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Aragonite crystallization on functionalized styrene–butadiene copolymer
Journal of Crystal Growth 254 (2003) 219–224
Aragonite crystallization on functionalized styrene–butadiene copolymer E. Dalas*, S.N. Koklas, V. Papakostas Department of Chemistry, University of Patras, GR-26504 Patras, Greece Received 18 February 2003; accepted 5 March 2003 Communicated by M. Schieber
Abstract A styrene (30% w/w)–butadiene copolymer (3-arm configuration), epoxidized and further functionalized with the – S(O)2OH groups was found to be a substrate favoring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and 25 C. The crystallization was studied by the constant composition technique, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order found from kinetic analysis was n ¼ 7:270:8; thus suggesting a polynuclear mechanism. A surface energy of 4475 mJ m2 was calculated for the growing aragonite phase and a four-ion cluster forming the critical nucleus, according to the classical nucleation theory. r 2003 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn; 87.15.V Keywords: A1. Nucleation; A2. Growth from solutions; B1. Minerals
1. Introduction Biogenic calcium carbonate is produced as calcite, aragonite, or less commonly vaterite with preferred crystal orientation. Cross-section through a mollusc shell reveals a wall built of calcium carbonate crystals, all stacked in one direction (a brick-like structure) [1–4]. Organic material is also present at the junctions between the adjacent crystals with concentrations between 1% and 5% by weight of the dry shell, and is *Corresponding author. Tel.: +30-61-0997-145; fax: +3061-0997-118. E-mail address: [email protected] (E. Dalas).
through to be essential mediator of shell formation [5–7]. This organic material may control crystal nucleation, polymorph selection and crystal orientation. Proteins extracted from aragonitic prismatic shell layers of molluscs are very acidic and have large amounts of covalently bound sulfated polysaccharides [8,9]. The aim of the present work is to investigate the possibility of calcium carbonate overgrowth on a trichain styrene–butadiene (SB) copolymer containing –S(O)2OH functional groups, to study the mineral polymorph as well as the mechanism of formation by the constant composition method [10–12]. This is particularly suited for studying the formation of a new phase on a substrate since the
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(03)01155-2
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E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
initial conditions of precipitation are maintained. The conditions of the experiments selected in the present work were such that the supersaturated solutions employed were stable for periods up to 2 days, their stability verified by the constant pH and calcium concentration. The low degree of supersaturation is a better representation of the physiological environment, where the free calcium concentration is low due to complexation with the macromolecules in biological fluid. The solutions were seeded with functionalized polymers in powder form and the calcium carbonate overgrowth was followed.
2. Experimental procedure The experiments were performed in a thermostated double-wall Pyrex vessel at 2570.1 C. The total volume of the working solution was 0.200 dm3 and the stable supersaturated solutions employed were prepared by mixing equal volumes of calcium nitrate and sodium bicarbonate solutions made from standardized stock solutions as described in detail elsewhere [13,14]. The pH of the solutions was measured by a glass/saturated calomel electrode (Metrohm), standardized before and after each experiment with NBS buffer solutions. The pH of the working solution was adjusted to 8.5 by the addition of standard potassium hydroxide (Merch, Titrisol). Prior to the pH adjustment, a quantity of 0.1 g polymer powder was added to the solution. The basic polymeric material was obtained form Phillips % n ¼ 246:000 g mol1, M % w =M %n ¼ (x (SB)3 trichain), M 1:08 lt is styrene (30% w/w)–butadiene copolymer (3-arm) and consists of chains in star arrangement, where three polybutadiene chains start at a point, extended for a certain length and the polystyrene chains continue. Epoxidation was carried out with the in situ formation of performic acid in toluene solution (5% m/m solids) using an excess (110%) of the epoxidizing agent, an equimolar mixture of formic acid and hydrogen peroxide. After the addition of H2O2 at 0 C, the temperature was allowed to rise and was controlled at 20 C for the duration of the epoxidation reaction. Other details on the proce-
dure have been described before [15]. The epoxidized polymer was characterized using H NMR spectroscopy, C13 NMR spectroscopy, FTIR spectroscopy and volumetric analysis. The absence of H NMR absorptions at 3.2–3.3 and 3.8 ppm indicates the absence of a diol grouping and a furan ring, respectively. Similarly, no IR absorptions were detected for the above groups, correspondingly at 3430 and 1067 cm1. In this work, more weight was given to NMR analysis since at higher degrees of conversion neighboring oxirane groups may vitiate chemical analysis results through side reactions, leading to higher-member rings. The epoxide rings were opened by the addition of sulfuric acid in a 1,4-dioxane solution, and the acid residues were titrated with ammonium hydroxide solution [16]. The functionalized copolymers were precipitated in methanol, treated in a blender, suspended in distilled water and dried at 25 C for 48 h under vacuum. The powders were suspended in 0.1 mol dm3 calcium nitrate solution for 24 h under continuous stirring. They were then washed in 1 dm3 of distilled water. Analysis of the washings showed no calcium desorption. The degree of copolymer functionalization was 27% mol-S(O)2OH functional groups per mol butadiene, determined by titration [17], and the BET specific surface area was 24 m2 g1 (Perkin-Elmer sorptometer, Model 212 D). A pH drop as small as 0.005 pH units was sufficient to trigger the addition of titrant from two mechanically coupled burettes of an appropriately modified pH-stat (Metrohm Dosigraph with 614 Impulsomat). The two burettes contained calcium nitrate and sodium carbonate titrants having the stoichiometry of the precipitating salt (calcium: carbonate=1:1). By inclusion in the titrants of the appropriate calcium nitrate and sodium bicarbonate concentration, as well as the appropriate potassium nitrate concentration for the ionic strength, the initial conditions could be maintained constant throughout the precipitation process. Calcium nitrate, potassium nitrate, sodium bicarbonate and sodium carbonate were purchased from Merck (pro analysis). The crystal growth rates, R; were easily and accurately obtained from the recorder traces of the titrants (corresponding to the moles of calcite
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
precipitating) as a function of time [10–13]. The constancy of the solution composition was checked by sampling regularly, filtering the samples through membrane filters (0.22 mm Gelman), and analyzing the filtrates for calcium by atomic absorption spectroscopy (Varian 1200). At the end of each experiment, the solids were collected by filtration and examined by scanning electron microscopy (JEOL JSM 5200), infrared spectroscopy (Perkin-Elmer 16-PC FT-IR using KBr pellets), powder X-ray diffraction (Phillips PW 1830/1840 using Cuka radiation Ni filter), thermogravimetric analysis, and differential scanning calorimetry (Du Pont 910 system coupled with a 990 programmer recorder).
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slightly coral-like shapes having many strange projections. The solution speciation was computed from the pH, the total mass balance, the ion pair formation constants for calcium and carbonate and the
3. Results and discussion The experimental conditions are summarized in Table 1. The solid phase overgrowth to the functionalized polymer substrate was found to be aragonite [18] from the examination of: powder Xray diffraction spectra (showed only the characteristic peaks corresponding to aragonite, h k l: 111, 121, 112, 200, 112, 130, 220 and 221, 041, 202, 132 and 230, 113); from the FT-IR spectra (showed only the absorption band corresponding to aragonite, 1785, 1480, 1080, 857, 842, 714, 698 cm1 [19,20]; from TGA analysis (exclude the existence of hydrated calcium carbonate salts); and from scanning electron microscopy (Fig. 1) [20–22]. In Fig. 1(a), hexagonal shapes elongated along the c-axis are usually observed for aragonite crystals with normal crystallographic symmetry. In Fig. 1(b), aragonite rods had changed into a platy morphology along with spheres and
Fig. 1. (a) and (b) Scanning electron micrographs of aragonite precipitated on functionalized (SB) trichain copolymer.
Table 1 Crystal growth of aragonite on S(O)2OH functionalized copolymer at sustained superasaturation, 25 C, pH 8.50, 0.5 mg of polymer powder/ml, total calcium T Ca=total carbonate TC TCa (103 mol dm3)
Ionic strength (102 mol dm3)
DGa (KJ mol1)
Induction time, ta (min)
R (105 mol min1 m2)
3 2.75 2.5 2.25
7.19 6.6 6.0 5.4
2.34 2.16 1.97 1.76
6 8 11 15
2.40 1.37 0.88 0.15
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
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electroneutrality conditions by successive approximations for the ionic strength [23]. The driving force for the crystal growth of the calcium carbonate polymorph is the change in Gibbs free energy, DG; for the transfer from the supersaturated solution to equilibrium " # Rg T ðCa2þ Þ ðCO2 3 Þ DGa ln 0 2 Ks;a " # Rg T Rg T IP ln ln Oa : ¼ ð1Þ ¼ 0 Ks;a 2 2 In Eq. (1), IP is the activity product ðCa2þ Þ ðCa2 3 Þ; where parentheses denote activities 0 Ks;a the thermodynamic solubility product of 0 aragonite (Ks;a ¼ 4:613 109 [24], Rg the gas constant, T the absolute temperature and Oa the supersaturation ratio with respect to aragonite). The relative solution supersaturation Sa ; is defined by Sa ¼
0 1=2 ðIPÞ1=2 ðKs;a Þ 0 Þ1=2 ðKs;a
¼ O1=2 a 1:
ð2Þ
The reproducibility of the measured rates reported in Table 1 was 75% (a mean of five experiments) and of the induction periods 79%. The induction times, t; observed in our experiments varied inversely proportional to the supersaturation and reflects the time needed for the formation of the critical nucleus. It is sometimes recommended that, induction periods are powers of the initial concentration, [Ca2+]0, [25] t ¼ kt ½Ca2þ 1p ;
ð3Þ
where kt is a constant and p number of ions forming the critical nucleus. In Fig. 2, such a plot is shown from which a value of p ¼ 4 was estimated. Although the meaning of the number of ions forming the critical nucleus, p; has been subjected to considerable criticism, it may be a useful parameter for the comparison of various substrates for the deposition of sparingly soluble salts, a process which although quantitatively is not in agreement with the classical nucleation theory, qualitatively it may be described in the same terms [26,27].
Fig. 2. Induction times, preceding the overgrowth of aragonite on functionalized (SB) copolymer.
The subsequent rates of crystallization were found to increase with supersaturation. Doubling or tripling the amounts of polymer powder introduced in the supersaturated solution had no effect on the initial rates normalized per unit of surface area of the substrate. It should be noted that the rates we have used in the kinetic analysis of our experiments were obtained from the slopes of the curves of titrant addition (reflecting the amount of solid precipitating at time zero. This is justified by the fact that the amount precipitated continuously increased, thus changing the total surface area. Changes in the stirring rate (between 60 and 350 rpm) had no effect on the kinetics parameters. It may therefore be suggested that aragonite overgrowth was induced by the polymer substrate by heterogeneous nucleation [26,28,29] and that the number of active growth sites remained constant. The dependence of the rates, R; on the solution supersaturation with respect to aragonite may be written as R ¼ kp San :
ð4Þ
In Eq. (4), kp is the rate constant and Sa the relative supersaturation with respect to aragonite. Logarithmic plots according to the semiempirical Eq. (4) gave a satisfactory linear fit (Fig. 3) with slope n ¼ 7:270:8: Since the supersaturation of solution employed in the present work is relatively
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
Fig. 3. Rate of crystallization of aragonite on functionalized (SB) polymer substrate as a function of the relative solution supersaturation.
low, it may be assumed that a polynuclear mechanism is operative [30]. According to this model, the dependence of the measured rates of crystallization on the solution supersaturation is 2=3 R ¼ kO7=6 ðln Oa Þ1=6 a ðOa 1Þ " # 4=3 2=3 bs2 Vm NA exp : 3ðRg TÞ2 n ln Oa
Fig. 4. Kinetics of aragonite crystallization on functionalized (SB) copolymer, pH 8.50; 25 C. Polynuclear growth model.
4475 mJ m2 was obtained for the overgrowing aragonite on polymer substrate. A value of 145 mJ m2 for the surface energy of aragonite was obtained by using the empirical relationship [32,33] s ¼ a2 ½11:6 1:12 ln ðCa2þ Þa 1021 J m2 :
ð5Þ
In Eq. (4), k is the rate constant, b a geometric shape factor (b ¼ 50; 81 for prismatic aragonite overgrowth, Fig. 1(a) [31], s the surface energy of the growing phase, NA the Avogadro’s number, Vm the molar volume of the overgrowing aragonite (3.19 1029 m3) and n the number of ions in the calcium carbonate solid (n ¼ 2), Eq. (5) may be written in the form B R=f ðOa Þ ¼ k exp ; ð6Þ ln Oa where 2=3 ðln Oa Þ1=6 ; fðOa Þ ¼ O7=6 a ðOa 1Þ " # 4=3 2=3 bs2 Vm NA B¼ : 3ðRg TÞ2 n
Logarithmic plots of R=f ðOa Þ as a function of 1=ln Oa yielded a straight line, as may be seen in Fig. 4, from the slope of which a surface energy of
223
2+
ð7Þ
In Eq. (6), [Ca ]a is the solubility of aragonite at 25 C (0.0679 mol m3) and a the molecular 1=3 diameter usually calculated as a ¼ Vm ; a ¼ 3:17 1010 m. The surface energy is a composite quantity which includes the development of interface between the nucleus and both the aquatic medium and the substrate upon which it is formed. The high value predicted by Eq. (7) pertains to homogeneous nucleation conditions in contrast to our experiments, where the new phase is grown on a foreign substrate with a definite number of active growth sites. Similar values of surface energy for the overgrowth of aragonite on other substrates have been published and summarized in Table 2 along with the kinetic order, n; of Eq. (4). In conclusion, it can be said that the SB copolymer substrate in ‘‘star’’ configuration containing –S(O)2OH functional groups stabilized the formation of aragonite polymorph. The consideration of the polarity of the above-mentioned groups in which the negative charge is shifted towards the oxygen atom [5,8,17,34,35] suggests that the
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
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Table 2 Kinetic order n (Eq. (4)) and surface energy for aragonite overgrowth over various substrates, 25 C, pH 8.5 Substrate
Kinetic order n
Surface energy s (mJ m2)
Ref.
Calcite Vaterite Xiphoid SB trichain copolymer with –S(O)2OH Functional groups
7.8 4.5 4.1 —
51 28 24 —
[20] [20] [21] —
7.2
44
This work
positively charged calcium ion was attracted by the electric field, fixing the Ca2+ ions near the – S(O)2OH groups that along with the diffused 2+ CO2 cations may 3 anions towards the fixed Ca be the critical nuclei that initiate the crystallization process.
References [1] S. Mann, Struct. Bonding 54 (1983) 245. [2] S. Mann, Nature 332 (1988) 119. [3] M.A. Grenshaw, in: G.H. Nancollas (Ed.), Biological Mineralization and Demineralization, Springer, Berlin, 1982, p. 43. [4] S. Mann, New Sci. 10 (1990) 42. [5] L. Addadi, A. Berman, J. Moradian Oldak, S. Weiner, Connect. Tissue Res. 21 (1989) 127. [6] S. Weiner, CRC Crit. Rev. Biochem. 20 (4) (1989) 365. [7] J.B. Thompson, G.T. Paloczi, J.H. Kindt, M. Michenfelder, B.L. Smith, G. Stucky, D.E. Morse, P.K. Hansma, Biophys. J. 79 (6) (2000) 3307. [8] E.M. Grenfield, D.C. Wilson, M.A. Grenshaw, Am. Zool. 24 (1984) 225. [9] R.B. Lorens, M.L. Bender, Geochim. Cosmochim. Acta 44 (1980) 1265. [10] T.F. Kazmierczak, M.B. Tomson, G.H. Nancollas, J. Phys. Chem. 86 (1982) 103. [11] M.B. Tomson, G.H. Nancollas, Science 200 (1977) 1059.
[12] P.G. Koutsoukos, Z. Amjad, M.B. Tomson, G.H. Nancollas, J. Am. Chem. Soc. 102 (1980) 1553. [13] E. Dalas, P.G. Koutsoukos, Langmuir 4 (1988) 907. [14] F. Manoli, S. Koutsopoulos, E. Dalas, J. Crystal Growth 182 (1997) 116. [15] A.G. Margaritis, J.K. Kallitsis, N.K. Kalfoglou, Polymer 30 (1989) 2253. [16] J.E. Davey, M.J.R. Loadman, Br. Polym. J. 16 (1984) 134. [17] E. Dalas, J. Mater. Chem. 1 (1991) 473. [18] ASTM card file No. 5–0453. [19] R.A. Berner, J.T. Westirich, R. Graber, J. Smith, S. Martens, Am. J. Sci. 278 (1978) 816. [20] T.G. Sabbides, P.G. Koutsoukos, J. Crystal Growth 133 (9) (1993) 13. [21] F. Manoli, E. Dalas, J. Crystal Growth 217 (2000) 422. [22] H. Imai, T. Terada, T. Miura, S. Yamabi, J. Crystal Growth 244 (2002) 200. [23] G.H. Nancollas, Interactions in Electrolyte Solutions, Elsevier, Amsterdam, 1966. [24] N.L. Plummer, T.M.L. Wigley, D.C. Parkkhurst, Am. J. Sci. 278 (1978) 179. [25] J. Nyvlt, O. Sohnel, M. Matuchova, M. Broul, The kinetics of Industrial Crystallization, Elsevier, Amsterdam, 1985, pp. 301, 284–286. [26] J. Nyvlt, O. Sohnel, M. Matuchova, M. Broul, The Kinetics of Industrial Crystallization, Vol. 284, Elsevier, Amsterdam, 1985, p. 68. [27] A.E. Nielsen, J. Christofersen, in: G.H. Nancollas (Ed.), Biological Mineralization and Demineralization, Springer, Berlin, 1982, p. 7. [28] N. Watabe, in: B. Pamplin (Ed.), Progress in Crystal Growth and Characterization, Vol. 4, Pergamon Press, Oxford, 1981. [29] H.M. Ang, P.I.W. Loh, in: A.S. Myeson, K. Toyokura (Eds.), Crystallization as a Separation Process, American Chemical Society, Washington, DC, 1990, p. 329. [30] A.E. Nielsen, Kinetics of Precipitation, Pergamon, Oxford, 1964. [31] C.I. Steefel, P. van Cappellen, Geochim. Cosmocim. Acta 54 (1990) 2657. [32] A.E. Nielsen, Pure Appl. Chem. 53 (1981) 2025. [33] A.E. Nielsen, J. Crystal Growth 67 (1984) 289. [34] E. Dalas, J. Mater. Sci. Lett. 11 (1992) 1408. [35] P. Malkaj, A. Chrissanthopoulos, E. Dalas, J. Crystal Growth 242 (2002) 233.
Aragonite crystallization on functionalized styrene–butadiene copolymer E. Dalas*, S.N. Koklas, V. Papakostas Department of Chemistry, University of Patras, GR-26504 Patras, Greece Received 18 February 2003; accepted 5 March 2003 Communicated by M. Schieber
Abstract A styrene (30% w/w)–butadiene copolymer (3-arm configuration), epoxidized and further functionalized with the – S(O)2OH groups was found to be a substrate favoring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and 25 C. The crystallization was studied by the constant composition technique, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order found from kinetic analysis was n ¼ 7:270:8; thus suggesting a polynuclear mechanism. A surface energy of 4475 mJ m2 was calculated for the growing aragonite phase and a four-ion cluster forming the critical nucleus, according to the classical nucleation theory. r 2003 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn; 87.15.V Keywords: A1. Nucleation; A2. Growth from solutions; B1. Minerals
1. Introduction Biogenic calcium carbonate is produced as calcite, aragonite, or less commonly vaterite with preferred crystal orientation. Cross-section through a mollusc shell reveals a wall built of calcium carbonate crystals, all stacked in one direction (a brick-like structure) [1–4]. Organic material is also present at the junctions between the adjacent crystals with concentrations between 1% and 5% by weight of the dry shell, and is *Corresponding author. Tel.: +30-61-0997-145; fax: +3061-0997-118. E-mail address: [email protected] (E. Dalas).
through to be essential mediator of shell formation [5–7]. This organic material may control crystal nucleation, polymorph selection and crystal orientation. Proteins extracted from aragonitic prismatic shell layers of molluscs are very acidic and have large amounts of covalently bound sulfated polysaccharides [8,9]. The aim of the present work is to investigate the possibility of calcium carbonate overgrowth on a trichain styrene–butadiene (SB) copolymer containing –S(O)2OH functional groups, to study the mineral polymorph as well as the mechanism of formation by the constant composition method [10–12]. This is particularly suited for studying the formation of a new phase on a substrate since the
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(03)01155-2
220
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
initial conditions of precipitation are maintained. The conditions of the experiments selected in the present work were such that the supersaturated solutions employed were stable for periods up to 2 days, their stability verified by the constant pH and calcium concentration. The low degree of supersaturation is a better representation of the physiological environment, where the free calcium concentration is low due to complexation with the macromolecules in biological fluid. The solutions were seeded with functionalized polymers in powder form and the calcium carbonate overgrowth was followed.
2. Experimental procedure The experiments were performed in a thermostated double-wall Pyrex vessel at 2570.1 C. The total volume of the working solution was 0.200 dm3 and the stable supersaturated solutions employed were prepared by mixing equal volumes of calcium nitrate and sodium bicarbonate solutions made from standardized stock solutions as described in detail elsewhere [13,14]. The pH of the solutions was measured by a glass/saturated calomel electrode (Metrohm), standardized before and after each experiment with NBS buffer solutions. The pH of the working solution was adjusted to 8.5 by the addition of standard potassium hydroxide (Merch, Titrisol). Prior to the pH adjustment, a quantity of 0.1 g polymer powder was added to the solution. The basic polymeric material was obtained form Phillips % n ¼ 246:000 g mol1, M % w =M %n ¼ (x (SB)3 trichain), M 1:08 lt is styrene (30% w/w)–butadiene copolymer (3-arm) and consists of chains in star arrangement, where three polybutadiene chains start at a point, extended for a certain length and the polystyrene chains continue. Epoxidation was carried out with the in situ formation of performic acid in toluene solution (5% m/m solids) using an excess (110%) of the epoxidizing agent, an equimolar mixture of formic acid and hydrogen peroxide. After the addition of H2O2 at 0 C, the temperature was allowed to rise and was controlled at 20 C for the duration of the epoxidation reaction. Other details on the proce-
dure have been described before [15]. The epoxidized polymer was characterized using H NMR spectroscopy, C13 NMR spectroscopy, FTIR spectroscopy and volumetric analysis. The absence of H NMR absorptions at 3.2–3.3 and 3.8 ppm indicates the absence of a diol grouping and a furan ring, respectively. Similarly, no IR absorptions were detected for the above groups, correspondingly at 3430 and 1067 cm1. In this work, more weight was given to NMR analysis since at higher degrees of conversion neighboring oxirane groups may vitiate chemical analysis results through side reactions, leading to higher-member rings. The epoxide rings were opened by the addition of sulfuric acid in a 1,4-dioxane solution, and the acid residues were titrated with ammonium hydroxide solution [16]. The functionalized copolymers were precipitated in methanol, treated in a blender, suspended in distilled water and dried at 25 C for 48 h under vacuum. The powders were suspended in 0.1 mol dm3 calcium nitrate solution for 24 h under continuous stirring. They were then washed in 1 dm3 of distilled water. Analysis of the washings showed no calcium desorption. The degree of copolymer functionalization was 27% mol-S(O)2OH functional groups per mol butadiene, determined by titration [17], and the BET specific surface area was 24 m2 g1 (Perkin-Elmer sorptometer, Model 212 D). A pH drop as small as 0.005 pH units was sufficient to trigger the addition of titrant from two mechanically coupled burettes of an appropriately modified pH-stat (Metrohm Dosigraph with 614 Impulsomat). The two burettes contained calcium nitrate and sodium carbonate titrants having the stoichiometry of the precipitating salt (calcium: carbonate=1:1). By inclusion in the titrants of the appropriate calcium nitrate and sodium bicarbonate concentration, as well as the appropriate potassium nitrate concentration for the ionic strength, the initial conditions could be maintained constant throughout the precipitation process. Calcium nitrate, potassium nitrate, sodium bicarbonate and sodium carbonate were purchased from Merck (pro analysis). The crystal growth rates, R; were easily and accurately obtained from the recorder traces of the titrants (corresponding to the moles of calcite
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
precipitating) as a function of time [10–13]. The constancy of the solution composition was checked by sampling regularly, filtering the samples through membrane filters (0.22 mm Gelman), and analyzing the filtrates for calcium by atomic absorption spectroscopy (Varian 1200). At the end of each experiment, the solids were collected by filtration and examined by scanning electron microscopy (JEOL JSM 5200), infrared spectroscopy (Perkin-Elmer 16-PC FT-IR using KBr pellets), powder X-ray diffraction (Phillips PW 1830/1840 using Cuka radiation Ni filter), thermogravimetric analysis, and differential scanning calorimetry (Du Pont 910 system coupled with a 990 programmer recorder).
221
slightly coral-like shapes having many strange projections. The solution speciation was computed from the pH, the total mass balance, the ion pair formation constants for calcium and carbonate and the
3. Results and discussion The experimental conditions are summarized in Table 1. The solid phase overgrowth to the functionalized polymer substrate was found to be aragonite [18] from the examination of: powder Xray diffraction spectra (showed only the characteristic peaks corresponding to aragonite, h k l: 111, 121, 112, 200, 112, 130, 220 and 221, 041, 202, 132 and 230, 113); from the FT-IR spectra (showed only the absorption band corresponding to aragonite, 1785, 1480, 1080, 857, 842, 714, 698 cm1 [19,20]; from TGA analysis (exclude the existence of hydrated calcium carbonate salts); and from scanning electron microscopy (Fig. 1) [20–22]. In Fig. 1(a), hexagonal shapes elongated along the c-axis are usually observed for aragonite crystals with normal crystallographic symmetry. In Fig. 1(b), aragonite rods had changed into a platy morphology along with spheres and
Fig. 1. (a) and (b) Scanning electron micrographs of aragonite precipitated on functionalized (SB) trichain copolymer.
Table 1 Crystal growth of aragonite on S(O)2OH functionalized copolymer at sustained superasaturation, 25 C, pH 8.50, 0.5 mg of polymer powder/ml, total calcium T Ca=total carbonate TC TCa (103 mol dm3)
Ionic strength (102 mol dm3)
DGa (KJ mol1)
Induction time, ta (min)
R (105 mol min1 m2)
3 2.75 2.5 2.25
7.19 6.6 6.0 5.4
2.34 2.16 1.97 1.76
6 8 11 15
2.40 1.37 0.88 0.15
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
222
electroneutrality conditions by successive approximations for the ionic strength [23]. The driving force for the crystal growth of the calcium carbonate polymorph is the change in Gibbs free energy, DG; for the transfer from the supersaturated solution to equilibrium " # Rg T ðCa2þ Þ ðCO2 3 Þ DGa ln 0 2 Ks;a " # Rg T Rg T IP ln ln Oa : ¼ ð1Þ ¼ 0 Ks;a 2 2 In Eq. (1), IP is the activity product ðCa2þ Þ ðCa2 3 Þ; where parentheses denote activities 0 Ks;a the thermodynamic solubility product of 0 aragonite (Ks;a ¼ 4:613 109 [24], Rg the gas constant, T the absolute temperature and Oa the supersaturation ratio with respect to aragonite). The relative solution supersaturation Sa ; is defined by Sa ¼
0 1=2 ðIPÞ1=2 ðKs;a Þ 0 Þ1=2 ðKs;a
¼ O1=2 a 1:
ð2Þ
The reproducibility of the measured rates reported in Table 1 was 75% (a mean of five experiments) and of the induction periods 79%. The induction times, t; observed in our experiments varied inversely proportional to the supersaturation and reflects the time needed for the formation of the critical nucleus. It is sometimes recommended that, induction periods are powers of the initial concentration, [Ca2+]0, [25] t ¼ kt ½Ca2þ 1p ;
ð3Þ
where kt is a constant and p number of ions forming the critical nucleus. In Fig. 2, such a plot is shown from which a value of p ¼ 4 was estimated. Although the meaning of the number of ions forming the critical nucleus, p; has been subjected to considerable criticism, it may be a useful parameter for the comparison of various substrates for the deposition of sparingly soluble salts, a process which although quantitatively is not in agreement with the classical nucleation theory, qualitatively it may be described in the same terms [26,27].
Fig. 2. Induction times, preceding the overgrowth of aragonite on functionalized (SB) copolymer.
The subsequent rates of crystallization were found to increase with supersaturation. Doubling or tripling the amounts of polymer powder introduced in the supersaturated solution had no effect on the initial rates normalized per unit of surface area of the substrate. It should be noted that the rates we have used in the kinetic analysis of our experiments were obtained from the slopes of the curves of titrant addition (reflecting the amount of solid precipitating at time zero. This is justified by the fact that the amount precipitated continuously increased, thus changing the total surface area. Changes in the stirring rate (between 60 and 350 rpm) had no effect on the kinetics parameters. It may therefore be suggested that aragonite overgrowth was induced by the polymer substrate by heterogeneous nucleation [26,28,29] and that the number of active growth sites remained constant. The dependence of the rates, R; on the solution supersaturation with respect to aragonite may be written as R ¼ kp San :
ð4Þ
In Eq. (4), kp is the rate constant and Sa the relative supersaturation with respect to aragonite. Logarithmic plots according to the semiempirical Eq. (4) gave a satisfactory linear fit (Fig. 3) with slope n ¼ 7:270:8: Since the supersaturation of solution employed in the present work is relatively
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
Fig. 3. Rate of crystallization of aragonite on functionalized (SB) polymer substrate as a function of the relative solution supersaturation.
low, it may be assumed that a polynuclear mechanism is operative [30]. According to this model, the dependence of the measured rates of crystallization on the solution supersaturation is 2=3 R ¼ kO7=6 ðln Oa Þ1=6 a ðOa 1Þ " # 4=3 2=3 bs2 Vm NA exp : 3ðRg TÞ2 n ln Oa
Fig. 4. Kinetics of aragonite crystallization on functionalized (SB) copolymer, pH 8.50; 25 C. Polynuclear growth model.
4475 mJ m2 was obtained for the overgrowing aragonite on polymer substrate. A value of 145 mJ m2 for the surface energy of aragonite was obtained by using the empirical relationship [32,33] s ¼ a2 ½11:6 1:12 ln ðCa2þ Þa 1021 J m2 :
ð5Þ
In Eq. (4), k is the rate constant, b a geometric shape factor (b ¼ 50; 81 for prismatic aragonite overgrowth, Fig. 1(a) [31], s the surface energy of the growing phase, NA the Avogadro’s number, Vm the molar volume of the overgrowing aragonite (3.19 1029 m3) and n the number of ions in the calcium carbonate solid (n ¼ 2), Eq. (5) may be written in the form B R=f ðOa Þ ¼ k exp ; ð6Þ ln Oa where 2=3 ðln Oa Þ1=6 ; fðOa Þ ¼ O7=6 a ðOa 1Þ " # 4=3 2=3 bs2 Vm NA B¼ : 3ðRg TÞ2 n
Logarithmic plots of R=f ðOa Þ as a function of 1=ln Oa yielded a straight line, as may be seen in Fig. 4, from the slope of which a surface energy of
223
2+
ð7Þ
In Eq. (6), [Ca ]a is the solubility of aragonite at 25 C (0.0679 mol m3) and a the molecular 1=3 diameter usually calculated as a ¼ Vm ; a ¼ 3:17 1010 m. The surface energy is a composite quantity which includes the development of interface between the nucleus and both the aquatic medium and the substrate upon which it is formed. The high value predicted by Eq. (7) pertains to homogeneous nucleation conditions in contrast to our experiments, where the new phase is grown on a foreign substrate with a definite number of active growth sites. Similar values of surface energy for the overgrowth of aragonite on other substrates have been published and summarized in Table 2 along with the kinetic order, n; of Eq. (4). In conclusion, it can be said that the SB copolymer substrate in ‘‘star’’ configuration containing –S(O)2OH functional groups stabilized the formation of aragonite polymorph. The consideration of the polarity of the above-mentioned groups in which the negative charge is shifted towards the oxygen atom [5,8,17,34,35] suggests that the
E. Dalas et al. / Journal of Crystal Growth 254 (2003) 219–224
224
Table 2 Kinetic order n (Eq. (4)) and surface energy for aragonite overgrowth over various substrates, 25 C, pH 8.5 Substrate
Kinetic order n
Surface energy s (mJ m2)
Ref.
Calcite Vaterite Xiphoid SB trichain copolymer with –S(O)2OH Functional groups
7.8 4.5 4.1 —
51 28 24 —
[20] [20] [21] —
7.2
44
This work
positively charged calcium ion was attracted by the electric field, fixing the Ca2+ ions near the – S(O)2OH groups that along with the diffused 2+ CO2 cations may 3 anions towards the fixed Ca be the critical nuclei that initiate the crystallization process.
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