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Spontaneous precipitation of calcium carbonate in the presence of chondroitin sulfate
Journal of Crystal Growth 217 (2000) 416}421
Spontaneous precipitation of calcium carbonate in the presence of chondroitin sulfate F. Manoli, E. Dalas* Department of Chemistry, Section of Physical, Inorganic and Nuclear Chemistry, University of Patras, GR-26500 Patras, Greece Received 17 April 2000; accepted 2 May 2000 Communicated by M. Schieber
Abstract The kinetics of spontaneous precipitation of vaterite (CaCO ) from an aqueous solution in the presence of chondroitin sulfates (CSA, CSB, CSC) was investigated by the constant composition method. The presence of chondroitin sulfate in the supersaturated solution resulted in a reduction of the crystal growth rate by 23}65%. Induction times preceding vaterite precipitation were inversely proportional to the solution's supersaturation and a surface energy of 52 mJ m\ was calculated according to the classical nucleation theory. Chondroitin sulfate in#uences the particle size distribution of the vaterite crystals formed and stabilizing this mineral phase, preventing the transformation to calcite. The apparent order found from kinetics data was n'2, thus suggesting a surface nucleation mechanism. 2000 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Calcium carbonate; Biomineralization; Chondroitin sulfate
1. Introduction The formation of calcium carbonate polymorphs, mainly calcite, aragonite and vaterite, has been reported in a number of cases such as gallstones [1], pancreatic stones in both humans and cattle [2,3], to animal phyla, algae, and in mollusk shells [4]. It has been found that supersaturation is critical in determining the calcium carbonate polymorph precipitating. Thus, at high degrees of supersaturation, where spontaneous precipitation occurs, vaterite forms predominantly even at 253C
* Corresponding author. Tel.: #30-61-997-145; fax: #3061-997-118. E-mail address: [email protected] (E. Dalas).
[5]. Since vaterite transforms easily to the more thermodynamically stable calcite when in contact with water, there are only a few data about it in the literature. It was found in gallstones, "sh otoliths and mollusk shells. The organic matrices are considered to play a principal role in biomineralization, but their function is still unclear. The chondroitin sulfates are among the principal mucopolysaccharides in the ground substance of mammalian tissues and cartilage, and occur combined with proteins. Three chondroitin sulfates have been isolated and designated A, B and C [6]. Chondroitin sulfate A is the chief one present in cartilage, adult bone, and cornea; chondroitin sulfate B is present in skin, heart valves, and tendons; and chondroitin sulfate C is found in cartilage and tendons. The chondroitin sulfates have been
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 1 4 - 5
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
di!erentiated on the basis of optical rotation and their behavior towards testicular hyaluronidase. The aim of the present work is to investigate the e!ect of chondroition sulfates on the spontaneously precipitated calcium carbonate by the constant composition technique [7,8], and attempting to answer the following question: Do chondroitin sulfates a!ect the nature, the rate or the particle size of the calcium carbonate phases forming? The methodology of the constant solution composition was applied for the study of the crystallization process of calcium carbonate, because of the advantages the method presents in accurately assessing the rates of crystallization and the nature of the precipitating crystalline polymorphs [9].
2. Experimental procedure Calcium carbonate supersaturated solutions, volume totalling 0.2 dm were prepared in a thermostated double-walled, water-jacketed pyrex glass reactor, at 25.0$0.13C, by simultaneously mixing equal volumes of calcium nitrate and sodium bicarbonate solutions, along with 20 mg of chondroitin sulfate. Calcium nitrate and sodium bicarbonate solutions were made fresh for each experiment and both stock solutions were standardized as described in Ref. [10]. The chondroitin sulfates were purchased from Sigma [chondroitin sulfate A (CSA), from Borine Trachea; chondroitin sulfate B (CSB), dermatan sulfate from Borine Mucosa; and chondroitin sulfate C (CSC) from Shark Cartilage] and used without further puri"cation. The pH of the working, supersaturated solutions was next adjusted to 8.50 by the addition (Merck, titrisol). pH measurements were done by a combination of glass/saturated calomel electrode (Metrohm) standardized before and after each experiment with NBS bu!er solutions (pH 7.41 and 9.18 at 25.03C). By using a reactor such that the air space above the solution was minimal and a high pH, rapid equilibration with the gaseous CO and constancy of its partial pressure was ensured [8]. The stability of the supersaturated solutions was indicated by the constancy of their pH. After the lapse of a certain induction period, the formation of calcium carbonate was started, resulting in a pH
417
decrease in the supersaturated solutions. A pH drop as small as 0.003 pH units, triggered the addition of calcium nitrate, sodium bicarbonate and sodium carbonate titrant solutions from two mechanical couple burettes of an appropriately modi"ed automatic titrator (Metrohm) [11,12], so that the solution composition could be maintained and the process could be studied at plethostatic conditions. The constancy of the solution composition was veri"ed by withdrawing samples from the reactor, "ltering through membrane "lters (Gelman 0.1 lm) and analyzing the "ltrates for calcium by atomic absorption spectroscopy (Varian 1200). The solid residues were examined by powder X-ray di!raction (Phillips PW 1830/1840 using Cu K radiation a Ni "lter), scanning electron microscopy (Jeol GSM 5200), FT}IR spectroscopy (Perkin}Elmer 16-PC FT}IR using KBr pellets) and di!erential scanning calorimetry (Du Pont 910 system, coupled with a 990 programmer recorder). The induction periods were measured as the time lapsed between the establishment of the working solution and the onset of titrant additions. The rate of calcium carbonate formation were taken from the plots of titrant addition as a function of time.
3. Results and discussion In all cases, an induction period preceded the onset of calcium carbonate formation. Spectroscopic examination by X-ray di!raction [13,14] (exhibit the characteristic re#ections for vaterite with d-spacing 3.57, 3.30, 2.73, 2.065 and 1.823) and FT}IR spectroscopy (exhibit the characteristic absorptions for vaterite at 1480, 1070, 873, 848 and 745 cm\) con"rmed the exclusive formation of vaterite. The absence of the hydrated polymorphs was also ruled out by di!erential scanning calorimetry [15]. The induction period, q, observed varied inversely proportional to supersaturation. The reproducibility of the reported rates was $7% (a mean of "ve experiments) and of the induction periods $15%. The solution speci"cation in all experiments was computed from pH, the total calcium mass balance, and the electroneutrality conditions assuming a system in which the partial pressure of CO is
418
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
Fig. 1. Induction time, q, of vaterite formation in the presence of CSB as a function of the initial calcium concentration in solution.
Fig. 2. Induction time, q , of vaterite formation in the presence of CSA as a function of the solution supersaturation according to the classical nucleation theory (Eq. (3)).
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
kept constant. Since the solution pH is high, and the volume of the working solution is such that air over the solution is minimal, this assumption is valid [8]. The driving force for the formation of vaterite, which is the change in Gibbs-free energy for the transition from the supersaturated solution to equilibrium, *G , was calculated from (Ca>)(CO\) R ¹ *G "! ln K 2 R ¹ "! ln X , (1) 2
where R is the gas constant, ¹ the absolute tem perature, parentheses denote the activities of the ions and K is the thermodynamic solubility of vaterite (1.222;10\) [16] and X the supersatura tion ratio. The initial conditions of the experiments reported herein were: total calcium, Ca "4.75, 4.5, 4.25, 4 mM and X "6.74, 6.18, 5.65, 5.13, respec tively; total calcium (Ca )"total carbonate (C ). The induction period observed in our experiments probably re#ects the formation of the critical nucleus. It is sometimes recommended that, induction periods are powers of the initial solution concentration, [Ca>] , [17]: s" k [Ca>]\N, (2) s where, k is a constant and p the number of ions s forming the critical nucleus. In Fig. 1, such a plot is shown. The value of p which is obtained from the slope was found to be 6, 8, 15 for vaterite formation in the presence of CSA, CSB and CSC, respectively. A value of p"6 has been reported in the literature for vaterite [18]. The dependence of the induction period on supersaturation may be used for the estimate of surface energies by plotting ln q as a function of (1/ln X ) according to the classical nucleation theory [19]. 0.4bpu 1 (3) ln q"B# k¹ (ln X ) where B is a constant, b the spape factor ("16p/3 in this case, assuming a sphere shape nuclei), k the Boltzmann constant, ¹ the absolute temperature and u the molecular volume of the phase forming (2u"molar volume/Avogadro's number), u" 3.129;10\ m). A plot of ln R against (1/ln X )
419
Table 1 Surface energy for vaterite formation Conditions
p (mJ m\)
Reference
On cholesterol On carboxylated copolymer On heated metal surfaces (503C) Spontaneous precipitation by the free drift method Spontaneous precipitation at pH 9.0 and 10.0, 253C by the constant composition method Spontaneous precipitation in the presence of chondroitin sulfate (mean value)
11 24 77 34
[14] [18] [24] [20]
73
[25]
52
This work
Fig. 3. Scanning electron microscopy of vaterite formation: (a) in the presence of CSA; (b) in the absence of any additive; Ca "4.75;10\ mol dm\, pH 8.5.
420
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
according to Eq. (3), results in a straight line from the slope of which a value for the surface energy may be obtained. Such a plot is shown in Fig. 2 Surface energy values obtained are 46, 49 and 62 mJ m\ for vaterite formation in the presence of CSA, CSB and CSC, respectively. Similar values for vaterite formation has been published as summarized in Table 1. The theoretical value for the surface energy computed for homogeneous vaterite nucleation is p"90 mJ m\ [20,21]. The rate of vaterite formation was reduced in the presence of chondroitin sulfates by 23}65% with mean values 48%, 29% and 46% for CSA, CSB and CSC, respectively. At Ca concentration equal to 4.75 M the relative reduction of vaterite formation rates (%) (R !R )/R (where R is the rate in the absence of many additive and R is the rate of the inhibitor (chondroitin sulfates)) was 50%, 32% and 51% for CSA, CSB and CSC, respectively. Chondroitin sulfates not only reduce the crystal growth rate but in#uence the particle size distribution of vaterite crystals as shown in scanning electron micrographs in Fig. 3(a) and stabilizing vaterite phase, preventing the transformation of vaterite to calcite (Fig. 3(b)). It was found that the
thermodynamically unstable vaterite transforms into the stable calcite at pH 8.5, 253C. It was suggested that, the transformation takes place through dissolution of vaterite, preferably of the small crystals followed by the crystallization of calcite [22] (Fig. 3(b)). The presence of CSA in the spontaneous precipitation process of vaterite leads to the formation of almost monodisperse crystals of about 3.6 lm (Fig. 3(a)). Spontaneous precipitation of vaterite in the presence of CSB and CSC leads to a particle size distribution 1.5}4 lm and 3.3}5.3 lm, respectively. The vaterite crystals are well formed and stable in the presence of chondroitin sulfates as compared to the crystals formed in the absence of
Table 2 Kinetic order values for spontaneous precipitation of vaterite in the presence and absence of chondroitin sulfates Conditions
Kinetic order, n
Blank experiments In the presence of CSA In the presence of CSB In the presence of CSC
6.9$0.3 8.6$0.4 6.4$0.6 8.0$0.8
Fig. 4. Rate of vaterite precipitation as a function of the relative solution supersaturation in the presence of CSC.
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
them, probably due to the lower crystal growth rate to the blocking of certain active growth sites and reducing the dissolution rate of small vaterite crystals [22]. Logarithmic plots of the rates of vaterite formation R, as a function of the relative solution supersaturation, S (S "X !1) according to R"kSL (4) yielded a straight line from the slope of which the apparent order, n, of the reaction was calculated (Table 2). Such a plot is shown in Fig. 4. These values are indicative of a surface nucleation mechanism [23]. In conclusion, the presence of chondroition sulfates in the supersaturated solutions with respect to vaterite a!ect: (a) the particle size; (b) the rates of spontaneously precipitated vaterite; and (c) stabilize this polymorph and prevent the transformation to the thermodynamically stable calcite, a process that take place even during the spontaneous precipitation experiments as shown in Fig. 3(b). There is no e!ect on the mechanism of vaterite formation. References [1] H.S. Kanfman, T.H. Magnuson, H.A. Pitt, P. Frasca, K.D. Lillemoe, Hepatology 19 (5) (1994) 1124. [2] H.J. Verine, Bull Comp. Pathol. 3 (1973) 5. [3] E.W. Moore, H.J. Verine, J. Am. Phys. Soc. G707 (1987) 5. [4] M.A. Grenshaw, in: G.H. Nancollas (Ed.), Biological Mineralization and Demineralization, Springer, Berlin, 1982, p. 243.
421
[5] A.G. Xyla, J. Microyannidis, P.G. Koutsoukos, J. Colloid Interface Sci. 153 (2) (1992) 537. [6] E.S. West, W.R. Todd, Textbook of Biochemistry, Macmillan, New York, 1963, p. 255. [7] M.B. Tomson, G.H. Nancollas, Science 200 (1977) 1059. [8] T.F. Kazmierczak, M.B. Tomson, G.H. Nancollas, J. Phys. Chem. 86 (1982) 103. [9] E.K. Giannimaras, P.G. Koutsoukos, J. Colloid Int. Sci. Langmuir 4 (1988) 855. [10] P.G. Koutsoukos, C.G. Kontoyannis, J. Chem. Soc. Faraday Trans. I 79 (1984) 617. [11] E. Dalas, P.G. Koutsoukos, Langmuir 4 (1988) 705. [12] F. Manoli, S. Koutsopoulos, E. Dalas, J. Crystal Growth 182 (1997) 116. [13] ASTM card "le No. 25}127. [14] E. Dalas, P.G. Koutsoukos, J. Colloid Interface Sci. 127 (1) (1989) 273. [15] E. Dalas, J. Kallitsis, P.G. Koutsoukos, J. Crystal Growth 89 (1988) 287. [16] N.L. Plummer, T.M.L. Wigley, D.C. Parkhurst, Am. J. Sci. 278 (1978) 179. [17] J. Nyvlt, O. Sohnel, M. Matuchova, M. Broul, The Kinetics of Industrial Crystallization, Elsevier, Amsterdam, 1985, p. 301. [18] E. Dalas, P. Klepetsanis, P.G. Koutsoukos, Langmuir 15 (23) (1999) 8322. [19] A.E. Nielsen, in: H.S. Peiser (Ed.), Crystal Growth, Pergamon Press, Oxford, 1967, p. 419. [20] D. Kralz, L. Brecevic, A.E. Nielsen, J. Crystal Growth 104 (1990) 793. [21] A.E. Nielsen, J. Crystal Growth 67 (1984) 289. [22] N. Spanos, P.G. Koutsoukos, J. Crystal Growth 1919 (1998) 783. [23] H.E.L. Madsen, J. Crystal Growth 80 (1987) 450. [24] E. Dalas, P.G. Koutsoukos, Desalination 78 (1990) 403. [25] N. Spanos, P.G. Koutsoukos, J. Phys. Chem. B 102 (1998) 6679.
Spontaneous precipitation of calcium carbonate in the presence of chondroitin sulfate F. Manoli, E. Dalas* Department of Chemistry, Section of Physical, Inorganic and Nuclear Chemistry, University of Patras, GR-26500 Patras, Greece Received 17 April 2000; accepted 2 May 2000 Communicated by M. Schieber
Abstract The kinetics of spontaneous precipitation of vaterite (CaCO ) from an aqueous solution in the presence of chondroitin sulfates (CSA, CSB, CSC) was investigated by the constant composition method. The presence of chondroitin sulfate in the supersaturated solution resulted in a reduction of the crystal growth rate by 23}65%. Induction times preceding vaterite precipitation were inversely proportional to the solution's supersaturation and a surface energy of 52 mJ m\ was calculated according to the classical nucleation theory. Chondroitin sulfate in#uences the particle size distribution of the vaterite crystals formed and stabilizing this mineral phase, preventing the transformation to calcite. The apparent order found from kinetics data was n'2, thus suggesting a surface nucleation mechanism. 2000 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Calcium carbonate; Biomineralization; Chondroitin sulfate
1. Introduction The formation of calcium carbonate polymorphs, mainly calcite, aragonite and vaterite, has been reported in a number of cases such as gallstones [1], pancreatic stones in both humans and cattle [2,3], to animal phyla, algae, and in mollusk shells [4]. It has been found that supersaturation is critical in determining the calcium carbonate polymorph precipitating. Thus, at high degrees of supersaturation, where spontaneous precipitation occurs, vaterite forms predominantly even at 253C
* Corresponding author. Tel.: #30-61-997-145; fax: #3061-997-118. E-mail address: [email protected] (E. Dalas).
[5]. Since vaterite transforms easily to the more thermodynamically stable calcite when in contact with water, there are only a few data about it in the literature. It was found in gallstones, "sh otoliths and mollusk shells. The organic matrices are considered to play a principal role in biomineralization, but their function is still unclear. The chondroitin sulfates are among the principal mucopolysaccharides in the ground substance of mammalian tissues and cartilage, and occur combined with proteins. Three chondroitin sulfates have been isolated and designated A, B and C [6]. Chondroitin sulfate A is the chief one present in cartilage, adult bone, and cornea; chondroitin sulfate B is present in skin, heart valves, and tendons; and chondroitin sulfate C is found in cartilage and tendons. The chondroitin sulfates have been
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 1 4 - 5
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
di!erentiated on the basis of optical rotation and their behavior towards testicular hyaluronidase. The aim of the present work is to investigate the e!ect of chondroition sulfates on the spontaneously precipitated calcium carbonate by the constant composition technique [7,8], and attempting to answer the following question: Do chondroitin sulfates a!ect the nature, the rate or the particle size of the calcium carbonate phases forming? The methodology of the constant solution composition was applied for the study of the crystallization process of calcium carbonate, because of the advantages the method presents in accurately assessing the rates of crystallization and the nature of the precipitating crystalline polymorphs [9].
2. Experimental procedure Calcium carbonate supersaturated solutions, volume totalling 0.2 dm were prepared in a thermostated double-walled, water-jacketed pyrex glass reactor, at 25.0$0.13C, by simultaneously mixing equal volumes of calcium nitrate and sodium bicarbonate solutions, along with 20 mg of chondroitin sulfate. Calcium nitrate and sodium bicarbonate solutions were made fresh for each experiment and both stock solutions were standardized as described in Ref. [10]. The chondroitin sulfates were purchased from Sigma [chondroitin sulfate A (CSA), from Borine Trachea; chondroitin sulfate B (CSB), dermatan sulfate from Borine Mucosa; and chondroitin sulfate C (CSC) from Shark Cartilage] and used without further puri"cation. The pH of the working, supersaturated solutions was next adjusted to 8.50 by the addition (Merck, titrisol). pH measurements were done by a combination of glass/saturated calomel electrode (Metrohm) standardized before and after each experiment with NBS bu!er solutions (pH 7.41 and 9.18 at 25.03C). By using a reactor such that the air space above the solution was minimal and a high pH, rapid equilibration with the gaseous CO and constancy of its partial pressure was ensured [8]. The stability of the supersaturated solutions was indicated by the constancy of their pH. After the lapse of a certain induction period, the formation of calcium carbonate was started, resulting in a pH
417
decrease in the supersaturated solutions. A pH drop as small as 0.003 pH units, triggered the addition of calcium nitrate, sodium bicarbonate and sodium carbonate titrant solutions from two mechanical couple burettes of an appropriately modi"ed automatic titrator (Metrohm) [11,12], so that the solution composition could be maintained and the process could be studied at plethostatic conditions. The constancy of the solution composition was veri"ed by withdrawing samples from the reactor, "ltering through membrane "lters (Gelman 0.1 lm) and analyzing the "ltrates for calcium by atomic absorption spectroscopy (Varian 1200). The solid residues were examined by powder X-ray di!raction (Phillips PW 1830/1840 using Cu K radiation a Ni "lter), scanning electron microscopy (Jeol GSM 5200), FT}IR spectroscopy (Perkin}Elmer 16-PC FT}IR using KBr pellets) and di!erential scanning calorimetry (Du Pont 910 system, coupled with a 990 programmer recorder). The induction periods were measured as the time lapsed between the establishment of the working solution and the onset of titrant additions. The rate of calcium carbonate formation were taken from the plots of titrant addition as a function of time.
3. Results and discussion In all cases, an induction period preceded the onset of calcium carbonate formation. Spectroscopic examination by X-ray di!raction [13,14] (exhibit the characteristic re#ections for vaterite with d-spacing 3.57, 3.30, 2.73, 2.065 and 1.823) and FT}IR spectroscopy (exhibit the characteristic absorptions for vaterite at 1480, 1070, 873, 848 and 745 cm\) con"rmed the exclusive formation of vaterite. The absence of the hydrated polymorphs was also ruled out by di!erential scanning calorimetry [15]. The induction period, q, observed varied inversely proportional to supersaturation. The reproducibility of the reported rates was $7% (a mean of "ve experiments) and of the induction periods $15%. The solution speci"cation in all experiments was computed from pH, the total calcium mass balance, and the electroneutrality conditions assuming a system in which the partial pressure of CO is
418
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
Fig. 1. Induction time, q, of vaterite formation in the presence of CSB as a function of the initial calcium concentration in solution.
Fig. 2. Induction time, q , of vaterite formation in the presence of CSA as a function of the solution supersaturation according to the classical nucleation theory (Eq. (3)).
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
kept constant. Since the solution pH is high, and the volume of the working solution is such that air over the solution is minimal, this assumption is valid [8]. The driving force for the formation of vaterite, which is the change in Gibbs-free energy for the transition from the supersaturated solution to equilibrium, *G , was calculated from (Ca>)(CO\) R ¹ *G "! ln K 2 R ¹ "! ln X , (1) 2
where R is the gas constant, ¹ the absolute tem perature, parentheses denote the activities of the ions and K is the thermodynamic solubility of vaterite (1.222;10\) [16] and X the supersatura tion ratio. The initial conditions of the experiments reported herein were: total calcium, Ca "4.75, 4.5, 4.25, 4 mM and X "6.74, 6.18, 5.65, 5.13, respec tively; total calcium (Ca )"total carbonate (C ). The induction period observed in our experiments probably re#ects the formation of the critical nucleus. It is sometimes recommended that, induction periods are powers of the initial solution concentration, [Ca>] , [17]: s" k [Ca>]\N, (2) s where, k is a constant and p the number of ions s forming the critical nucleus. In Fig. 1, such a plot is shown. The value of p which is obtained from the slope was found to be 6, 8, 15 for vaterite formation in the presence of CSA, CSB and CSC, respectively. A value of p"6 has been reported in the literature for vaterite [18]. The dependence of the induction period on supersaturation may be used for the estimate of surface energies by plotting ln q as a function of (1/ln X ) according to the classical nucleation theory [19]. 0.4bpu 1 (3) ln q"B# k¹ (ln X ) where B is a constant, b the spape factor ("16p/3 in this case, assuming a sphere shape nuclei), k the Boltzmann constant, ¹ the absolute temperature and u the molecular volume of the phase forming (2u"molar volume/Avogadro's number), u" 3.129;10\ m). A plot of ln R against (1/ln X )
419
Table 1 Surface energy for vaterite formation Conditions
p (mJ m\)
Reference
On cholesterol On carboxylated copolymer On heated metal surfaces (503C) Spontaneous precipitation by the free drift method Spontaneous precipitation at pH 9.0 and 10.0, 253C by the constant composition method Spontaneous precipitation in the presence of chondroitin sulfate (mean value)
11 24 77 34
[14] [18] [24] [20]
73
[25]
52
This work
Fig. 3. Scanning electron microscopy of vaterite formation: (a) in the presence of CSA; (b) in the absence of any additive; Ca "4.75;10\ mol dm\, pH 8.5.
420
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
according to Eq. (3), results in a straight line from the slope of which a value for the surface energy may be obtained. Such a plot is shown in Fig. 2 Surface energy values obtained are 46, 49 and 62 mJ m\ for vaterite formation in the presence of CSA, CSB and CSC, respectively. Similar values for vaterite formation has been published as summarized in Table 1. The theoretical value for the surface energy computed for homogeneous vaterite nucleation is p"90 mJ m\ [20,21]. The rate of vaterite formation was reduced in the presence of chondroitin sulfates by 23}65% with mean values 48%, 29% and 46% for CSA, CSB and CSC, respectively. At Ca concentration equal to 4.75 M the relative reduction of vaterite formation rates (%) (R !R )/R (where R is the rate in the absence of many additive and R is the rate of the inhibitor (chondroitin sulfates)) was 50%, 32% and 51% for CSA, CSB and CSC, respectively. Chondroitin sulfates not only reduce the crystal growth rate but in#uence the particle size distribution of vaterite crystals as shown in scanning electron micrographs in Fig. 3(a) and stabilizing vaterite phase, preventing the transformation of vaterite to calcite (Fig. 3(b)). It was found that the
thermodynamically unstable vaterite transforms into the stable calcite at pH 8.5, 253C. It was suggested that, the transformation takes place through dissolution of vaterite, preferably of the small crystals followed by the crystallization of calcite [22] (Fig. 3(b)). The presence of CSA in the spontaneous precipitation process of vaterite leads to the formation of almost monodisperse crystals of about 3.6 lm (Fig. 3(a)). Spontaneous precipitation of vaterite in the presence of CSB and CSC leads to a particle size distribution 1.5}4 lm and 3.3}5.3 lm, respectively. The vaterite crystals are well formed and stable in the presence of chondroitin sulfates as compared to the crystals formed in the absence of
Table 2 Kinetic order values for spontaneous precipitation of vaterite in the presence and absence of chondroitin sulfates Conditions
Kinetic order, n
Blank experiments In the presence of CSA In the presence of CSB In the presence of CSC
6.9$0.3 8.6$0.4 6.4$0.6 8.0$0.8
Fig. 4. Rate of vaterite precipitation as a function of the relative solution supersaturation in the presence of CSC.
F. Manoli, E. Dalas / Journal of Crystal Growth 217 (2000) 416}421
them, probably due to the lower crystal growth rate to the blocking of certain active growth sites and reducing the dissolution rate of small vaterite crystals [22]. Logarithmic plots of the rates of vaterite formation R, as a function of the relative solution supersaturation, S (S "X !1) according to R"kSL (4) yielded a straight line from the slope of which the apparent order, n, of the reaction was calculated (Table 2). Such a plot is shown in Fig. 4. These values are indicative of a surface nucleation mechanism [23]. In conclusion, the presence of chondroition sulfates in the supersaturated solutions with respect to vaterite a!ect: (a) the particle size; (b) the rates of spontaneously precipitated vaterite; and (c) stabilize this polymorph and prevent the transformation to the thermodynamically stable calcite, a process that take place even during the spontaneous precipitation experiments as shown in Fig. 3(b). There is no e!ect on the mechanism of vaterite formation. References [1] H.S. Kanfman, T.H. Magnuson, H.A. Pitt, P. Frasca, K.D. Lillemoe, Hepatology 19 (5) (1994) 1124. [2] H.J. Verine, Bull Comp. Pathol. 3 (1973) 5. [3] E.W. Moore, H.J. Verine, J. Am. Phys. Soc. G707 (1987) 5. [4] M.A. Grenshaw, in: G.H. Nancollas (Ed.), Biological Mineralization and Demineralization, Springer, Berlin, 1982, p. 243.
421
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