

9.419 9.422 9.428 9.435 9.445

0.0 2.0 5.5 6.0 11.2

0.000 2.040 5.930 6.380 12.610

6.880 6.885 6.892 6.902 6.913

+

0.000 0.182 0.546 0.909 1.272

Ca5(PO4)3OH Ca4.9Sr0,I (PO,),OH Ca4.72 srO. 28 ("4 ),OH Ca4.7 Sr0.$(Po4),OH Ca4.44Sr0. 56("4) ,OH

1

06

t

4000

Ca/ P

Flgure 6. Influence of the Ca/P molar ratio on the intensity of the P-OH band in the apatites precipitated in experiments 248-251 (Table 11): (A) data from ref 39; (0)present work.

=500

*boo

'

800

I

400

F r e q u e n c y (crn-'1

Figure 7. Infrared spectra of stoichiometric HAP and the apatites precipitated in experiments 248-251 (Table 11).

other phosphates containing HP042-groups, the 870-cm-l band was assigned to the v5(ul) symmetrical stretching vibration of the P-O(H) g r o ~ p s . ~As~ can ? ~ be ~ seen in Figure 6, for Ca/P ratios > 1.5, the slope of the linear plot of absorbance at ,,v as a function of Ca/P was approximately half that for Ca/P < 1.5. This is in agreement with the quantitative studies of Berry,43who showed that for Ca/P < 1.5 twice as many HP0:- groups were formed per calcium defect as for Ca/P > 1.5. Incorporation of strontium into the HAP lattice also had a marked effect on the infrared band attributed to the motion of the lattice hydroxyl groups. The broad absorption band in the 3800-3000-~m-~range with a highintensity peak at 3400 cm-' is attributed to adsorbed water. In Figure 7 it can be seen that the peak due to the -OH group was shifted by the incorporation of the larger strontium ion in the apatite lattice. The intensity of the 3400-cm-l peak also increased with decreasing Ca/P ratio, an effect probably due to the small size of the crystalline particles. The infrared band at 630 cm-' attributed to the librational mode of the -OH group in HAPu was shifted to lower frequency in the strontium-containing apatite. In this phase, the larger lattice constants allow the -OH groups to librate more readily, and the hydrogen bonding of oxygen atoms to neighboring phosphate groups is also reduced.41 Figure 8 shows that the frequencies assigned ~

~

(41)E.E.Berrv and K. B. Baddiel. SDectrochirn. Acta. Part A . 23, 1781 (1967). (42) A. C. Chaoman. D. A. Lone. and D. T. L. Jones. Smctrochirn. Acta, Part A , 21,633(1965). (43) E. E. Berry, Bull. SOC.Chirn. Fr., 1765 (1968). (44)J. J. Stutman, J. D. Termine, and A. S. Posner, Trans. N . Y. Acad. Sci., 27,669 (1965). I

Figure 8. Change of relative frequencies of infrared absorption bands with increasing incorporation of Sr2+ ions in the HAP lattice: (A) ref 38; (0)present work.

to the phosphate group vibrations (vp and v4) were also influenced by the strontium substitution for calcium, the change being linearly related to the strontium content of the solid. This dependence is indicative of the domi-

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Kou1soukos and Nancoilas

TABLE V : Size of HAP Crystals Grown in Solutions Containing Strontium av

expt

particle

method of measurement

lo'[Sr], mol dm-'

Coulter counter 0.902 0.00 Coulter counter 3.603 0.00 0.835 1.00 calcda calcd" 0.918 3.00 calcd0.908 5.00 calcda 0.612 7.00 Calculations based upon broadening of X-ray powder diffraction peaks. seed

110 248 249 250 251

Figure 9. Scanning electron micrographs of HAP grown 00 HAP seed crystais (100% growth with respect to wlginal seed). I O O O O X magnification.

Figure IO. Scanning electron micrograph of HAP grown on HAP in %hapresence of 7 X ' 0 1 M sr?+ (111% gowh with [email protected] to cfighal

tected not only microscopically but also from the hroadening of the peaks of the powder X-ray diffraction spectra. Sizes were calculated by Scherrer's method46from the broadening of the (002) reflection using a highly crystalline HAP sample as standard. The results are summarized in Table V, and it can be seen in Figures 9 and 10 that, in the presence of strontium, the HAP crystallites which normally grew as needles with increasing length4?showed a much smaller increase in size (experiment 251) for the same amount of crystallization. The results of the present work indicate that, in the presence of strontium, the crystallization of HAP from metastable calcium phosphate solutions of low supersaturation results in the creation of cationic vacancies in the lattice and in significant incorporation of the strontium ions. Solid solutions of calciun-strontium apatite are formed, and the dependence of strontium content upon the Sr/Ca ratio in solution suggests a method for preparing solids of desired calcium and strontium contents by using the constant solution composition method. This method has recently been successfully used in order to achieve a strontium substitution in DCPD of as much as 20?J0.~ Acknowledgment We thank the National Institute of Dental Research of the National Institute of Health for Grant DE03223 in support of this work.

seed).

nanting influence which the nearest neighbors exert on the vibrations of the phosphate groups." The presence of strontium in the crystallizing solutions also influenced the size of crystals formed. This was de(45)

w.E. Klee and G. Engel, J. Inorg. Nucl. Chem., 32, 1837 (1970).

(46) H. P. Klug and L. E. Alexander, "X-rayDiffraction hoeedures", Wifey, New York, 1970. (47) P. Koutsoukos and G. H. Nancollas, J. [email protected] Growth, in press. (48) H. Hohl and G. H. Naneollas, unpublished data. (49) H. Guept, 0. Giibeli, and G. Schwarzenbsch,Xelu. Chim. Acta, 45, 1171 (1962). (50) H. S. Harned and T. R. Paaton, J. Pkys. Chem., 57,531 (1953).