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The Overgrowth of Calcium Carbonate on Poly(vinyl chloride-co-vinyl acetate-co-maleic acid) E. Dalas,† P. Klepetsanis,‡,| and P. G. Koutsoukos*,§,| Department of Chemistry, Department of Pharmacy, and Department of Chemical Engineering, University of Patras, GR 265 00 Patras Greece, and Institute of Chemical Engineering and Processes at High Temperatures, P.O. Box 1414, GR 265 00 Patras, Greece Received October 1, 1998. In Final Form: July 5, 1999 Poly(vinyl chloride-co-vinyl acetate-co-maleic acid) (vinyl chloride 83%, vinyl acetate 13%, carboxylated 1%) was found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solutions at pH 8.50 and 25 °C. Induction times preceding calcium carbonate precipitation were inversely proportional to the solution supersaturation, and a surface energy of 23 mJm-2 was calculated according to classical nucleation theory. The relatively low value may be attributed to the heterogeneous character of vaterite nucleation. The linear dependence of the rates of vaterite formation on the solution supersaturation, in which the crystallization took place, in combination with the independence of the measured rates on the fluid dynamics, suggested that vaterite overgrowth was controlled by surface diffusion. This finding was in agreement with the results obtained for the crystallization of vaterite on cholesterol. Our results suggest that the kinetics of overgrowth may be very important for the stabilization of transient mineral phases. The structure of the polymeric substrates also plays a role, mainly through the development of active growth sites, which should show chemical and structural affinity to the mineral phase.
Introduction The nucleation and subsequent growth of sparingly soluble salts on substrates exhibiting a well-defined structure or molecular arrangement is very interesting, not only for the implications in the development of new composite materials, but also for understanding heterogeneous nucleation and growth processes taking place at interfaces. Of particular interest is the growth of inorganic salts on polymeric substrates, for which an important factor is the favorable geometry match between functional, binding groups on the polymers and ions of the mineral salts.1 The formation of crystals is preceded by the formation of nuclei, which grow until they reach a critical size. Only critical nuclei grow further to macroscopic crystalline formations. The formation of supercritical nuclei depends largely on the interactions between a crystal plane and a substrate. In the case in which the substrates are polymers, the surfaces of which are in most cases disordered, ion binding facilitated by specific polymer groups may catalyze nucleation and further growth.2 Among other insoluble salts, calcium carbonate is of primary importance because of the numerous applications it is finding as a filler in asphalt, paints, paper, plastics, and rubber, as a fix controller in the manufacture of steel, iron, and so forth.3 The formation of calcium carbonate on various crystalline substrates has been demonstrated and has been ascribed to the favorable lattice match between the overgrowth and the substrate.4,5 Experimental investigations have shown that polymeric substrates also * Corresponding author. † Department of Chemistry, University of Patras. ‡ Department of Pharmacy, University of Patras. § Department of Chemical Engineering, University of Patras. | Institute of Chemical Engineering and Processes at High Temperatures. (1) Mann, S.; Archibald, P. D.; Didymus, J.; Douglas, T.; Heywood, E. R.; Meldrum, F. C.; Reeves, N. J. Science, 1993, 261, 1286. (2) Wheeler, A. P.; Sikes, C. S. In Biomineralization; Mann, S., Webb, J., Williams, R. J. P., Eds.; VCH: Weinhem, 1989; pp 95-131. (3) Cowan, J. C.; Weintritt, D. J. Water Formed Scale Deposits; Gulf: Houston, TX, 1976; p 112.
selectively induce the formation of sparingly soluble salts.6-8 The lack of crystallinity of the polymers tested in these reports, an unfavorable factor for the formation of the salts, was compensated by the introduction of functional groups through chemical modification of the polymers. Thus, the presence of sulfonic groups on copolymers, for example, was found to promote the formation and stabilization of vaterite in aqueous supersaturated calcium carbonate solutions.9 Because the formation of the salts is preceded by nucleation, it is expected that surface phenomena play an important role in the regulation of the nature of crystalline material formed by the stabilization of unstable precursor phases. A typical example is the calcium carbonate system, which exhibits polymorphism. Calcium carbonate polymorphs include, in the order of increasing solubility, calcite, aragonite, and vaterite. Moreover, two hydrated forms, the mono- and hexahydrate calcium carbonates, have also been reported to form, depending on the chemical environment in which precipitation takes place.10,11 In the present work, we have examined the effectiveness of carboxylate group-containing polymers for the induction of the overgrowth of calcium carbonate from aqueous supersaturated solutions. The interesting aspect of carboxylate-containing polymers is that they are not only of technological interest but also may be used as model systems for the investigation of biological mineralization processes. In the experiments designed to investigate the nucleating capability of the polymeric substrates with respect to calcium carbonate, supersaturated solutions of (4) Lonsdale, D. K.; Sutor, D. J.; Wooley, W. E. Br. J. Urol. 1968, 40, 402. (5) Lonsdale, K. Science 1968, 159, 1199. (6) Dalas, E.; Kallitsis, J.; Koutsoukos, P. G. Colloids Surf. 1991, 53, 197. (7) Dalas, E.; Kallitsis, J.; Koutsoukos, P. G. J. Cryst. Growth 1988, 89, 287. (8) Kallitsis, J.; Koumanakos, E.; Dalas, E.; Sakkopoulos, S.; Koutsoukos, P. G. Chem. Commun. 1989, 1147. (9) Verdoes, D.; van Landschoot, R. C.; van Rosmalen, G. H. J. Cryst. Growth 1990, 99, 1124. (10) Brooks, R.; Clark, L. M.; Thurston, E. F. Philos. Trans. R. Soc. London, Ser. A 1950, 243, 145. (11) Carlson, W. D. Rev. Mineral. 1984, 11, 191.
10.1021/la981366g CCC: $18.00 © 1999 American Chemical Society Published on Web 09/21/1999
Overgrowth of Calcium Carbonate on a Copolymer
calcium carbonate were seeded with commercially obtained carboxylated vinyl chloride-vinyl acetate copolymers in the powder form. The growth kinetics of calcium carbonate were measured by the highly sensitive and reproducible technique of constant solution supersaturation adapted for the calcium carbonate system.12 Poly(vinyl chloride-co-vinyl acetate-co-maleic acid) [Aldrich, prod. no. 20030-1] consisting of 83%, 13%, and 1% of the respective monomers was used as provided in powder form without further purification or drying. Experimental Section All Experiments were done at 25.0 ( 0.1 °C in a 0.250 dm3, double-walled Pyrex vessel thermostatted by circulating water. Triply distilled, CO2-free water was used for the preparation of solutions. The supersaturated solutions, with a volume totaling 0.200 dm3, were prepared in the reaction vessel from calcium nitrate and sodium bicarbonate solutions, as described in detail elsewhere.13 The pH of the supersaturated solutions was adjusted to 8.50 by the addition of standard potassium hydroxide solution, as needed. The initial conditions of the working supersaturated solutions were chosen so that they were stable for at least 4 days. Solution pH was measured by a glass/saturated calomel pair of electrodes (Radiometer G202C and K 402, respectively), standardized before and after each experiment by NBS buffer solutions.14 Homogeneity of the species concentration and of the solids dispersion was achieved by magnetic stirring at ca. 350 rpm. Following the pH adjustment in the supersaturated solutions and verification of its stability, 200 mg of exactly weighed carboxylated copolymer was suspended in the supersaturated solution. The specific surface area of the polymer powder determined by nitrogen adsorption (multiple point BET, PerkinElmer Model 212 D sorptometer) was 3.0 m2 g-1. The pH of the supersaturated solutions remained constant following adjustment at the desired value, thus precluding the spontaneous precipitation of calcium carbonate which is accompanied by a drop in pH. Precipitation of calcium carbonate on the suspended copolymer began after the lapse of well-defined time periods and was accompanied by proton release in solution. Changes in pH as small as 0.005 pH unit triggered the addition of titrant solutions from two mechanically coupled glass burets of an appropriately modified pH-stat (Metrohm 614). The titrants added were calcium nitrate and sodium carbonate with a molar ratio of 1:1, that is, the ratio corresponding to the precipitating salt. More specifically, the two titrant solutions used, T1 and T2, had the following compositions, respectively:
T1 ) (NCCat + 2CCat) M Ca(NO3)2 T2 ) NCNa2CO3 M Na2CO3 + 2CNaHCO3 M NaHCO3 + 2CKOH M KOH where CCat and CKOH are the total concentrations of calcium nitrate, sodium bicarbonate, and potassium hydroxide in the working, supersaturated solutions, adjusted at the experimental pH value. CNa2CO3 is the concentration of Na2CO3, equal to that of calcium nitrate, and N a constant determined from preliminary experiments. The value of N depends on the rate of precipitation and determines the concentration of titrant solutions needed to replace the precipitated calcium carbonate. If the value of N is incorrect the supersaturation of the solutions will be either below the initial value or will be exceeded upon the addition of titrants. In the present experiments, the value of N was taken as equal to 10. The addition of titrant solutions controlled by proton release in solution, concomitant with the precipitation of calcium carbonate, permits investigation of the process under steadystate conditions. This methodology has the advantage not only of precisely monitoring very small rates of crystallization but (12) Kazmierczak, T. F.; Thomson, M. B.; Nancollas, G. H. J. Phys. Chem. 1982, 86, 103. (13) Koutsoukos, P. G.; Kontoyannis, C. G. J. Chem. Soc., Faraday Trans. 1 1984, 60, 1181. (14) Bates, R. G. Determination of pH; Wiley: New York, 1973.
Langmuir, Vol. 15, No. 23, 1999 8323 also of maintaining the initial experimental conditions. It was thus possible to grow substantial amounts of the initially formed crystalline phase that were sufficient for unambiguous physicochemical characterization.15 During the course of the precipitation of calcium carbonate on the polymer particles in suspension, samples were withdrawn and filtered through membrane filters (0.2 µm, Millipore), and the filtrates were analyzed for calcium by spectrophotometic titration with EDTA, using murexide as indicator. The analysis confirmed the maintenance of supersaturation in the solution (which was better than 2% in all experiments). The lapsed time between the suspension of the polymer powder in the supersaturated solutions and the beginning of the titrant additions was taken as the induction period for the nucleation of calcium carbonate on the carboxylated copolymer. From the slope of the recorder traces (titrant addition as a function of time) the rates of calcium carbonate deposition were calculated. Initial rates were used for the kinetics treatment. The solid phases formed during the course of precipitation were examined by infrared spectroscopy (Perkin-Elmer 467), powder X-ray diffraction (Phillips, 1300/00, Cu KR radiation), scanning electron microscopy (JEOL JSM 5200), thermogravimetric analysis, and differential scanning calorimetry (Du Pont 910 system coupled with a 990 programmer recorder).
Results and Discussion The driving force for the precipitation process is the chemical potential change for going from the supersaturated solution to equilibrium:
∆µ ) µ∞ - µs
(1)
In eq 1, µs and µ∞ are the chemical potentials of the solute (CaCO3) in the supersaturated solution and at equilibrium, respectively. However
µs ) µos + kT ln(RCaCO3)s
(2)
µ∞ ) µos + kT ln(RCaCO3)∞
(3)
where k is Boltzmann’s constant, T the absolute temperature, and R denotes activities. Assuming that µos ) µo∞ and substituting eqs 2 and 3 in eq 1 we have
∆µ ) kT ln(RCaCO3)∞ -kT ln(RCaCO3)s ) -kT ln
(RCaCO3)s (RCaCO3)∞
(RCa2+RCO32-)s1/2
) -kT ln
(RCa2+RCO32-)∞1/2
hence
∆µ ) -
kT RCa2+RCO32ln 2 Ko
(4)
s
In eq 4, the ratio (RCa2+RCO32-)/Kos is defined as the saturation ratio:
Ω)
RCa2+RCO32Kos
(5)
The relative supersaturation, σ, is defined as
σ ) Ω1/2 - 1
(6)
(15) Koutsoukos, P. G.; Amjad, Z.; Thomson, M. B.; Nancollas, G. H. J. Am. Chem. Soc. 1980, 102, 1553.
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Table 1. Equilibria and Equilibrium Constantsa of the Calcium Carbonate System Closed to the Atmosphereb log K° (25 °C) H2CO3* ) CO2(g) + H2O H2CO3 ) H+ + HCO3HCO3- ) H+ + CO32Ca2+ + C32- ) CaCO3° Ca2+ + HCO3- ) CaHCO3+ Ca2+ + OH- ) CaOH+ H2O ) H+ + OH-
-3.6 -6.352 -10.329 -1.89 -0.29 -1.30 -14.0
a Values from: Stumm, W., Morgan, J. J. Aquatic Chemistry, 2nd ed.; J. Wiley: New York, 1981. b [H2CO3*] ) [CO2(aq) + H2CO3] and CO2(g) + H2O ) CO2(aq).
Table 2. Crystal Growth of Calcium Carbonate on Vinyl Chloride-Vinyl Acetate Carboxylated Copolymer at Constant Solution Supersaturationa
exp no.
Cat (×10-3 mol dm-3)
1 2 3 4 5 6 7
3.00 2.75 2.50 2.40 2.25 2.00 1.90
∆Gv ∆GA ∆GC (kJ mol-1) 0.80 0.68 0.56 0.51 0.44 0.30 0.25
1.96 1.76 1.56 1.48 1.35 1.14 1.05
2.21 1.99 1.77 1.69 1.55 1.32 1.23
induction time, τ (min)
rate of crystal growth (RG) (×10-8 mol min-1 m-2)
52 120 204 250 300 421 527
1.9 1.5 1.2 1.0 1.0 0.7 0.4
a pH 8.5, 25 °C; total calcium (Ca ) ) total carbonate (C ) (system t t closed to the atmosphere). Experimental conditions, induction times preceding precipitation, and rates of crystal growth on the polymer substrate are given.
It is obvious that for the calculation of the solution supersaturation, knowledge of the activity coefficients and of the analytical ion concentrations is needed. The solution speciation was computed from the total calcium, Cat, and total carbonate, Ct, mass balance equations, the solution pH, and the electroneutrality condition. For the activity coefficients, the Davies formulation16 was used, and the system of equations was solved by successive approximations for the ionic strength.17 The partial pressure of CO2 was practically constant, as the pH of the solutions was high and the air above the solution was minimized, thus ensuring closed system conditions. The equilibria considered and the corresponding stability constants are summarized in Table 1. The thermodynamic solubility products used for the calculation of the solution supersaturation with respect to the various phases were: 1.22 × 10-8 M2 for vaterite, 4.61 × 10-9 M2 for aragonite and 3.31 × 10-9 M2 for calcite.18 The initial conditions of the experiments described herein are summarized in Table 2. As may be seen from Table 2, the induction periods were inversely proportional to the relative solution supersaturation. The subsequent rates of crystallization were found, on the other hand, to increase with supersaturation. Doubling or tripling the amounts of copolymer introduced into the supersaturated solutions had no effect on the induction period or the initial rates, normalized per unit area of the substrate. These facts suggested that calcium carbonate overgrowth was induced selectively by the organic substrate by heterogeneous nucleation. The phase formed was identified as vaterite by spectroscopic methods. The infrared and powder X-ray dif(16) Davies, C. W. Ion Association; Butterworths: Washington, 1962. (17) Nancollas, G. H. Interactions in Electrolyte Solutions; Elsevier: Amsterdam, 1966. (18) Plummer, L. N.; Busenberg, E. Geochim. Cosmochim. Acta 1982, 46, 1011.
Figure 1. Infrared spectra: (a) poly(vinyl chloride-co-vinyl acetate-co-maleic acid); (b) calcium carbonate (vaterite) grown on copolymer substrates.
fraction spectra are shown in Figures 1 and 2, respectively. In the FTIR spectrum of the copolymer (Figure 1a) the presence of carboxyl groups was verified by expansion of the spectrum around 1600 cm-1, and the band at 1633 cm-1, corresponding to the carboxyl groups of maleic acid could be clearly seen. The formation of vaterite was identified by the characteristic bands at 745 and 1070 cm-1 (Figure 1b marked by *). Moreover, the spherulitic vaterite formations can be clearly seen in the electron micrographs shown in Figure 3. The vaterite polymorph forming on the powdered copolymer was gradually transformed to the thermodynamically more stable calcite phase.19 The identification of vaterite as the precursor phase was possible only because the employed experimental technique of constant solution supersaturation provided for the possibility of formation of adequate quantities at preselected degrees of solution supersaturation. Correct identification of the precursor phase is of paramount importance in understanding the mechanism of polymorphic salt nucleation on polymeric substrates.20 The mechanism that is most likely for the initiation of vaterite nucleation on the carboxylate copolymers used occurs through binding of the calcium ions at the ionizable carboxylic groups:
It was verified by potentiometric titrations of copolymer suspensions that, at the working pH (8.50), the carboxylic groups are ionized, facilitating electrostatic interactions with free calcium ions. The carboxyl-group content of the
Overgrowth of Calcium Carbonate on a Copolymer
Figure 2. Powder X-ray diffraction spectra: (a) carboxylated copolymer; (b) calcium carbonate (vaterite) overgrowth.
Figure 3. Scanning electron micrographs: (a) carboxylated copolymer; (b) calcium carbonate overgrowth on carboxylated copolymer substrates.
tested copolymer was found to be of key importance in inducing calcium carbonate precipitation when it was suspended in supersaturated solutions. To test this finding
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further, experiments were conducted using vinyl chloridevinyl acetate copolymer (Aldrich, cat. no. 18297-4) as a substrate. This copolymer, which did not contain ionizable carboxyl groups, was highly hydrophobic and failed to induce calcium carbonate precipitation. It should be noted that calculations on a model carboxylated polymer containing carboxylate sticker groups have shown that there is a critical concentration of 3 mol % of the carboxyl groups at which the fracture energy for the polymer-solid interface is attained.21 Because nucleation is most likely initiated at the carboxylic acid component of the copolymer, we may consider these points as the anchoring parts of the composite polymer-mineral. At the contact point, the maleic acid units may be assumed to be linear. The distance between two adjacent carboxylic groups in the carboxylated copolymer used as substrate was taken as 3.0 Å,22 whereas the distance between calcium ions in the vaterite lattice, which are arranged at the corners of equilateral triangles, is 4.2 Å.23 Favorable matching can therefore be obtained every four pairs of carboxylate groups and every three pairs of calcium ions of the overgrowing vaterite. Similarly, we used carboxylated poly(vinyl chloride) polymer containing 1.8% ionizable carboxyl groups (Aldrich 18955-3). Again, in this case the polymer failed to induce precipitation of calcium carbonate. This finding suggested that it is not only the ionizable carboxylategroup content that determines the capability of a polymeric substrate to induce mineral formation; the stereochemical factor is important for the affinity of the polymeric substrate to the forming nuclei.24 Finally, we tested a styrene/maleic anhydrite copolymer (Aldrich cat. no. 18293-1) with 50% maleic anhydride content, as determined by FTIR analysis. The polymer was fully hydrolyzed so that a content of 50% ionizable carboxylate groups was present. This polymer, when dispersed in water, was fully dissolved and acted as a calcium sequestrant, not allowing any overgrowth of calcium carbonate. In the tests of the role of the carboxylate groups, we used polymers of similar molecular weight to avoid complications arising from this very important structural factor. The advantage of the methodology employed to test the nucleation capability of the various substrates is that at conditions of constant supersaturation in a closed system, nucleation driven by CO2 exhange at the air/water interface is ruled out.25 The rates we used in the kinetics analysis of our experiments were obtained from the slopes of the titrant volume addition-time curves (reflecting the amount of solid precipitating) at time zero. This is justified not only by the fact that the amount precipitated continuously increased, thus changing the total surface area, but also from the conversion of the vaterite initially formed to the thermodynamically more stable calcite. This conversion is not accompanied by pH changes and may be monitored either by spectroscopic (powder XRD, FTIR) methods or by morphological examination (SEM). As a consequence, phase transformation of vaterite to calcite does not affect measurements of the rates that are done at the initial stages of the process. The thermodynamic quantities used in the kinetics analysis of the results corresponded to (19) Spanos, N.; Koutsoukos, P. G. J. Phys. Chem. 1998, 102, 6679. (20) Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2732. (21) Gong, L.; Friend, A. D.; Wool, R. P. Macromolecules 1998, 31, 3706. (22) Gill, J. S.; Varsanik, R. G. J. Cryst. Growth 1986, 76, 57. (23) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Proc. R. Soc. London 1989, A423, 457. (24) Oner, M.; Calvert, P. Mater. Sci. Eng., C2 1994, 93. (25) Calvert, P.; Riecke, P. Chem. Mater. 1996, 8, 1715.
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Figure 4. Plot of the rate of vaterite formation on polyvinylchloride vinyl acetate carboxylated (1%) copolymer as a function of the solution supersaturation; pH 8.50, 25 °C.
vaterite. From the inverse proportionality of the induction times, τ, measured on the initial solution concentration of the calcium ions expressed as in eq 7,26
τ ) kp[Ca2+] 1-p 0
(7)
where kp is a constant and p an integer corresponding to the number of ions in the critical nucleus, it is possible to calculate the value of p. The linear fit according to eq 7 is shown in Figure 4. From the slope of this line a value of p ) 6 was obtained. Because eq 7 is an empirical relation used for the description of the nucleation process, the validity of the calculated values of p may be restricted only to comparisons among various substrates. Thus, for the overgrowth of calcite on collagen, a value of p ) 3 was obtained,27 whereas for the overgrowth of calcium carbonate monohydrate on sulfonated polystyrene and polystyrene divinyl benzene, p was 5 and 3, respectively.7 For the induced overgrowth of vaterite on cholesterol, a value of p ) 4 was obtained.28 Concerning the mechanism of formation of vaterite on the polymeric substrate, we have assumed that it may proceed via the formation of ion pairs between calcium ions and the ionized carboxylate groups on the polymer. The dependence of the rate of formation of sparingly soluble salts, RG, on the supersaturation, σ, may be expressed by eq 8:
RG ) kGσn
(8)
where kG is the apparent rate constant, which depends on the surface concentration of the active growth sites of the substrate, and n is the apparent order of the growth process. For n ) 1 or 2, eq 8 provides a similar prediction for the dependence of the rates of growth on supersaturation with the Burton, Cabrera, and Frank (BCF) relationship:29
RG ) Aσ2 tanh(B/σ)
(9)
where A and B are constants that depend on temperature and step spacings. According to eq 9, RG ) Aσ2 for low values of σ, whereas for high values Rσ ) Aσ. First-order dependence of the rate on the supersaturation is, however, (26) Mullin, J. W. Crystallization, 3rd ed.; Butterworth/Heinemann: Oxford, 1993; p 178. (27) Dalas, E.; Koutsoukos, P. G. Langmuir 1988, 4, 907. (28) Dalas, E.; Koutsoukos, P. G. J. Colloid Interface Sci. 1989, 127, 273. (29) Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. Soc. London 1951, A243, 299.
Figure 5. Plot of the dependence of the logarithm of the induction time, preceding the formation of vaterite, on the copolymer as a function of the square of the inverse of the logarithm of the relative supersaturation with respect to vaterite; pH 8.50, 25 °C.
predicted in cases in which mass transport is ratedetermining. Because, however, no dependence was found on the solution fluid dynamics, this mechanism was ruled out. The kinetics results were fitted according to eq 8, and the fit is shown in Figure 4. The first-order dependence of the rates measured on the relative solution supersaturation suggested a surface diffusion-controlled mechanism. According to classical nucleation theory, the induction time depends on the solution supersaturation, and the quantitative relationship:
log τ ) C
γ3s T3(log Ω)2
(10)
where C is a constant and γ3s the interfacial tension of the growing phase. A plot of the induction time measurements for vaterite overgrowth on the carbonated copolymer as a function of the solution supersaturation, according to eq 10, is shown in Figure 5. As may be seen, two different lines may be drawn through the experimental points. The change of slope, corresponding to σ ) 0.52 may be considered as the threshold point separating homogeneous from heterogeneous nucleation.27-30 From the slope, a, of the steepest line, the surface tension may be calculated:31
γs ) 0.029(ν2a/υ2)1/3
(11)
where ν ) 2 (the number of ions in CaCO3) and υ, the molar volume, ) 1.88 × 10-5 m3 mol-1. For vaterite, a value of 24 mJ m-2 was calculated. It is interesting to note that formation of vaterite was also reported when cholesterol was suspended in calcium carbonate supersaturated solutions.28 The copolymers examined in the present work do not have structural similarities with cholesterol, and they are more hydrophilic due to the presence of ionizable carbonate groups. The selectivity of the mineral phase developed, therefore, may be ascribed to the kinetics rather than to geometrical affinities, which may be important for the formation of the critical nuclei that grow further. Therefore, the kinetics of growth of the supercritical nuclei may be the important factors that determine the nature of the calcium carbonate minerals forming on these substrates. The structural relationship between polymers and mineral overgrowth (30) Mullin, J. W.; Ang, H. M. Faraday Discuss. 1976, 61, 141. (31) Nielsen, A. E.; So¨hnel, O. J. Cryst. Growth 1971, 11, 233.
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may be important in determining the adherence of the inorganic particles to the organic substrates. In the present work, it was shown that the presence of carboxylate groups on copolymers suspended in stable, aqueous supersaturated solutions resulted in the formation of vaterite on the polymer. The kinetics of the overgrowth of the mineral phase on the polymer showed linear dependence of the rates on the solution supersaturation, suggesting that the rate-determining step is the surface diffusion of the growth units on the supercritical nuclei that are formed at the active growth sites provided
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by the polymeric substrates. The formation of vaterite was preceded by induction times that were inversely proportional to the solution supersaturation. Induction times corresponded to the times needed for growth of the critical nuclei to reach dimensions detectable indirectly from the changes of the physicochemical properties of the supersaturated solutions. The surface energy of vaterite was computed from measurements of the induction times. LA981366G
