Effects of sodium fluoride, potassium fluoride and ammonium fluoride solutions on the hydrolysis of CaHPO4 at 37.4°C

j. . . . . . . .

CRYSTAL G R O W T H

ELSEVIER

Journal of Crystal Growth 183 (1998)417 426

Effects of sodium fluoride, potassium fluoride and ammonium fluoride solutions on the hydrolysis of CaHPO4 at 37.4°C R.I. Martin, P.W. Brown* Intereolh, ge Materials Research Laboralot3", Pennxvh~ania State Uni~ersi(v, Unirersi O, Park, PA 16802, USA

Received 13 March 1997; accepted 9 July 1997

Abstract The variations in pH during CaHPO4 hydrolysis in water and in 18.75 600raM NaF, K F and N H a F solutions were determined at a 20.0 liquid-to-solids weight ratio and 37.4C and the equilibrium solution chemistry and morphology were determined after 3 months. Hydrolysis kinetics were rapid in all solutions containing fluoride. The pH variations demonstrate complex kinetic behavior. The cations with fluoride alter the reaction mechanisms over the range of concentrations studied. CaHPOa dissolution changes from incongruent to congruent as the concentrations of cation-phosphate complexes increased regardless of whether the cation is Na, K or NH4. When hydrolysis is carried out in fluoride concentrations below 75 mM, CaHPO¢ and fluoroapatite coexist at equilibrium. The fluoroapatite formed discrete needles on the surface of the CaHPO,~. The density of fluoroapatite needles on the CaHPO4 surfaces and edges was not uniform. Vluoroapatite was the only phase present when hydrolysis was carried out in 75 mM fluoride. At fluoride concentrations above 75 mM, fluoroapatite and CaF2 coexist at equilibrium. Thus, the evolution of phases when CaHPO~ is hydrolyzed in increasing fluoride concentrations is CaHPO4 + fluoroapatite, fluoroapatite and fluoroapatite + CaF2. The fluoroapatite and CaF2 coexist as components of pseudomorphs of the original CaHPOa crystallites. Pseudomorphs of the original CaHPO4 crystallites were the only morphology observed when CaHPO4 was hydrolyzed in 600 mM fluoride. ,~', 1998 Elsevier Science B.V. All rights reserved. K e v w o r d s : CaHPO4; Hydrolysis; Kinetics; Fluoroapatite; lncongruency

1. Introduction The hydrolysis of dicalcium phosphate, C a H P O 4 , in w a t e r forms calcium-deficient hyd r o x y a p a t i t e ( C D H A p ) , C a 9 H P O ~ ( P O 4 ) 5 O H , by

* Corresponding author.

its i n c o n g r u e n t dissolution [1]. C a H P O 4 is in e q u i l i b r i u m with calcium-deficient h y d r o x y a p a tite. Stoichiometric hydroxyapatite (SHAp), Calo(PO4)6(OH)2 is the o t h e r terminal c o m p o s i tion. In the absence of c o m m o n ions 01- neutral salts (the t e r n a r y P 2 O s - C a O H 2 0 system [2]) C D H A p readily o v e r g r o w s the surfaces of h y d r o l y z i n g C a H P O ~ crystallites a n d subsequent C a H P O a

0022-0248/98/S19.00 ,d 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 4 2 0 - X

418

R.L Martin, P. {K Brown / Journal q[" Crystal Growth 183 (1998) 417 426

dissolution becomes diffusionally controlled [1]. The dissolution kinetics of CaHPO4" 2H20, which has solubility characteristics similar to CaHPO4, have been thoroughly characterized by Zhang and Nancollas [3]. There are numerous investigations [4 7] describing the role of volume diffusion and surface processes in controlling C a H P O ~ ' 2 H 2 0 dissolution under conditions where it appears to be congruent. Because of solubility similarities, CaHPO4 dissolution may also become congruent. When CaHPO4 is hydrolyzed in NaOH and CaCI2 solutions introduced for controlling pH and ionic strength, the crystal size decreases with the degree of calcium deficiency in HAp and decreasing equilibrium pH [8]. Under these solution conditions, the formation of sodium phosphate species minimizes or eliminates incongruency of CaHPO4 dissolution [9]. Thus, solution factors may dominate the mechanism by which CaHPO4 converts to hydroxyapatite [8]. Monovalent cations in the soluble fluoride salts NaF, KF and NH4F may also effect the mechanism of CaHPO4 hydrolysis and the composition and crystal growth morphology of the apatite formed. These factors are assessed in this study. Sodium [10] and potassium [11] ions may incorporate into the apatite whereas ammonium ions are likely excluded by size limitations. In this paper, we consider whether potassium and ammonium are similar to sodium in eliminating the incongruency of CaHPO4 dissolution. Fluoride adds further variability to the composition of apatite where fluoride substitution for hydroxyl achieves a complete solid-solution series [12]. The solid-solution series is defined by Cal0(PO4)6(OH)f2_2y)(F)2y , where y varies from 0 to 1. Calcium deficiency and fluoride substitution occur in the same homogeneous apatite phase. Hence, the variable apatite composition may be described using two parametric variables: Ca10 ~-(HPO4)x(PO4-)6 :,(OH)I1 -~.)I2-x)(F)2y, where x and y vary from 0 to 1. This formula describes a surface in the quaternary phase space (PzOs-CaO-CaF2 H20 system). Fluoride-substituted apatite layers growing on the surfaces of the CaHPO4 crystallites may exhibit characteristics different from that of a CDHAp diffusion barrier. The crystallinity of fluoride-substituted HAp formed at 80'C initially decreases

with initial substitution and then increases as the composition approaches FAp [13]. However, it has not been established if this effect occurs at 37.4cC. HAp formed at 37.4~'C is poorly crystalline compared to that formed at higher temperatures and fluoride substitution increases crystallinity (P. Leamy, P.W. Brown, K.S. TenHuisen, and C. Randall, unpublished observation). The corresponding variation in surface area of the fluoride-substituted CDHAp growing on the CaHPO4 surfaces may alter its diffusion characteristics. The solubility of fluoride-substituted SHAp formed at 22'C [14] goes through a maximum [15] and then decreases below the solubility of SHAp as the composition approaches stoichiometric ftuoroapatite (SFAp), Calo(PO,~)6(F)2. At the calciumdeficient limit, Ca/P ~ 1.5, Okazaki et al. [13] found the solubility of fluoride-substituted CDHAp formed at a 4.0pH value and 25:C decreased in proportion to the logarithm of the fluoride content in agreement with Larsen's findings [15]. Equilibrium between CaHPO,~ and fluoridesubstituted CDHAp is then characterized by a decreasing pH and Ca/PO~ ratio in the solution. The driving force for CaHPO~ hydrolysis and the degree of CaHPO~ incongruency are also affected by the decreased solubility of the fluoride-substituted CDHAp. This was observed for CaHPO4 hydrolysis in NaF solutions (Martin and Brown, unpublished observation) and can be attributed to the formation of soluble sodium phosphate species. As with the sodium ion, potassium and ammonium ions form soluble phosphates, thus, it is an objective of this paper to establish the generality of the effect of soluble phosphate complex formation on the mechanism of Call PO4 dissolution. When CaHPO4 is hydrolyzed in fluoride solution of sufficient concentration, CaF2 is also formed. It is the final objective of this paper to establish whether the concentrations at which CaF2 coprecipitation is detected depends on the cation associated with fluoride. The work of van den Hoek et al. [12] established a modified Doerner-Hoskins equation that accurately describes the crystal-growth stage of the overall fluoride-substituted SHAp formation at fluoride concentrations below 0.25 mM in their chemical system. Above 0.25 mM, the three-way ratio of

R.L Martin, P. ~ Brown / Journal of OTstal G~vwth 183 (1998) 417--426

calcium, phosphate and fluoride may approach conditions where the solution also becomes saturated with respect to CaF2 and causes the system to loose a degree of freedom. Coprecipitation of CaF2 and a partially calcium-deficient FAp will result and crystal growth will then appear to proceed in a manner opposite to that predicted by the modified Doerner Hoskins equation. Such a solution condition can readily be achieved since SFAp and most of the fluoride-substituted SHAp solidsolution series dissolves incongruently with respect to CaFz [16]. If coprecipitation is considered in the model, the FAp crystal growth may continue above a 0.25 m M fluoride concentration. It is the final objective of this paper to establish the concentrations at which CaF 2 coprecipitation is detected.

2. Materials and methods

2.1. Synthesis of CaHPO~ Ca(OH)2 was synthesized from CaCO3 (Fisher Scientific Co. reagent grade). Deionized water was heated to a boil and taken off the hot plate (i.e. this avoids bumping). C a O was immediately added. C a O was prepared by calcining CaCO3 at 1000°C for 1 h. After the boiling reaction had calmed the Ca(OH)2 was dried for two days at 120°C. C a H P O 4 was precipitated from an acid-base reaction of H3PO~ and Ca(OH)2 at 85~C. Phosphoric acid H3PO4 was obtained as 85.7 wt% solution (Fisher Scientific Co. reagent grade). A 500 ml volume of deionized water was acidified with phosphoric acid to a pH value of approximately 3.5 and divided into two equal parts. One part was poured into a round bottom flask and 27.23 g of finely divided Ca(OH)2 were added and dispersed while heated to 85°C. The other part was combined with 41.83 g of 85.7 wt% H3PO4 in a bottle mounted above a 50 ml burette. The acid solution was siphoned into the burette and the stop-clock adjusted for acid addition into the stirring Ca(OH)2 slurry over approximately 2 h at 85"~C. The acid was added at a slower rate during the second hour. The C a H P O ~ slurry which formed was continuously stirred for 16 h at 85°C to ensure complete reaction. The C a H P O 4 was separated from the liquid by

419

filtration using a # 2 W h a t m a n filter paper in a Biichner funnel. The powder was dried for 2 days at 102°C in a ventilated oven.

2.2. CaHP04 particle-size analysis The particle volume distribution of the C a H P O 4 was determined by a forward-scatter technique by a Microtrac small particle analyzer. The particles were mildly sonicated in a dilute sodium hexametaphosphate solution and circulated through the analyzer. Data were collected for 60 s in triplicate and averaged together.

2.3. CaHP04 hydrolysis A 25 ml capacity jacketed reaction vessel was held at 37.4~'C by a RM6 Brinkman circulating bath. A 20 ml volume of water or fluoride solution was poured into the vessel and covered with a plastic lid which also functioned as a holder for a gas inlet tube and a pH electrode. The tolerance of the lid permitted gas to escape the vessel. The pH electrode was calibrated with 4.01 and 7.00 pH buffers before each hydrolysis. The solution was stirred by a magnetic stir bar and equilibrated with a nitrogen atmosphere and 37.4°C. Hydrolysis initiated when C a H P O 4 particles were added to the solution. All hydrolysis reactions were carried out at a 20.0 liquid-to-solids ratio. Every 3 s thereafter, a pH datum was transferred from an Orion 920 pH meter to a 4.7 M H z D O S computer for 24 h. Subsequently, the solution and powder were transferred to 40 ml septum bottles, placed in a water bath at 37.4°C and allowed to equilibrate for at least 3 months.

2.4. Solution chemistry analysis The supernatants from these equilibrated samples were analyzed to determine their pH values and the concentrations of calcium, phosphate, sodium and fluoride. 10 cc of supernatant was extracted from each slurry by decanting and filtering the solution through Gelman syringe filters, 13 m m in diameter with 0 . 2 g m pores (Acrodisc 13 CR PTFE). Atomic emission spectroscopy (AES) was used to determine the cation concentrations and

420

R.I. Marlin. P.W. Brown : Journal of Crystal Growlh 183 (1998] 417 426

ion chromatography (IC) was used to determine the anion concentrations.

pH '

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2.5. Solids" characlerization

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//----

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The solids harvested from the slurries were washed in acetone in a Bfichner funnel on Whatman # 42 filter paper. The powder was packed into a recessed glass slide. The glass slide was mounted on a Philips or Scintag X-ray diffraction system. The diffractometers were run at 2 / m i n between 20' and 35 ~' 2 0 with a continuous step size of 0.02 20. The X-ray source was a copper target X-ray tube operated at 40 KV and 35 mA (Philips) or 45 KV and 4 0 m A (Scintag) which produced a 1.54059 wavelength k~ radiation.

I

0.6 M NH4F

~

0 45 M

.---0.15 M

0.075 M

2. 6. Microstructural analyses 1

The morphologies of the C a H P O 4 and the hydrolysis product(s) were observed in the first stage of a dual-stage ISI-DS130 scanning electron microscope (SEM). Each sample was thinly dispersed on the surface of a brass sample holder with acetone, air dried and then gold-coated for 2.5 rain in a glow discharge SEM specimen coater at an approximate 1 inch deposition distance with 900 V. The accelerating voltage used on the SEM was 9 kV.

3. Results

and

discussion

The pH variations during C a H P O ¢ hydrolysis in water and in NH4F solutions having concentrations to 0.6 M are shown in Fig. 1. Hydrolysis in water at L : S = 20 does not rapidly result in equilibrium (Martin and Brown, unpublished observations); the pH does not reach 4.3, the value at the C a H P O 4 - C D H A p invariant point at 37.4'C. If complete hydrolysis occurred, the reaction would be H:O

9CaHPO4 + H 2 0 --+ CagHPO4(PO4)5OH q- 3 H 3 P O 4 i a q ~.

(1)

When hydrolysis is carried out in N H 4 F solutions having concentrations to 0.075 M, the pH

2

3

4

5

Hours Fig. 1. C a H P O a hydrolysis ill a m m o n i m n fluoride solutions at 37.4 C.

attained reaches lower values. Equilibration, as indicated by no fnrther apparent pH change, is reached within approximately 411. The solution chemistry data permit the analysis of the mechanistic path taken as C a H P O 4 is hydrolyzed in N H 4 F solutions. In this analysis it is assumed that stoichiometric FAp forms. However, Refs. [13, 15] have shown that the formation of calcium-deficient FAp cannot be precluded. Complete dissolution of C a H P O ~ at a L/S = 20 would produce a solution having calcium and phosphate concentrations of 0.368 M. If the following reaction producing FAp is assumed: 11~0

10CaHPO4 + 2NH~F

--+ C a l o ( P O 4 ) 6 F

2

+ 2(N H~,H 2PO,,),,q + 2H 3 PO41~,q).

(2)

10 tool of C a H P O 4 react with 2 tool of NH4F to produce FAp. This will occur in 0.074 mol NH4F solution. Thus, if reacting C a H P O ~ and N H 4 F at this molar ratio produces only FAp, C a H P O 4 hydrolysis will produce the highest concentration of protons and should result in a pH minimum. As

R.I. Martin, P.I,K Brown/Journal ol 03wtal Growth 183 (1998) 417 426

Fig. 1 shows, C a H P O ~ hydrolysis in 0.075 M N H 4 F results in a pH minimum of approximately 3.15. X-ray diffraction analyses confirm the absence of C a H P O 4 indicating its complete hydrolysis. Xray diffraction also confirms the presence of FAp. CaF2 is not observed. Thus, Eq. (2), the solution chemistry data, and the X-ray diffraction analyses are consistent in indicating the pH minimum observed occurs when CaHPO+ hydrolysis is complete and when FAp is the only solid product. Although X-ray diffraction analysis is not tile optimal method for detecting CaF2 or for observing small proportions of phases, X-ray diffraction analyses consistently confirm solution chemistry analyses. Hydrolysis of C a H P O 4 (at a L : S of 20) in higher concentrations of NH+F will produce CaF2 as a second phase. As Fig. 1 shows, this results in the pH increasing again. For example, when the molar ratio of C a H P O 4 to NH4F is 11 : 4, one mole of FAp and one mole of CaF2 form at complete reaction. In this case H~O

l l C a H P O , t + 4NH4_F ~ Calo(PO~,)6F2 + CaF2

421

bH

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Hours Fig. 2. CaHPO4 hydrolysis in potassium fluoride solutions at 37.4 C.

+ 4(NH:,H2PO4aq) ~H

q- H3PO4Iaq ).

(3)

Calculation indicates this should occur in 0.133 M NH~F solution; X-ray diffraction analysis indicates the presence of CaF2 when hydrolysis is carried out at this NH4F concentration. The pH of the solution would be higher than that obtained as a result of Eq. (2) because only a single mole of H3PO¢I~,q) is formed. This analysis is also in accord with the results in Fig. 1 which indicate an equilibrium pH near 3.6 is attained. When C a H P O ~ hydrolysis is carried out in N H 4 F solution having a molar ratio higher than 11 : 4, there is a continuing reduction in the concentrations of H3PO41aq) produced and the proportion of CaF 2 produced increases. For example, when CaHPO,~ hydrolysis is carried out in N H a F solution in which the C a H P O 4 : N H 4 F molar ratio is 2 : l, the following reaction occurs: tl:O

12CaHPO,, + 6 N H 4 F --+ Calo(PO,,)~,F2 + 2CaF2 + 6(NH4_H2PO.,Lq.

(4)

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0 NaF •

,

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I 0.4

KF

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, 0.6

Fig. 3. The variations in pH when CaHPO4 hydrolyzed in various NaF. KF and NH4F solutions at L / S - 20 and 37.4 C.

R.1. Martin, P. gvi Brown/Journal o/'Co,stal Growth 183 (1998) 417-426

422

CaHPO~ hydrolysis in solutions more highly concentrated in NH4F should result in further increases in pH as dibasic ammonium phosphate forms. This is illustrated in Eqs. (5) and (6) for

In this instance the NH4F concentration is 0.184 M. The pH attained at this NH4F concentration should be higher than those observed at low molar ratios because n o "H3PO4{aq)" are generated.

m M (Ca) 35

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Fig. 4. The e q u i l i b r i u m c o n c e n t r a t i o n s of(a) calcium, (b) fluoride salt cation, (c) p h o s p h a t e and (d) fluoride from sodium, p o t a s s i u m and a m m o n i u m fluoride sources after 3 months.

423

R.1. Martin, P.W. Brown/' Journal of C~'stal Growth 183 (1998) 417 426

hydrolysis at C a H P O 4 : NH4F molar ratios of 13 : 8 and 14 : 10, respectively.

CPS '

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H20

13CaHPO4 + 8NH4F ~ Calo(PO,~)6F2 + 3CaF2 + 6(NH4H2PO4aq) + ((NH4)zHPO4)aq.

(5)

for hydrolysis in 0,226 M NH4F solution, and H,O

14CaHPO4 + 10NHgF ~ Calo(POg)6F2 + 4CaF2 + 6(NH4H2PO4)aq + (2(NH4)2HPO4)aq. (6) for hydrolysis in 0.262 M NH4F solution. Comparison of Eqs. (5) and (6) indicates the ratio of dibasic ammonium phosphate to monobasic ammonium phosphate increase, suggesting a commensurate increase in pH. The data in Fig. 1 show the equilibrium pH to continuously increase as the NH4F concentration increases from 0.15 M to 0.3 M and beyond. CaHPO4 hydrolyses in K F solutions with initial concentrations ranging to 0.6 M effect the pH as shown in Fig. 2. The trends are very similar to those observed in Fig. 1, CaHPO4 hydrolysis in N a F solutions (Martin and Brown, unpublished observations) also results in pH variations similar to those observed in NH4F and K F solutions. Fig. 3 shows the equilibrium pH values after 3 months incubation at 37.4 C. The equilibrium pH values remain at minima when the initial fluoride concentration is near 0.075 M. These pH values are 2.79, 2.75 and 2.92 for hydrolysis in NaF, K F and NH4F solutions, respectively. Thus, there are slow reductions in the pH occurring between 4 h and 3 months. The equilibrium pH values rise to approximately 6.25 at initial fluoride concentrations of 0.3 M and then to approximately 7.25 for initial fluoride concentrations of 0.6 M. The curves for each fluoride source nearly overlay. The equilibrium concentrations of calcium, phosphate, fluoride and cation are shown in Fig. 4. The calcium concentrations approach maxima when the initial fluoride concentrations are 0.075 M (Fig. 4a). The calcium concentrations at these maxima are 31.5 mM after hydrolysis in K F

37.5 mM

18.75mM

J~ I

25

27

29

31 T w o Theta

33

35

Fig. 5. X-ray diffraction patterns of the solid(s) present after hydrolyzedin 18.75 600 mM NH~F solutions for 3 months at 37.4 C.

C a H P O 4 was

and N H , F solutions and 23.7 mM after hydrolysis in NaF solution. For hydrolysis in fluoride solutions having concentration of 0.3 M and above, the calcium concentrations fall below the limits of detection. The concentrations of the Na, K, and NH4 are shown in Fig. 4b. They increase linearly. The

424

R.L Martin. P. ~1~ Brown/Jottrna/o/C1ystal Growth 183 (1998) 417 426

Fig. 6. (a) The morphology of apatite formed in 18.75 m M K F solution on the CaHPO., SHI'['accs at a 20 liquid-to-solids ratio and 37.4 C. (hi The morphology of apatite formed in 75 m M N a F solution. (c) The morphology of apatite formcd in 75 m M NH,~F solution at a 20 liquid-to-solids ratio and 37.4 C. (d) The pseudomorphs composcd of apatite and CaF2 formed in 0.6 M K F or NH4F solution from the original C a H P O 4 morphology at a 20 liquid-to-solids ratio and 37.4 C. (e) The detailed morphology of the apatite and CaF2 at the surfaces of the pseudomorphs.

R.L Martin. P. g{ Brown / Journal o! Co,stal Growth 183 (1998) 417 426

phosphate concentrations rise rapidly to approximately 155 mM in solutions with initial fluoride concentrations of 0.075 M regardless of the fluoride source, Fig. 4c. Above this initial fluoride concentration, the phosphate concentration continues to increase but more slowly. Equilibrium fluoride concentrations are near the detection limit for initial F concentrations below 15 mM, Fig. 4d. As these figures show, Ca rises as the initial fluoride concentration rises, reaching a peak at about 0.075 M when CaHPO4 has been consumed and FAp is the only solid. As CaE2 forms, the Ca concentrations in the solutions decrease, eventually falling below the limit of detection. Phosphate also rises with Ca over the range of concentrations where Call PO~ is converting to EAp. Breaks in the curves are seen when the initial fluoride concentrations are about 0.075 M. Call PO4 hydrolysis in higher initial concentrations results in the formation of both FAp and C a F 2 and the phosphate concentration increases in response to the increasing proportion of CaF2 present (Fig. 5). CaHPO+ hydrolysis in 0.6 M NaF solution resuits in a fluoride concentration of 126 mM after equilibration for 3 months. Hydrolysis in 0.6 M KF results in a fluoride concentration of 79 mM. Hydrolysis in 0.6 M NH¢E results in fluoride concentration of only 61 mM. Thus, the fluoride concentrations shows a strong dependence on the cation. As Fig. 4c illustrates, the phosphate concentrations show only a mild dependence on the cation being 245, 239 and 262mM for NaF, KF and NHaF, respectively. Because CaF 2 is formed in each instance (Fig. 5), calcium ion concentrations remain below the limit of detection. Significantly, Fig. 4b shows that essentially all of the ammonium ion is retained in solution. There are, however, minor reductions in the Na and K concentrations. It is difficult to quantitatively account for the extensive differences in fluoride uptake other than to suggest cation-dependent differences in adsorption phenomena. Fluoride is well known to adsorb to apatite. Extensive fluoride adsorption was observed in our prior work when apatite was formed by reaction between C a H P O a and Ca4(PO4)20 El7]. CaHPO~ hydrolysis in 18.75 mM NaF, KF or NH~F solutions produces a dense layer of apatite

425

crystallites on the surfaces of the CaHPO4 crystallites. At this concentration, the cation associated with fluoride did not effect the crystallite size. Fig. 6a shows these crystallites when the cation was potassium. The lengths of most crystallites are 0.2 0.3 ~tm and their thicknesses are approximately 0.1 btm or less. Most of the crystallites appear to have formed by discrete nucleation events and are not present as aggregates of crystallites. When the initial fluoride solution is 75 mM, the Ca/F ratio is approximately 10 : 2 and apatite is the only crystalline-phase identified by X-ray diffraction analysis. The apatite crystallites form as discrete entities as shown in Fig. 6b and Fig. 6c. The crystallite size is similar to that of the apatite formed on the CaHPO~ surfaces at lower fluoride concentrations. Fig. 6b shows the apatite morphology when the cation associated with fluoride was sodium and Fig. 6c shows the apatite morphology when potassium or ammonium is the cation. The apatite morphology is more prismatic and there are six distinct hexagonal sides to the apatite rods when the cation is potassium or ammonium. When the initial fluoride concentration is increased to 0.6 M, the morphology is pseudomorphs of the original CaHPO4 morphology as shown generally in Fig. 6d and in detail in Fig. 6e. The pseudomorphs have cobblestone surfaces with no signs of morphologies typical of fluoroapatite or CaVe. Pseudomorph formation was not dependent on the cation associated with fluoride. The CaHPO4 was completely hydrolyzed and apatite and CaE 2 were the only crystalline phases identified by X-ray diffraction analysis. Crushing the pseudomorphs revealed a solid structure throughout. The pseudomorphs are composed of tightly packed granules slightly larger inside than the size of the cobblestone effect on the surface. Crushing had no effect on the X-ray diffraction patterns indicating the absence of a Call PO# core.

4. Conclusions Unlike hydrolysis in water {Martin and Brown, unpublished observation), CaHPO4 rapidly hydrolyzes in fluoride solutions. The fluoride source (NaF, KF or NH4E) does not significantly effect

426

R.L Martin, P. ~ Brown/Journal of Cm,stal Growth 183 (1998) 417 426

hydrolysis kinetics. H y d r o l y z i n g C a H P O 4 in the presence of cations which form soluble p h o s p h a t e complexes a n d fluoride decrease the i n c o n g r u e n c y of its dissolution. If the fluoride c o n c e n t r a t i o n is sufficient, the m e c h a n i s m of C a H P O 4 dissolution becomes congruent. H y d r o l y s i s in solutions having fluoride concent r a t i o n s below 75 m M , f l u o r o a p a t i t e crystallites form on the surfaces of C a H P O ~ crystallites but are ineffective in isolating t h e m from the solution. Hydrolysis in 0.075 M fluoride solutions results in the c o m p l e t e c o n v e r s i o n of C a H P O 4 to F A p . H y d r o l y sis in m o r e c o n c e n t r a t e d solutions results in increasing p r o p o r t i o n s of CaF2. The m o r p h o l o g y of the F A p which forms does show a d e p e n d e n c e on the c a t i o n associated with the fluoride. Thus, the extent to which fluoride remains in solution when hydrolysis is carried out in c o n c e n t r a t e d solutions m a y be related to F A p m o r p h o l o g y . W h e n hydrolysis is carried out in solutions having fluoride c o n c e n t r a t i o n s a b o v e 0.3 M, p s e u d o m o r p h s of the C a H P O 4 crystals, which were c o m p o s e d of C a F 2 a n d fluoroapatite, form.

Acknowledgements The a u t h o r s gratefully a c k n o w l e d g e the s u p p o r t of N I D R G r a n t DE09421.

References [1] P.W. Brown, J. Am. Ceram. Soc. 75 (1) (1992) 17. [2] R.I. Martin. P.W. Brown, J. Am. Ceram. Soc. 80 (1997) 12630. [3] J. Zhang, G.H. Nancollas, J. Crystal Growth 125 (1992) 251. [4] G.H. Nancollas, R.W. Marshall, J. Dent. Res. 50 (1971} 1268. [5] J. Christoffersen, M.R. Christoffersen, J. Crystal Growth 87 (1988) 41. [6] J. Christoffersen, M.R. Christoffersen, J. Crystal Growth 87 (1988) 51. [7] J. Zhang, G.H. Nancollas, J. Dent. Res. 66 i1987} 219. [8] K. Ishikawa, E.D. Eanes, J. Dent. Res. 72 (2) i1993) 474. [9] M.J. Fulmer, P.W. Brown, J. Biomed. Mater. Res. 27 (1993) 1095. [10] R.Z. LeGeros, in: P.W. Brown, B. Constantz (Eds.), Hydroxyapatite and Related Materials, CRC, Boca Raton, 1994, p. 6. [11] E.J. Duff, J. Chem. Soc. A (1971) 33. [12] W.G.M. van den Hock, T.P. Feenstra, P.L. de Bruyn, J. Phys. Chem. 84 (1980) 3312. [13] M. Okazaki, T. Aoba, Y. Doi, J. Takahashi, Y. Moriwaki, J. Dent. Res. 60 (4) (1981} 845. [14] M.J. Larsen, Arch. Oral. Biol. 31 (1986) 757. [151 M.J. Larsen, J. Dent. Res. 69 (special issue) (1990) 575. [16] T.D. Faro G. Tarbutton, H.T. Lewis Jr., J. Phys. Chem. 66 (1962) 315. [17] M.J. Fulmer, P.W. Brown, J. Am. Ceram. Soc. 75 (1992~ 3401.