Langmuir 1991, 7, 511-583
517
Induced Crystal Growth of Calcium Oxalate Monohydrate at Hydroxyapatite Surfaces. The Influence of Human Serum Albumin, Citrate, and Magnesium A. Ebrahimpour, L. Perez, and G . H. Nancollas' Departments of Chemistry and Biomaterials, State University of New York at Buffalo, Buffalo, New York 14214 Received March 7, 1990. I n Final Form: M a y 7, 1990 The growth of calcium oxalate monohydrate (COM)crystals on hydroxyapatite (HAP) surfaces has been investigated by using the constant composition method. The effect of human serum albumin (HSA), citrate, and magnesium ions on the COM overgrowth was studied. HSA promoted the growth of COM crystals on HAP surfaces while citrate and magnesium acted as inhibitors. Although COM crystals grown on HAP displayed the same morphology as those grown in the absence of additives, the addition of citrate ions resulted in a change in the morphology of the overgrown COM.
Introduction Epitaxy, or the oriented overgrowth of one crystalline phase upon another, is believed to play an important role in the formation of bones and teeth as well as in pathological processes such as the formation of stones in the urinary tract.' In many urinary stones, there is a relatively abrupt layer to layer change in composition involving mineral phases such as the calcium phosphates and calcium oxalates as well as the brganic macromolecular components.2 On the basis of the degree of crystal lattice misfit between the mineral layers, Lonsdale3 suggested that these urinary stones were formed by the epitaxial growth of one phase upon another. Although most urines are normally supersaturated in calcium oxalate but undersaturated with respect to other stone-forming minerals, the concentration of calcium phosphate and uric acid may fluctuate after the ingestion of food between undersaturation and supersaturation several times a day. In the case of calcium oxalate, in vitro experiments have shown that homogeneous nucleation does not occur until a relative supersaturation greater than 70 has been a ~ h i e v e d .Since ~ the relative supersaturation of calcium oxalate in mammalian voided urine does not exceed 30, it has been suggested that its formation may take place by heterogeneous nucleation on a suitable substrate surface that offers an acceptable epitaxial relationship. In many calcium oxalate urinary stones calcium phosphate phases are found at the center, and since good lattice matches exist between these crystalline phases, as shown in Table I, it is quite possible that the epitaxial growth of calcium oxalate monohydrate (COM) on hydroxyapatite (HAP) takes place during stone formation. Potential inhibitors of this process may therefore adsorb on the HAP substrate surface and prevent the nucleation of the second mineral phase (COM). Although Koutsoukos et al.5 and Baumann et a1.6 have demonstrated the possibility of COM growth on HAP, the (1) Schneider, H. G. Advances in Epitaxy and Endotaxy; Schneider, H. G., Ruth, V., Ed.; VEB Deutscher Verlag fur Grundstoffindustrie:
Leipzig, 1971; p 14. (2) Ciffuentes Delatte, L. Composicion y estructura de los calculos renales; Salvat: Barcelona, 1984; p 171. (3) Lonsdale, K. Nature 1968, 215, 56. (4) Finlayson, B. Kidney Int. 1978, 13, 344. (5) Koutsoukos, P. G.; Sheehan, M. E.; Nancollas, G. H. Invest. Urol. 1981, 28, 358. (6) Baumann, J. M.; Ackermann, D.; Affolter, B. U r d . Res. 1989,17, 153.
0743-7463/91/2401-0517$02.50/0
Table I. Crystallographic Data for COM-HAP Epitaxial crystal crystal COM
system
monoclinic
System usual
space group C;,,
habits {lOO){OlO} {OCjl)(OO~} 1111111211
unit cell lattice parameters a0 = 6.24 A bo = 14.58 A Cn = 9.89 A
{1122)~2081) a = p = 90" (3142) y = 120" Linear Net Dimensions for the (010) Faces of COM and HAP COM: 6.24 A X 9.46 A (90') HAP: 6.88 A X 9.419 A (90") percentage misfit: 10% and 0.441 respectively in the two orthogonal directions, a0 and bo
surfaces were insufficiently well characterized to confirm COM nucleation and overgrowth as distinct from homogeneous nucleation of this phase. In the present work the possible epitaxial growth of COM on HAP has been investigated by using the constant composition seeding technique. The growing crystals were investigated by scanning electron microscopy and energy-dispersive X-ray analysis. The influence of some of the species present in urines, namely human serum albumin (HSA),citrate, and magnesium ions upon this process has also been studied.
Materials and Methods Solutions were prepared by using reagent grade calcium chloride dihydrate, potassium oxalate monohydrate, dibasic calcium phosphate, sodium citrate, magnesium chloride (Fisher Scientific Co.), and triple-distilled carbon dioxide free water. Solutionswere standardizedby exchanging the respective cations for hydrogen ions on a Dowex-50ion exchange resin (50-20mesh) and titrating the liberated acid with standard potassium hydroxide solution. HAP seed crystals were prepared at boiling temperature while under reflux in a nitrogen atmosphere. HAP (seed A) was prepared according to the method of Bell et al.' by the slow mixing of 1.0 L of 5 X 10-3 mol L-* calcium hydroxide solution with an equal volume of 2.1 X 10-3 mol L-'dibasiccalcium phosphate solution in a 5-L flask followed by refluxing for an additional 48 h. Seed A suspensionwas cooled to 37 "C, adjusted to pH = 7.00, and aged for 6 months. Thereafter the solid was filtered (0.22 pm Millipore), dried in vacuo at 110 "C, and stored (7) Bell, L. C.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1973, 42, 250.
0 1991 American Chemical Society
578 Langmuir, Vol. 7, No. 3, 1991
Figure 1. Scanning electron micrograph of HAP (seed B) crystals.
Figure 2. Transmission electron micrograph of HAP (seed B) crystals, 80 OOOX magnification a t 100 kV. at room temperature. HAP (seed B) was prepared by the dropwise addition of 2.0 L of a solution 0.15 mol L-l in diammonium hydrogen phosphate and 1.5 mol L-l in ammonium hydroxide into 7.0 L of a solution 0.08 mol L-l in calcium nitrate and 0.2 mol L-l in ammonium hydroxide. Followingthe addition, which was completed in 5 h with the solution reaching a pH of 10,0.060 L of 7.4 mol L-l ammonium hydroxide solution was added and the suspension was refluxed for 12 h. The solid was filtered (Watman no. 1 filter), washed with 2 L of triple-distilled water, and aged in a saturated solution of HAP a t pH 7.4 and 37.0 "C for more than 3 months. X-ray powder diffraction (Nicolet/ NIC with a position sensitive detector in transmission mode along with the STOE attachments, Cu Kcu radiation with Ni filter) and infrared spectroscopic (Perkin-Elmer 467 grating spectrophotometer) data for both HAP preparations agreed with those reported for HAP.8,9 Chemical analysis of the seed crystals A and B gave calcium to phosphate molar ratios of 1.66 f 0.03 and 1.64 f 0.03, respectively. Scanning electron micrographs (SEM, Cambridge S-90B)and transmission electron micrographs (TEM, JEOL lOOCXII) for both HAP seed A and B were similar and showed the characteristic rodlike crystals. The SEM and TEM micrographs for HAP seed B are shown in Figures 1 and 2, respectively. The specific surface areas of HAP seed A and B crystals were determined as 21.0 f 0.5 and 21.5 f 0.5 m2 g-l, respectively(30/70 He/N2, Quantasorb 11,Quantachrome, Greenvale, NY). HAP seed B was used for all experiments with the exception of E-2 through E-8 for which seed A was used. (8) ASTM file card No. 9-432. (9) Baddiel, C. B.; Berry, B. E. Spectrochim. Acta 1966,22, 1407.
Ebrahimpour et al. Crystallization experiments were made in a water-jacketed Pyrex cell, maintained a t 37.0 f 0.1 "C, by the slow addition of potassium oxalate solution to a calcium chloride solution stirred by means of a magnetic stirrer a t 360 rpm. The stirrer bar was raised from the cell base to avoid the grinding of crystals, and nitrogen gas, saturated with water vapor a t 37.0 "C, was bubbled through the solution to exclude carbon dioxide. Following verification of the stability of the supersaturated solutions, known amounts of HAP seed slurry were added to induce the heterogeneous nucleation and subsequent growth of COM. The activities of the lattice ions were maintained constant by the potentiostatically controlled addition of solutions containing calcium chloride and potassium oxalate using a Metrohm Herisau pH-stat (Model Combititrator 3D, Brinkmann Instrument Co.). Titrant control was achieved by the use of a calcium-specific ion electrode (Calcium Ion Selectrode Model F2112Ca, Radiometer Instrument Co.) and a silver/silver chloride reference electrode. In order to maintain a constant ionic strength of 0.150 mol L-l, sodium chloride was present in both supersaturated and titrant solutions. In addition to monitoring the volume of mixed titrant solutions required to maintain constant ionic activities, samples of slurry were periodically withdrawn from the reaction cell and filtered (0.22-pm Millipore filters) and the filtrates analyzed for calcium and oxalate ions. The former was done by atomic absorption spectroscopy (Perkin-Elmer, Model 503)and the latter by high-performance ion chromatography (DionexQIC analyzer). The solid phases were examined by X-ray powder diffraction, SEM, energy-dispersive X-ray (EDX) analysis (EDAX 9100/70 energy-dispersive X-ray spectrometer), and differential scanning calorimetry (DSC; Thermal Analyzer, Du Pont 2000). The concentrations of titrant solutions were such as to exactly compensate for changes in anion and cation activities as well as dilution during crystallization. In experiments involvingcalcium binding additives, these were added prior to the addition of potassium oxalate solution so that the extent of complex formation with the calcium ions could be taken into account. Furthermore a speciation computer program was utilized in order to calculate the extent of oxalate ion pair formation with the monovalent and divalent cations. In this computer program the ionic activity coefficients and concentrations of the ionic species in the working solutions were calculated by using thermodynamic solubility products for COM, (mol K,, = 2.47 X (mol L-1)2,10and HAP, K,, = 2.35 X L-1)9.11 Values of the ion association constants were K(HOX-) = 2.188 X lo4 L mol-l,12 K(Na0X-) = 13.2 L mol-',13 K(K0X-) = 13.4 L mol-l,13 K(Ca20X2+)= 71.4 L mol-l,14 K(CaOX22-) = 17.3 L mol-l, K(Ca0X) = 2.746 X 103 L mol-l,14 K(Mg0X) = 4.020 X 103Lmol-l,l6K(H3Cit) = 1.270 X lo3L mol-l,15K(H2Cit-) = 5.61 X lo4L mol-l,15K(HCit2-)= 2.72 X lo6L mol-l,15K(CaCit-) and = 6.00 X lo4 L K(CaHCit) = 5.05 X lo2 L K(CaH&it+) = 1.25 X lo3L mol-'.'' Small corrections were also made for calcium phosphate ion pair formation.le21 All crystal growth experiments could therefore be performed a t identical calcium oxalate activities. The titrants also contained the additives at the same concentrations as those of the supersaturated solution. HAP seed crystals were pretreated with the additives before inoculation of the supersaturated solutions by (10) White, D. Ph.D. Thesis, State University of New York at Buffalo, 1983. (11) Salimi, M. H. Ph.D. Thesis, State University of New York at Buffalo, 1985. (12) Bates, R. G.; Pinching, G. D. J . Res. Natl. Bur. Stand. (U.S.) 1948, 40, 405. (13) Tomazic, B.; Nancollas, G. H. J . Cryst. Growth 1979, 46, 355. (14) Finlayson, B.; Roth, R. A.; DuBois,L. G. In Urinary Calculi;Cifuentes Delatte, L., Rapado, A., Hodgkinson, A., Ed.; Karger: Basel, 1973; pp 126,193. (15) Bates, R. G.; Pinching, G. D. J . Am. Chem. SOC.1949, 71,1274. (16) Finlayson, B.; Smith, A.; DuBois, L. Invest. Urol. 1975, 13, 20. (17) Davies, C. W.; Hoyle, B. E. J . Chem. SOC.1955, 1038. (18) Chughtai, A.; Marshal, R.; Nancollas, G. H. J . Phys. Chem. 1968, 72, 208. (19) Bates, R. G.; Acree, S. F. J . Res. Natl. Bur. Stand. (U.S.) 1943, 30, 129; 1945, 34, 395. (20) Bates, R. G.; J . Res. Natl. Bur. Stand. (U.S.) 1951,47, 2236. (21) Bjerrum,N.; Unmack,A.; K. Dan. Videnk.Selsk. Mat.-Fys.Medd. 1929, 9, 1.
Induced Crystal Growth of COM at HAP Surfaces
Langmuir, Vol. 7, No. 3, 1991 579
Table 11. Growth of COM Crystals on HAP (Seed A) Surfaces at 37 "C, pH = 7.40, IS = 0.150 mol L-I, and Different Supersaturations
5.3 -
COM growth rate expt no.
induction time/min
Tc,(=T0,)/lO-4 mol L-1
(In S)-2
on HAP/10* mol m-2 min-1
E-2 E-3 E-5 E-7 E-8
345 270 207 132 90
3.90 4.20 4.35 4.50 5.00
2.56 2.10 1.93 1.78 1.41
5.9 8.5 8.5 12.3 18.2
.
aJ
c
rZ 5.000 -I I
4.7 -
I
45 4.0
2.5
I
41
, 4.2 -LOCI A s
1
I
4.3
Figure 4. Plot of -In (rate) against -In (AG)for crystallization of COM on HAP surfaces.
b 2.2 CT 0
't
1.8 I 1.3
I
I
1.5
2.0 1 A L o q SIZ
I
I
2.5
Figure 3. Plot of In ( T ) against (In S)-2for nucleation of COM crystals on HAP (seed A) at 37 "C, pH = 7.40 and ionic strength = 0.150 mol L-1. adding a known amount of dried seed crystals to a saturated solution of the seed (pH = 7.40 and ionic strength = 0.150 mol L-' (NaC1)) in the presence of the respective additives. The concentration of the additive was then monitored until equilibrium had been reached. The equilibrated HAP crystal suspensions were used as seed in the crystallization experiments. The supersaturated solutions contained the same equilibrium concentrations of the additives as those for the pretreated HAP seed.
Results and Discussion The results of the crystal growth experiments are summarized in Table 11. It can be seen that in all experiments there was a delay or induction period, 7,in titrant addition preceding COM overgrowth. In Figure 3 In (7)is plotted against (In S)-2where the supersaturation ratio, S, is defined by eq 1. In eq 1,ai is the activity of
species i, niai'i is the ionic product, vi the number of ions, i, in the formula unit, v = Civi, and K,, the solubility product. The linear relationship in Figure 3 suggests that the induction period reflects the time necessary for the formation of critical nuclei of the depositing In terms of classical nucleation the0ry,~~--~6 7 may be expressed by = A exp[2 AG*/(5kT)]
(2) where AG*, the free energy for nucleation, is inversely proportional to In (S)and k, A, and T a r e the Boltzmann constant, preexponential constant, and absolute temperature, respectively. The surface energy, 5.93 X J m-2, calculated from the slope of the line in Figure 3 may be T
(22) Barone, J. P.; Nancollas, G. H. J. Colloid Interface Sci. 1977,62, 421. (23) Neilsen, A. E. In Crystal Growth; Peiser, H . S., Ed.; Pergamon: Oxford, 1967; p 419. (24) McDonald, J. E. Am. J . Phys. 1963,32, 31. (25) Turnbull, D.; Fisher, J. C. J. Chem. Phys. 1949, 27,71. (26) Frenkel, J. Kinetic Theory of Liquids; Oxford University Press: London, 1946.
Figure 5. Scanning electron micrograph of COM crystals a t large extent of standard growth on HAP surfaces.
compared with the value, 6.32 x J m-2, reported by Koutsoukos et al.5 for the growth of COM crystals on HAP surfaces. Following the induction period, the rates of crystallization of COM on the HAP surfaces were calculated from the slopes of plots of number of moles of COM grown as a function of time. In terms of the crystal growth kinetics equation given by eq 3, rate of growth = cW/dt = K,(S - l)n (3) rate plots are shown in Figure 4. In eq 3, dNis the number of moles of COM precipitated in time dt, K , the rate constant, and n the effective order of reaction. The slopes of the rate plots in Figure 4, following the induction period, correspond to an apparent reaction order, n = 2.4, in eq 3. This is somewhat greater than the value, 2.0, for the parabolic growth of COM on COM crystallites.1° However, it should be noted that the present data reflect the rates of COM crystallization a t the HAP surfaces so that appreciable changes in surface area may be expected due to both heterogeneous nucleation and concomitant growth. X-ray powder diffractograms of the final products showed peaks characteristic of both COM and HAP. A typical scanning electron micrograph taken at large extent of standard COM crystal overgrowth on HAP surfaces is shown in Figure 5. The development of COM crystals on the HAP surfaces after the induction periods is clearly seen and EDX analyses of samples taken a t these times are shown in Figure 6. In Figure 6a the EDX spectrum of HAP crystals (curve A) is compared with that of sample 5, 2.4 h after seed inoculation (curve B). The marked
580 Langmuir, Vol. 7, No. 3, 1991
Ebrahimpour et al.
RATE: CPS TIME 5OLSEC 0 0 - 2 O K E V : 1O E V / C H P R S T : 5OLSEC A : # 5 COM ON H A P B : # 7 COM ON H A P FS= 2313 MEM: A / B FS= 1956
102
-
Table 111. Growth of COM Crystals on HAP (Seed B ) Surfaces at TcP= TOX= 5.00 X lo-' mol L-l, pH = 7.40, IS = 0.150 mol L-l, and 37 "C in Presence of Additives
COM growth
104
I
I
expt no. 66, 72, 106"
expt no.
71 70 69 68 67 0
P
solution induction equilibrium time/min concn
90
adsorbed
0
HSA/
induction time/min 67 60 55 86 86
ppm
0 0 0
36 360
0
HSA/ mg
0.33 0.67 1.35 2.51 3.17
C A
expt no. RATE: C P S TIME SOLSEC 0 0 - 2 0 K E V : l O E V / C H PRST: 50LSEC A : 7 COM ON H A P B: 1 COM ON H A P FS= 2086 MEM: A/B FS= 1803
107 109 111 112
induction
time/min
[citrate]/ 10-4 mol
120 150 170 170
L-1
0.34 1.11
1.83 2.58
rate on
additive/ HAP/10" m-2 HAP mol m-2min-l
citrate/ 10" mol 1.1
2.8 3.2 3.2
0.97
COM growth rate on HAP/104 mol m-2 min-1 2.8 2.0 1.5 4.5 1.5
COM growth rate on HAP/ 10" mol m-2 min-1 1.5 1.0 0.83 0.83 COM growth
expt no.
induction
time/min
103 102 101 100
110 110 120 170
[magnesium]/ mol L-' 1.55 2.33 3.87 5.44
magnesium/ 10-7 mol
2.7
4.0
6.7 9.5
rate on HAP/104 mol m-2 min-1 0.96 0.93 0.91 0.87
No additives present.
decrease in the intensity of the phosphorus peak reflects the deposition of COM directly on the HAP crystallites. A further decrease of this peak is seen in Figure 6b where sample 5 is compared with sample 7 in the same experiment after 3.2 h of reaction. Although previous kinetics results were interpreted in terms of the possible secondary nucleation of COM in these COM-HAP systems, the EDX spectra in Figure 6 clearly show the gradual growth of COM crystals on the HAP surfaces. These results confirm that in kidney stone formation, the overgrowth of COM on apatite-like substrates is quite possible despite the fact that spontaneous precipitation of COM would not be expected a t such supersaturations in urines free from nucleating surfaces. Moreover the reverse process, growth of HAP on COM, does not occur under typical urine conditions.27.28 The results of COM overgrowth studies in the presence of additives using HAP seed B are summarized in Table I11 and plotted in Figure 7. It can be seen (standard experiments 66,72, and 106 where additives were absent)
that for the pure seed, the rate of growth is considerably smaller using seed B than seed A (experiment E-8, Table 11). This is probably due to the higher level of crystal perfection of HAP seed B. In the overgrowth experiments, HAP (seed B) crystals were equilibrated in saturated solutions of HAP containing the additive, adjusted to a pH of 7.4 and ionic strength 0.150 mol L-' (NaCl). The concentrations of the additives were measured after the suspensions had reached equilibrium and were normalized with respect to the HAP surface area. The experimental data for the adsorption isotherms of HSA, citrate, and magnesium ions are summarized in Table I11 and plotted in Figure 8. HSA, the major urinary protein, has been identified in renal stone matrices a t very low concentration^.^"^^ There is therefore considerable interest in elucidating the kinetics of growth of COM on HAP in the presence of HSA. It can be seen in Figure 7a that for the HAP seed crystals pretreated with HSA, the induction period preceding COM growth was less than that of the standard experiment, and the rate of subsequent COM crystallization was greater than that of the pure system. When HSA was also added to the supersaturated solution (experiments 67 and 68) prior to seed addition, the induction period was increased but it was still less than that obtained in the absence of additive. Moreover the subsequent rate of COM growth was larger in the presence of HSA. Of significance, is the fact that HAP surfaces, pretreated with HSA, appear to be more effective in inducing the growth of COM than
(27) Koutsoukos, P. G. Ph.D. Thesis, State University of New York at Buffalo, 1980. (28) Meyer, J. L.; Bergert, J. H.; Smith, L. H. Clin. Sci. Mol. Med. 1975, 49, 369.
(29) Morse, R. M.; Resnick, M. I. J. Urol. 1988, 139, 869. (30) Morse, R. M.; Resnick, M. I. J . Urol. 1989, 141, 641. (31) Iwata, H.; Kamei, 0.; Abe, Y.; Nishio, S.; Wakatsuki, A.; Ochi, K.; Takeuchi, M. J . Urol. 1988, 139, 607.
I
C U R S O R (KEV) =O3. 050
A EDAX
Figure 6. Energy dispersiveX-rayspectra of COM crystal growth on HAP surfaces at (a) 0.0 and 2.4 h and (b) 2.4 and 3.2 h after seeding the solution.
Induced Crystal Growth of COM at HAP Surfaces
Langmuir, Vol. 7, No. 3, 1991 581
' E
5 I, E
100
300
200
1 .o
t
400
time / minutes
time / minutes
/
-
7r
Figure 8. Adsorption isotherm of the additive on HAP (seed B) surfaces at 37 "C, pH = 7.40, and ionic strength = 0.150 mol L-I with the additive equilibrium concentration plotted against the surface excess of the adsorbed additives: (a) HSA, (b) citrate, and (c) magnesium.
100
200
300
400
time / minutes
Figure 7. Growth rates of COM crystals on HAP (seed B) surfaces at 37 "C, pH = 7.40, ionic strength = 0.150 mol L-I, and the additive (a) HSA, (b) citrate, and (c) magnesium. In experiments 66, 7 2 , and 106, no additives were present. untreated HAP. The adsorbed HSA on the HAP surface provides suitable sites for the initiation of COM nucleation, while at the same time retarding the rate of expansion of the nucleus. The possibility that impurities may act as both crystal growth inhibitors and promoters has been discussed p r e v i ~ u s l y If. ~the ~ ~overgrowth ~~ of COM on HAP surfaces in the presence of HSA follows a polynucleation m e ~ h a n i s m , ~critical ~ - ~ ' nuclei, formed at the surface, will (32) Davey, R. J. J . Cryst. Growth 1976, 34, 109. (33) Davey, R. J. In Industrial Crystallization 78, De Jong, E. J., Jancic, S. J., Eds.; North Holland Publishing Co.: Amsterdam, 1979; pp l69-18X - .. - - -.
(34) Weeks, J. D.; Gilmer, G. H. Adu. Chem. Phys. 1979, 40, 157. (35) Hillig, W. B. Acta Metall. 1966, 14, 1868.
grow at finite rates. New nuclei can be initiated either on an incomplete layer or a growing nucleus. The exponential growth rate law as proposed by N i e l ~ e and n ~ ~the constants he and K , involved in these calculations are described by eqs 4-6, respectively, where R , a, vin, y, and Kad are the
R = keS7l6(S- 1)2/3(In S)lI6exp[-K,/(ln S)] (4)
k, = ~ ~ v ~ ~ ( Kexp[-y/(kT)I ~ ~ C ~ V ~ )(5)~ / ~ K , = (.lr/3)(r/W2
(6)
growth rate, mean ionic diameter, integration rate frequency, edge free energy, and lattice ion adsorption coefficient, respectively. When In S >> K,, the exponential term in eq 4 approaches unity and the creation of surface nuclei is no longer the rate-determining step. Thus the rate would be expected to decrease in the presence of additives since the growth rate will be determined by the (36) Nielsen, A. E. J. Cryst. Growth 1984, 67, 289. (37) O'Hara, M.; Reid, R. C. Modelling Crystal Growth Rates from Solution; Prentice-Hall, Inc.: Englewood, Cliffs, NJ, 1973; p 39.
582 Langmuir, Vol. 7, No. 3, 1991
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s.91c PUE CALCIW O U T E MoNotwm4 six.: 2 . w g Ythod S C / M I N D O C - ~
Ebrahimpour et al.
Rn Dot.:
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1
-0.6-
-
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-l-O-
.
LL Y
E
-le6-
-2.0-
a
-0.2-
-
F!
I
\\
- 0 ..44-4
Figure 10. Scanning electron micrograph of COM crystals growing on HAP surfaces at 37 "C, pH = 7.40, and ionic strength = 0.150 mol L-l and with citrate ions (1.83 X lo4 mol L-l) present as the additive.
1
\Iv
:1 Y
3
-0.8 -0.8-
I
-0.e-
eo2.
,
.
-1.0.
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im
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-
,
.
,
.
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step movement. However, in the present work, ln S (~1.18) is relatively small compared with K , ( ~ 4 . 2and ) ~ thus ~ the growth rate appears to be controlled by the number of nuclei formed per unit area and time. Under these conditions, the formation of COM critical nuclei is the rate-determining step that may be accelerated by the adsorption of additives that cause a decrease in y. It can be concluded that the growth of COM on HAP surfaces in the presence of HSA may follow a polynuclear (38) Nielsen, A. E.; Geochemical Processes at Mineral Surfaces;Davis, J. A., Hayes, K. F., Eds.;ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; p 600.
model which explains the increase in growth rate a t low concentrations of HSA and low supersaturations. This general increase in the rate of reaction is also reflected in the shorter induction time (Figure 7a). However, as the concentration of HSA is increased, when the adsorbed HSA molecules are sufficiently close to each other, the expansion of initiated COM nuclei may be inhibited by neighboring HSA molecules before they can reach critical size. The observed induction period minimum (experiments 71-67, Table 111)may be explained by the combined action of these effects. At high HSA concentration, adsorption again becomes more important and the growth rate decreases due to the blocking of the active sites by the adsorbed molecules. It is important to note that the morphology of COM-crystals grown on both pretreated and untreated HAP surfaces was essentially the same. Potassium and sodium citrate have found some use in the clinical management of idiopathic hypercalciuria and calcium u r ~ l i t h i a s i s .The ~ ~ present ~~ work shows, as seen in Figure" 7b and in Table 111, that citrate ions inhibited the heterogeneous nucleation process as well as the initial rate of COM growth on HAP. However, as the added ions were taken up by the developingcrystals,the rate of growth reached almost the same value as that in the absence of additive. DSC studies shown in Figure 9 suggest that the possible incorporation of citrate into the growing crystals lowers the enthalpy of dehydration of the overgrown COM in comparison to that grown on the HAP surfaces in the absence of citrate. It is significant that in the presence of citrate, the COM crystals developed with markedly different morphology as shown in Figure 10. However, (39) Preminger, G. M.; Sakhaee, K.; Pak, C. Y. C. J . Urol. 1988, 239, 240. (40) Schwille, P. 0.;Rumenapf, G.; Schwarzlander,H.; Kuch, P.; Berens, H. In Inhibitors of Crystallization in Renal Lithiasis and Their Clinical Application; Martelli, A., Ed.; Basel: Krager, 1988. (41) Ebrahimpour,A. Ph.D. Thesis, State University of New York at Buffalo, 1990. (42) Smesko, S. A. Ph.D. Thesis, State University of New York at Buffalo, 1989.
Induced Crystal Growth of COM at HAP Surfaces X-ray diffraction results showed no difference between the COM overgrown on HAP in the absence and presence of citrate. These observations may be explained by the preferential adsorption of citrate ions on certain crystal face(s), reducing their rate of growth while a t the same time allowing the growth of other faces a t near normal rates. It is interesting to note that other worker^^+^^ have also found citrate to be an effective inhibitor of both HAP and COM mineralization. The inhibition of crystallization of COM, HAP, and COM on HAP suggests an important role for citrate in management of renal stone formation. The addition of magnesium, a common urinary cation, also increased the induction time for the growth of COM on HAP surfaces as shown in Table 111. However in this case, the rate of growth of COM was the same as that in the absence of magnesium (Figure 7c). Thus magnesium ions apparently inhibit the rate of heterogeneous nucle-
Langmuir, Vol. 7, No. 3, 1991 583 ation of COM on HAP surfaces while having little effect upon the subsequent COM crystallization. It is interesting to note that in studies of the growth of COM on COM crystallites the presence of magnesium ion was also found to have little influence on the rate of reacti0n.~3
Acknowledgment. We acknowledge support from the National Institute of Health (DE03223),the Health Care Instruments and Devices Institute, the Center for Advanced Technology, and The Industry University Cooperative Center for Biosurfaces. Registry No. COM, 5794-28-5;citrate, 126-44-3;magnesium, 7439-95-4. (43)Ryall, R. L.;Harnett, R. M.; Marshal, V. R. Clin. Chim. Acta 1981,112, 349.
