Mechanism of I-lydroxyapatite Dissolution
n,
and/or XU,and V and/or VI. In contrast to earlier studies on aliphatic ani ne^,^^-^^ this indicates that ion energetics can be a useful tool in characterizing the structural isomers of C2HGN'- . In the cases of C&€50+ isomer I1 and isomer IIX in both sets, i t is seen that gaseous ion properties obtained from ion-molecule reaction chemistry studies are most useful in the interpretation of ion energetics data.
Acknowledgment. We wish to thank the National Science Foundation for providing part of the funds necessary for the purchase of the mass spectrometer used in this study. References and Notes (lj L. Friedman and J. Turkevich, J. Amer. Chem. SOC., 74, 1666 (1952). (2) D. Van Raalteand A. G. Harrison, Can. J. Chem., 41, 3118 (1963). (3) A. GI. Harrison and R. G , Keyes, J. Amer. Chem. Soc., 90, 5046 (1968). (4) T. MI. Shanron and F. W. McLafferty, J. Amer. Chem. SOC., 88, 5021 (1966). (5) F. W . McLafferty and VV. T. Pike, J. Amer. Chem. SOC.,89, 5951 (1967). (6) F. W. McLafferiy and H. D. R . Schuddemage, J. Amer. Chem. Soc., 91,'1866(:969). (7) J. L. Beauchamp and R. C . Dunbar, J. Amer. Chem. SOC.,92, 1477 (1970). (8) A. S. Blair and A. 6 .Harrison, Can. J. Chem., 51, 703 (1973). (9) H. Pritchard and A. G.Harrison, J. Chem. Phys., 48,5623 (1968). (10) K. M. A. Refaey and \ N . A. Chupka, J. Chem. Phys., 48, 5205 (1968). (11) M. S. El. Mtinson and J. L. Franklin, J. Phys. Chem., 68, 3191 (1964). (12) A, G. Harrison, A. Ivko, and D. Van Raalte, Can. J. Chem., 44, 1625 (1966). (13) R H. Martin, F. W. t-anipe, and R. W. Taft, J. Amer. Chem. Soc., 88,1353 (1 986).
1273 (14) C. D. Finney and A. G . Harrison, int. J . Mass Spectrom, ion Phys., 9,221 (1972). (15) E. G. Jones, J. H. Beynon, and R. G. Cooks, J. Chem. Phys., 57, 2656 (1972) (16) i. Friedman, F. A. Long, and M . Wolfsberg, &/, Chem. Phys., 27. 613 (1957). (17) J. M. Williamsand W. H. Harnill, J. Chem. Phys.. 49,4467 (1968). (18) M . A. Haney and J. L. Franklin, Trans. Faraday Soc., 65. 1794 (1969). (19) W. A. Chupka, J. Chem. Phys., 30, 191 (1959). (20) M. W. Siegel, R. J. Celotta, J . L. Hall, J. Levine, R . A. Bennett, Phys. Rev., A6, 607 (1972) (21) G. R. Phillips, 5. H. Solka, and M E. Russell, to be submitted for publication.
(22) 6. G . Keyes and A. G . Harrison, J . Amer. Chem. SOC., 90, 5671 (1968). (23) D. Amos, R. G. Gillis, J. L. Occolowitz, and J. F . Pisani, Org. Mass Spectrom , 2, 209 (1969) (24)J Collin Suli Soc Chim Selg , 62,411 (1953) (25) V H Dibeler, J L Franklin, and R M Reese, J Amer Chem SOC., 81,68 (1959). (26) R. G . Gowenlock, P. Pritchard-Jones, and R. J. Majer, Trans. FaradaySoc., 57, 23 (1961). (27) M. W. Akopyan and F. I. Vilesov, Kinet. Katal., 4, 39 (1963). (28)J. E. Collin and M. J. Franklin, Bull. SOC. Roy, Sci. Liege, 285 (1966). (29) K. Watanabe, J. Chem. Phys., 26, 1773 (1957) (30) K. Watanabe, T. Nakayama, and J. Mottl, J. Quanl. Spectrosc. Radiat. Transfer, 2, 369 (1962). (31)J. A. Kerr, Chem. Rey., 66,465(1966) (32) R. H. Staley, R. R. Corderrnan, M. S. Foster, and J. L. Beauchamp, J. Arner. Chern. SOC., 96,1260 (1974). (33)J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Werron, K. Diaxi, and F. H. Field, Nat, Stand. Ref. Data Sei., Nat. Bur. Stand., No. 26 (1969). (34) S.W . Benson, F. R. Cruickshank, D. M. Golden, G . R. Haugen, H. E. O'Neai, A. S. Rogers, R. Shan, and R. Walsh, Chem. Rev., 69,
279 (1969). (35)J. Collin, Bull. SOC.Chim. Belg., 67,549 (1958). (36) D. H. Aue, H. M. Webb, and M. 5. Bowers, presented at the Twentieth Annual Conference on Mass Spectrametry and Ailied 'Topics, Dallas, Texas, June4-9, 1972. (37) D. Van Raalte and A. G. Harrison: Can. J. Chem.. 41,2054 (1963) (38) F. P.Lossing and G. P. Semeluk, Can. J. Chem., 48,955 (1970) (39) D. P. Stevenson, Discuss. Faraday Soc.. 10, 35 (1951)
Mechanism of Hydroxyapatite Dissolution. The Synergistic Effects of Solution Fluoride, Strontium, and Phosphate
. Dedhiya, Fudah Young, and William I . Higuchi" College iof Pharmacy, University of Michigan, Ann Arbor, Michigan 48104 February 7 1974)
(Received June 4, 1973,Revised Manuscript Received
Publicabon casts assisted by the National Institute of Dental Research
The dissolution rates and apparent solubilities of synthetic hydroxyapatite in acetate buffers containing various concentrations of phosphate, strontium, and fluoride ions were determined. The synergistic effects of' these ions in retarding the apatite dissolution were critically examined with a physical model in which a surface strontium-fluoride-apatitic complex was assumed to govern the driving force for the dissolution reaction. I t is shown that a surface complex, (hjSr4(P04)t#2, with an activity product, K,, = acavt6as++4 aPo4:-6 aF-2 = 1 x 10-13152 is consistent with the experimental data.
Introduction It has been well established during recent years that dental caries is a disease involving the dissolution of dental
enamel in an ambient acid environment of bacterial origin. The enamel mineral is composed principally of hydroxyapatite, Calo(PO4)6(OH)z. Consequently, numerous The Journal of Physical Chemistry, Voi. 78, N o . 73, 1974
M. G. Dedhiya, F. Young, and W. t . Higuchi
1274,
research efforl s have been directed toward understanding the dissolution behavior of dental enamel and synthetic hydroxyapatite.1,2 During the last decade, in our laboratories, the dissolution kinetics of hydroxyapatite in acid buffer solutions has been studied using a physical model describing the solidliquid interfacial conditions and diffusion in the stagnant liquid layer adjacent to the solid s ~ r f a c e The . ~ ~ model ~ has been critically examined for. apatite dissolution under various conditions of solution pH, buffer type, buffer capacity, and common ions (calcium and phosphates). SelfconEiistent correlations between the model predictions and experimental results have been observed. The general physical model approach has also been used in studying calcium fluoride conversions in sitthe hydroxyapatite uations where compressed pellets of synthetic hydroxyapatite and human enamel blocks were exposed to a buffer solution containing high fluoride concentrations.5~6 More recent studies? were aimed at understanding the mechanism of action of fluoride ions at low concentrations (around 1 ppm) upon the dissolution rate of hydroxyapatite in acidic buffers. The experimental data obtained 3 v e ~a wide range of conditions using powdered TVA hydroxyapatite led to the interpretation in which a fluorideapacitic surface complex or a “surface phase” with a composition, Galo(PCB&(F)z, was assumed to govern the rate of dissolution. It was postulated? that this surface compler constantly re-forms rapidly on the dissolving hydroxyapatite crystal surface through the rapid exchange of the surface hydroxyls by fluoride and thereby essentially maintains a steady-state surface “coating” that is able to govern interfacial conditions at the site of dissolution. Thus this surface complex was viewed not as a true bulk pkase but as a surface “phase” probably not extending much beyond a single unit cell into the hydroxyapatite crystai at the site of dissolution. Recent work in these laboratories has shown that the efFects of solution strontium upon the dissolution rates of TWj4 hydroxyapatite might be explained by a similar mechanism. ‘The investigators8 have shown that the dissolution process in this case is consistent with a mechanism involving an interfacial strontium-apatitic complex, as the governing surface “phase” for @a~Sr4(P84)~(0II)2, dissolution. 111 this report, the synergistic effects of solution strontium, fluoride, and phosphate in reducing the dissolution rates and apparent solubilities of TVA hydroxyapatite in acetate buffers has been systematically investigated on the basis of a physical model derived from the earlier studies.’ ,8 A b will be seen, an interfacial strontium-fluoride-apatitic complex with a composition of CaeSre(P04)&‘z was found l o be consistent with the experimental data.
ionic strength maintained at 0.5 M using sodium chloride. The procedures and materials for preparing these buffer solutions have been described previously.4 Phosphate was determined by a colorimetric procedure and calcium by atomic absorption spectrophotometry as have been described elsewhere.4 Procedures for Dissolution Studies. The hydroxyapatite powder (100 mg) was transferred to a 500-ml volumetric flask clamped to the arm of a Burrell wrist action shakerlo immersed in the water bath maintained at 30”. The dissolution experiments were started by adding to the flask 200 ml of the required buffer solution preequilibrated a t 30”. The agitation of the flask was kept constant throughout the study. Samples of 5-ml buffer solutions were withdrawn from the flask at various time intervals by means of a hypodermic syringe and needle, and were filtered using Milliporell filters (0.22 pm pore size) with Swinny filter holders. These samples were then analyzed for calcium or phosphate in calculating the amount of hydroxyapatite dissolved.
~ ~ ~ ~ ~ rSection i r n e ~ t ~ ~
The basic model for these processes is shown in Figure 1. This is shown for dissolution in acid buffer containing strontium and fluoride. The acid species (H+,HB) diffuse toward the crystal-liquid interface ( x = O), while calcium and phosphate ions diffuse outward into the bulk through the liquid diffusion layer of thickness h. Both solution strontium and fluoride are neither generated nor consumed during dissolution according to ey 4. Thus, the model assumes that the dissolution of hydroxyapatite crystals in an acid buffer containing a constant level of strontium and fluoride ions is controlled by the activity product of the surface strontium-fluoride -apatitic complex and the diffusion rates of various species involved.
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The TVA hydroxyapatite used in this study was prepared by the procedures previously d e s ~ r i b e d .The ~ . ~ procedure for tbe preparation of this material involved the reaction of monocalcium phosphate monohydrate and calcium carbonate at high temperature (1200”) in an atmosphere of steam and nitrogen. These apatite crystals are characterized by the high degree of purity and well-annealed crystallinity. The acetate buffer solutions used in ail the experiments contain 0.05 M total acetate. These solutions containing various concentrations of strontium, phosphate, and fluoride were adjusted to pH 4.5 with The Journal of Pnysicai Chemrstry, Vol. 78, No. 13, 1974
Theoretical Section Strontium-Fluoride-Apatitic Surface Complex Model. The physical model (Figure 1) assumes that when hydroxyapatite crystals are exposed to acid buffers containing sufficiently high concentrations of strontium ions (> M ) in the presence of low fluoride (
