Dual constant composition studies of phase transformation of dicalcium phosphate dihydrate into octacalcium phosphate

Dual Constant Composition Studies of Phase Transformation of Dicalcium Phosphate Dihydrate into Octacalcium Phosphate JINGWU ZHANG, ARMAN EBRAHIMPOUR, AND GEORGE H. NANCOLLAS 1 Departments of Chemistry and Biomaterials, State University of New York at Buffalo, Buffalo, New York 14214 Received September 12, 1991; accepted November 22, 1991 The nucleation kinetics of octacalcium phosphate (OCP) in a suspension o f dissolving dicalcium phosphate dihydrate (DCPD) crystals has been investigated using the dual constant composition (DCC) technique at pH 7.00-7.40 and 37°C. The rate of DCPD dissolution is directly proportional to the relative undersaturation and the small magnitude of the rate constant suggests that desorption of the lattice ions from the surface is the primary rate-determining step. The rate of heterogeneous OCP nucleation on DCPD crystal surfaces is strongly dependent upon the solution pH and the nature of the supporting electrolytes. Magnesium ion markedly prolongs the induction period for OCP nucleation at concentrations above 1 × 10 _4 mol/liter, while a comparable inhibition is achieved with a zinc ion concentration as low as 1 × 10 -6 tool/liter. At pH 7, DCPD appears to be the first precipitating crystalline phase in the slightly supersaturated solutions. © 1992AcademicPress,Inc.

INTRODUCTION

pected to precede that of OCP according to the Ostwald rule of stages (7), most free-drift Detailed investigations of the precipitation in vitro studies fail to detect DCPD during the events in calcium phosphate solutions and the transformation from ACP to OCP (8). The interactions between solid phases have many involvement of DCPD has been suggested in important biological and industrial applicathe forming of embryonic chick and bovine tions ranging from water treatment to the bones and the developing of dentin (9), alstudy of vertebrate hard tissues such as bones though this has been disputed (10). With reland teeth ( 1-3 ). A number of calcium phosatively rapid growth and dissolution kinetics, phate phases such as amorphous calcium compared with the other calcium phosphate phosphate (ACP), dicalcium phosphate dihydrate (DCPD, CaHPO4.2H20), and octacal- phases, DCPD will probably be important in cium phosphate (OCP, Ca4H (PO4)3.2.5H20) governing the composition of the surrounding have been proposed as precursors to the for- solution. It is therefore of interest to study the mation of the thermodynamically most stable development of more basic calcium phosphate phase, hydroxyapatite (HAP, Ca5 (OH)- phases in the DCPD suspensions. The phase transformation from DCPD to (PO4)3), under physiological conditions (4). OCP and subsequently to more basic phases In neutral and acidic solutions, DCPD has has previously been investigated using a pobeen found to be the first crystalline phase to tentiostatic method at pH values maintained precipitate with the additional possibility of by automated addition of base (11-14). Durthe formation of an unstable amorphous phase ing these reactions the precipitation driving as a precursor (5, 6). At physiological pH forces decrease due to the formation of a new (7.4), although the formation of DCPD is exphase. In light of the fact that the nature of the precipitating phase depends not only on l To whom correspondence should be addressed. the pH but also on the supersaturation (5), it 132 0021-9797/92 $5.00 Copyright© 1992by AcademicPress,Inc. All rightsof reproductionin any formreserved.

Journalof Colloidand InterfaceScience, Vol. 152,No. 1, August1992

DCC STUDIES OF PHASE TRANSFORMATION

is important to study the phase transformation kinetics at constant solution composition. It has been suggested by a number of investigators that transformation between calcium phosphate phases is solution-mediated (6, 12). Thus we can regard the transformation as a combination of dissolution of the original phase and crystallization of the new one. In this study, the application of the highly reproducible constant composition (CC) method (15 ) has been extended to study the kinetics of these phase transformations. This was achieved by using two CC devices incorporating two different sensors to simultaneously control each of these reactions. The DCC method not only provides accurate kinetics data but also enables relatively large amounts of material to be dissolved or grown for physical chemical characterization. The applicability of this novel approach to studying simultaneous processes has been examined elsewhere (16). In the present work the kinetics of dissolution of DCPD and OCP nucleation at pH values of 7.00 and 7.40 was investigated. The influence of magnesium and zinc ions and of supporting electrolyte on the OCP nucleation was also studied. MATERIALS AND METHODS Reagent grade chemicals (Fisher Scientific ) and deionized, distilled, CO2-free water (DDW), were used to prepare stock solutions, which were filtered (0.22-~tm Millipore filters) twice before use. Calcium chloride and nitrate solutions were standardized by EDTA titration. Monobasic potassium phosphate, potassium nitrate, and sodium chloride salts were dried in v a c u o at 110°C. Stock solutions of magnesium and zinc chloride were standardized by exchanging the cations on an ion exchange column (Dowex-50, in the hydrogen ion form) and potentiometrically titrating the liberated acid against standard potassium hydroxide solutions. The latter were prepared from washed KOH pellets using DDW in a nitrogen atmosphere and standardized by ti-

133

tration against potassium hydrogen phthalate. Solutions were stored and dispensed under nitrogen in sealed buret units (Brinkmann Instruments). DCPD crystals, prepared according to the method developed previously (16), had a specific surface area of 1.0 +_ 0.1 m2/g (30 / 70 N2/He, BET, Quantasorb, Quantachrome). The DCC method and its applicability in studying simultaneous reactions have been discussed in detail elsewhere (16). Briefly, two potentiometric titrators (Metrohm Impulsomat 614, Multi-Dosimat 645, Dosigraph 625, and Potentiostat 605, Brinkmann Instruments) were utilized in conjunction with two different ion-selective electrodes to control the addition of suitable titrants for each of the reactions. In the present experiments, a calcium electrode (Orion series 93) and a glass electrode (Corning 476022 ), both coupled with a Br6nsted-type Ag/AgC1 reference electrode, were used to trigger the addition oftitrants for DCPD dissolution and OCP crystallization, respectively. A single titrant solution containing HCI and NaC1 (or HNO3 and KNO3 depending upon the supporting electrolyte in the working solution) was used for DCPD dissolution, while two titrant solutions were used for OCP nucleation, one containing KOH and K H 2 P O 4 and the other CaC12 and NaC1 (or Ca(NO3)2 and KNO3). The calculation oftitrant concentrations has been described in Ref. (16) and the effective concentrations for OCP crystallization, CeocP, are given in Table III. For the experiments involving additives, the same concentration of additives was included in the titrants (CeocP = 0.0200 M) to avoid dilution effects. Working solutions (200 ml) were prepared in a double-walled jacketed vessel thermostatted at 37.0 + 0.1°C. At least two standard buffer solutions (17), covering the required pH range, were used to calibrate the pH electrode. The solutions were maintained under a nitrogen atmosphere, saturated with water vapor at 37°C, in order to avoid contamination by atmospheric carbon dioxide. After iniJournal o/'Colloid and Inter~bee Scie;we, Vol. 152, No. 1, August 1992

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ZHANG, EBRAHIMPOUR, AND NANCOLLAS

tiation o f the reactions by the introduction o f 0.040 g o f dry D C P D crystals, the concentration o f each ionic species in the solution was maintained constant by the a u t o m a t e d addition o f suitable titrants. The solution was agitated by means o f a magnetic stirrer at 450 r p m to ensure a u n i f o r m suspension o f solids. During each experiment, calcium and phosphate concentrations in filtered aliquots were determined periodically in order to verify the constancy o f the solution composition (18). The solid samples were also examined by Xray powder diffraction, scanning electron microscopy, and chemical analysis. CALCULATIONS The relative supersaturation with respect to D C P D and O C P crystals is given by Eqs. [1] and [ 2 ], respectively (4, 19 ),

TABLE I Summation of Equilibrium Constants Equilibrium

Constant

Ref.

H2PO4 H3PO4 = H + + HPO42HPO42- = H+ + pO3H20 = H ÷ + OH Ca2+ + OH- = CaOH+ Ca2+ + HPO42- = CaHPO4 Ca2+ + PO43-= CaPO~ Caz+ + H2PO4 = CaH2PO~ Mg2+ + OH- = MgOH+ Mg2+ + HzPO2 = MgH2PO~ Mg2+ +.HPO4z = MgHPO4 Zn2+ + OH- = Zn(OH) + Zn2+ + H2PO4 = ZnH2PO~ Zn2+ + HPO2 = ZnHPO4

6 . 2 2 X 10 -3

(34) (35) (36) (37) (25) (25)

H 3 P O 4 = H + I-

6.59 × 10-s 6.6 × 10-~3 2.40 M 10-14 25 591 1.35 X 106 27.9 380a 27.9b 741 1.08 X 105 40a 2.0 X 103 ~

(25)

(25) (38) (25) (39) (40) (41) (41)

a Value at 25°C: b Approximated by the formation constant of the CaH2PO~ ion pair.

O'DCPD

ubility for the present experimental conditions with the available ion-pair formation constants crocP= [ ( C a 2 + ) 4 ( H + ) (24, 25). Moreover, it should be noted that )< (PO43-)3/KocP]l/8 _ 1, [2] Eq. [ 3] was originally proposed to calculate the m e a n activity coefficients for single elecwhere KDcpD and KocP are the solubility prod- trolytes at ionic strengths below 0.1 tool/liter. ucts o f D C P D and O C P respectively, and the For the mixed electrolytes and relatively high r o u n d brackets denote the activities o f the en- ionic strength ( I = 0.1 - 0.15 m o l / l i t e r ) as closed species. The ionic activity coefficients encountered in the present work, the accuracy were estimated using the semiempirical Davies o f Eq. [ 3 ] was limited. Therefore, the appropriate D C P D solubility values were deterequation ( 2 0 ) : m i n e d experimentally by preparing calcium phosphate solutions at various concentrations Igyi = - A z i 1 + - - -Ii / 2 0.3I [3] and potentiometrically monitoring the p H and calcium concentrations following the introIn Eq. [3], Yi is the activity coefficient o f an duction o f D C P D seed crystals during the enion o f charge zi, the constant A is taken as tire period preceding O C P nucleation. At p H 0.5231 at 37°C (21), and I is the ionic 7.00 it was verified that solubility equilibrium strength. I o n speciation calculations were had been achieved in the NaC1 m e d i u m since made using expressions for mass balance along the solution was stable for at least 1 week with the equilibrium constants for ion-pair and without the precipitation o f a m o r e basic complex formation, s u m m a r i z e d in Table I. phase. Using Eq. [3] and the equilibrium conThe solubility p r o d u c t o f O C P was that o f stants in Table I, the conditional solubility Shyu et al. (22). The available literature KlXrPD product, K~CPD , was calculated and these revalue (23) did not yield a reliable D C P D sol- suits are included in Table II. The difference = [(Ca2+)(HPO42-)/KDcPD]l/2 _ 1

Journal of CoUoid and lnteff'ace Seiance. Vol. 152, No. 1, August 1992

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135

DCC STUDIES OF PHASE TRANSFORMATION TABLE II Solubility of DCPD Crystals under the Experimental Conditions

pH

I (M)

Medium

Tc~ = Tp (mM)

K ~ D (M 2)

7.00 7.00 7.40

0.100 0.150 0.150

KNO3 NaCI NaC1

1.85 2.09 1.95

2.24 X 10-7 2.37 X 10-7 2.64 X 10-7

between these values and those reported in the literature at different ionic strengths and pH ranges (23, 24) may be attributed to uncertainties in the estimation of activity coefficients. In the presence of magnesium and zinc ions, ion-pair formation with phosphate ions was taken into account in the speciation calculations (Table I). In the kinetics experiments, the phosphate concentrations were adjusted so as to yield driving forces identical to those in the absence of additives. RESULTS AND DISCUSSION Experiments were first conducted at physiological pH 7.40, ionic strength 0.15 mol/liter

in NaCI, in solutions undersaturated with respect to DCPD but supersaturated in OCP. DCPD dissolution commenced immediately upon the introduction of DCPD seed crystals and the addition of OCP growth titrants was initiated after the induction periods, ~ocP, listed in Table III. The newly forming calcium phosphate phase, possessing the typical OCP crystal morphology (26), appeared at first near the edges of the DCPD crystals (Fig. l a), which thereafter were completely covered as the reaction proceeded (Fig. l b). X-ray powder diffraction and the constancy of solution composition strongly suggested that this new phase was OCP. From the recorded volumes of added titrants, the amounts of OCP formed and DCPD dissolved were calculated and are plotted as a function of time in Fig. 2. It can be seen that the rate of DCPD dissolution began to decrease markedly following OCP nucleation. This may be attributed to the reduction in exposed DCPD surface as OCP overgrowth increased as observed in the scanning electron micrographs of solid samples withdrawn during the reactions. The induction periods for heterogeneous

TABLE III Rates of DCPD Dissolution and Induction Periods of OCP Nucleaction in the Absence of Additives, Seeded with 0.040 Grams of DCPD Crystals

(10 -3 M )

O'DCr,D

aOCP

(10 -s M)

RDCPD (10 -s mol s-' m -2)

"rocp (min)

1.70 1.80 1.92 1.95

-0.120 -0.072 -0.015 0

2.35 2.51 2.69 2.74

20.0 20.0 20.0 20.0

10.2 7.10 1.06 0

59 58 51 45

1.84 1.97 2.00 2.04 2.06 2.09

-0.112 -0.053 -0.040 -0.022 -0.013 0

1.60 1.75 1.78 1.83 1.85 1.88

10.0 20.0 10.0 10.0 20.0 20.0

13.8 8.97 5.70 2.45 1.65 0

1.66 1.77 1.85

-0.096 -0.041 0

1.58 1.72 1.82

1.66 1.77 1.85

10.6 5.57 0

Tc~ = Tp

pH

0.150 M NaC1 7.40 7.40 7.40 7.40 0.150 MNaC1 7.00 7.00 7.00 7.00 7.00 7.00 0.100 M K N O 3 7.00 7.00 7.00

Ceocv

35 20 15

Journal q/'Colloid and lnle([h~e Science, Vol. 152. No. 1, August 1992

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ZHANG, EBRAHIMPOUR, AND NANCOLLAS

FIG. 1. Scanning electron micrographs of OCP nucleation on DCPD surfaces at pH 7.40: (a) initial stage of OCP nucleation, (b) covered DCPD surface.

Journal ~fColloid and lnte#ace Science. Vol. 152,No, I, August1992

137

DCC STUDIES OF PHASE T R A N S F O R M A T I O N 00

~"

'°°1

o O. . . . o~O.o-

o~

j

-

300

'ca

Io

60

~o

z

i

o"° o

a

0( $7 ~ 0

i

60 Time

! ti

AA 30

~

~

t

zoo

_,_,'" 90

o 120

(rnin)

FIG. 2. Extent of OCP formation (solid symbols) and D C P D dissolution (open symbols) during DCC experiments at Tca = Tp = 1.70 X 10 -s mol/liter (circles) and 1.80 X 10 -s mol/liter (triangles), I = 0.150 tool/liter with NaC1 as the background electrolyte, and pH 7.40.

nucleation of OCP in DCPD suspensions were determined in solutions having equimolar total calcium (Tc,) and phosphate (Tp) concentrations, ranging from 1.70 X 10 -3 to 1.95 X 10 -3 mol/liter. The kinetics data for OCP growth at pH 7.40 are summarized in Table III and the extents of growth are plotted against time in Fig. 3. It can be seen that as the calcium and phosphate concentrations increased, the induction period for OCP nucleation decreased and the subsequent growth rate increased. In solutions with lower Tca and Tp, the overall growth rate of OCP was reduced, due not only to a lower driving force for OCP formation, but also to the decrease in the available DCPD surface. For example, at the lowest Tca and Tp studied, the onset of OCP nucleation occurred after 80% of DCPD crystals had been dissolved (Fig. 2). At even lower concentrations, it is probable that no OCP would have formed before the dissolution of DCPD crystals had been completed. In contrast to the short induction periods observed at pH 7.40, no DCPD transformation could be detected at pH 7.00 for as long as 1 week perhaps due to the lower driving force for OCP crystallization (Table III). In order to confirm this suggestion, further experiments were made at higher calcium and phosphate concentrations (Tc, = Tp = 2.20 X 10 3 mol/liter) to reduce the supersatura-

tion difference. However, spontaneous nucleation occurred with an induction period of less than 4 h. The precipitating phase, with Ca/ P molar ratio of 1.0, strongly resembled DCPD in morphology. Because of the relative instability of these solutions, the influence of pH on the OCP nucleation rate could not be investigated at a given driving force. However, the established pH dependence of growth and dissolution kinetics of various calcium phosphate phases suggested that the marked difference in the induction periods at pH values of 7.00 and 7.40 was primarily a consequence of acidity rather than small difference in thermodynamic driving forces (27-29). An advantage of the DCC method is that it also allows the dissolution rate of the initiating phase to be determined at constant driving forces. Values of the initial dissolution rates of DCPD in these experiments are summarized in Table III. In Fig. 4, the logarithmic plot of rate as a function of relative undersaturation yields an effective order of 0.9 + 0.2 and a rate constant, KD = 7.8 X 10 - 7 tool s -1 m 2. The order suggests that DCPD dissolution is controlled by volume diffusion and/or desorption of calcium and phosphate ions from the crystal surfaces. If the rate is controlled entirely by volume diffusion, the rate constant may be written DvCs KD = - - ,

[41

6

400 1,70raM A - - A 1.80mM o--o

~--o

~ o 300

o--o

N

~

1.92mM

l l - - l l 1.92mM 1.95mM

? ~

] /

A I

!

~

A

J

!:i! /

200

100

n~ 0

/ ;J

~

oO o - ,° ° °

30

60

,

90

,

_ ,

120

T i m e (rain)

FIG. 3. The extent of OCP growth as a function of time at the indicated different equimolar concentrations of calcium and phosphate; pH 7.40.

Journal o/Colloid and Inte~'/hce Science, VoL 152, No. 1, August 1992

138

ZHANG, EBRAHIMPOUR, AND NANCOLLAS

o:.:o

~ 10 o u

/ o ,

, I

f

,

,

,

0.01

,

,

, f

0, I O"OCPO

FIG. 4. Logarithmic plot of DCPD dissolution rate against relative undersaturation.

where Dv is the diffusion coefficient, C~ the solubility and 6 the diffusion layer thickness (19). Using Dv ~ 1 X 10-9 m2 s -1 andC~ (K~)cpD) 1/2 ~ 0.5 m o l m -3, the diffusion layer is calculated to be 6 × 10 -4 m , which is unreasonably large compared with the dimensions of D C P D crystals. Thus it appears that desorption provides the major resistance to D C P D dissolution. In order to avoid phase transformation, other investigations on the kinetics of D C P D dissolution were carried out at lower p H values (30-32). Since the dissolution rate has been found to be pH-dependent (33), it is difficult to make a direct comparison with the results of these studies. Since the observed prolonged induction periods for D C P D transformation in NaC1 solution at p H 7.00 (Table III) appear to contradict the results of a previous inyestigation using a pH-stat method in potassium nitrate solutions (12), DCC experiments were made using this supporting electrolyte. It can be seen in Table III that, in agreement with the previous study, the induction periods for OCP nucleation were markedly reduced and became even shorter than those at p H 7.40, using NaC1 as background electrolyte. X-ray powder diffraction and chemical analysis confirmed that the growing phase was again OCP. The results suggest that NaCI may have an inhibitory effect on OCP nucleation, as compared with KNO3. Journal q/Colloid and lnwtjbce Science.

Vol.

152, No.

1, A u g u s t

1992

The stability of D C P D in neutral NaC1 solution is in agreement with the results of Cheng (5), who was unable to observe D C P D transformation to OCP within 21 days under similar experimental conditions. However, in the absence of sodium ions, D C P D hydrolysis was found by Tung et al. (13) to commence within an hour at p H 7.40. The inhibitory effect of sodium ions on OCP nucleation m a y be due to their relatively strong adsorption at the sites for calcium ion integration on the periphery of the subcritical OCP nuclei. A current investigation in our laboratory also revealed that compared with potassium ions, sodium ions retard the growth of calcium sulfate dihydrate crystals. Thus, "inert" electrolytes m a y have considerable influence on the rates of processes occurring at crystal-solution interface. The effect of magnesium and zinc ions on the nucleation of OCP was also investigated in D C P D suspensions at p H 7.40 at an ionic strength of 0.150 mol / liter (NaC1). For these experiments, the compositions of the working solutions were so close to the D C P D saturation equilibrium that no titrants for D C P D dissolution or growth were consumed. Thus, it was sufficient to use only OCP growth titrants to maintain the constant solution composition.

400 -~

©~

I [

• OI I

I

me e--e

ioot! |

i

zx

<> /

01ram

/

l

1.0ram

A

0--00mM

0--,C.

i.

~

f

:

~

~

v

~

O__~__~__~-A'¢

0

/

C.

J

120

~_¢-0-,

240 Time

°-

360

,

¢

4BO

,

.

600

(min)

FIG. 5. Effect of magnesium on the kinetics of OCP crystallization in DCPD suspensions at pH 7.40, ionic strength = 0.150 M (NaC1): (O) T~g = 0 mM, Tca= 1.92 raM, Tp = 1.92 raM; (e) TMg= 0.100 raM, Tca = 1.92 mM, Tp = 1.94 raM; (A) TM~= 0.500 raM, Tc~ = 1.92 mM, Tp = 1.98 mM, (~) TMg= 1.00 raM, Tca 1.92 mM, Tv = 2.07 mM.

DCC STUDIES OF PHASE TRANSFORMATION 400

----

v

0

--

.~

-~-

o

~

200

~ ~t

~t A

,,°° ' / , i ; 120

240

360 Time

o 480

600

720

(rain)

FIG. 6. The extent of OCP crystallization as a function of time in the presence of different zinc ion concentrations: (0) 0 ~M, ( e ) 0.500 ~zm, (E3) 1.00 ~m, (:7) 1.50 ~m, (A) 5.00 t~m, (()) 10.0 um; Tc, = Tp = 1.95 raM, pH 7.40, and ionic strength = 0.150 M (NaC1), seeded with 0.040 g DCPD.

The extents of OCP mineralization in the presence of magnesium and zinc ions are plotted against time in Figs. 5 and 6, respectively. It can be seen that magnesium at concentrations less than about 1 × 10 -4 mol/liter had little influence either on the induction period or the subsequent OCP growth rates. However, the growth rate decreased markedly while the induction period increased from 50 to 200 rain in the presence of 1 X 10 -3 tool/liter magnesium ion. The inhibition of OCP nucleation on D C P D surfaces by magnesium ions was also reported by Perez et al. (12). Studies in the presence of zinc ions in a range of 0.50010.0 X 10-6 mol/liter indicated that although at the lowest concentration no significant change in the induction period or OCP growth rate was observed, the highest zinc concentration resulted in a significant reduction in the growth rate and an increase in the induction period from 45 to 450 rain. In comparison with magnesium ions, zinc ions have a much stronger inhibitory effect on OCP nucleation. ACKNOWLEDGMENT We thank the National Institute of Health (Grant DE03223), the Procter and Gamble Company, and the Industry-University Center for Biosurfaces for financial support of this work.

139

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ZHANG, EBRAHIMPOUR, AND NANCOLLAS

28. Salimi, M. H., Ph.D Dissertation, State University of New York at Buffalo, 1985. 29. Zhang, J., Ph.D Dissertation, State University of New York at Buffalo, 1990. 30. Nancollas, G. H., and Marshall, R. W., J. Dent. Res. 50, 1268 (1971). 31. Christoffersen, M. R., and Christoffersen, J., J. Cryst. Growth 87, 51 (1988). 32. Zhang, J., and Nancollas, G. H., submitted. 33. Zhang, J., and Nancollas, G. H., to be published. 34. Bates, R. G., J. Res. Natl. Bur. Stand 47, 2236 ( 1951 ). 35. Bates, R. G., and Acree, S. F., J. Res. Natl. Bur. Stand 30, 129 ( 1943); 34, 395 (1945).

Joltrnal qfColloid and lnlerta~v Science, Vol. 152, No. I, August 1992

36. Bjerrum, N., and Unmack, A., Kgl. Danske Videnskabernes Selskab. Mat-fys. Medd. 9, 1 (1929). 37. Harned, H. S., and Owen, B, B., "The Physical Chemistry of Electrolytic Solutions," 3rd Ed., p. 645. Reinhold, New York, 1958. 38. Stock, D. I., and Davies, C. W., Trans. Faraday Soc. 44, 856 (1948). 39. Tabor, H., and Hastings, A. B., J. Biol. Chem. 148, 627 (1943). 40. Perrin, D. D., J. Chem. Soc., Part IV, 4500 (1962). 41. Nriagu, J. O., Geochim. Cosmochim. Acta 37, 2357 (1973).